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1 INTRODUCTION
Karl Fischer was the scientist who in 1935 developed the original Karl Fischer
method for water determination in various compounds
Water content or moisture content needs to be determined at all stages of the
manufacturing process from raw materials to finished goods Mostly it is determined in
Quality Control Labs Process Labs General Chemistry RampD Labs Analytical Labs and
Manufacturing department as quality of the product depends on it In the pharmaceutical
industry it is essential to know the amount of water contained in the ingredients of a drug in
order to correctly predict its lifetime stability and effectiveness In the food industry the
water content of both raw materials and the finished foodstuff needs to be carefully
monitored
The Karl Fischer titration is a direct method that is almost specific for water The
method is especially useful for low moisture levels (lt1) and also levels lt001 are
achievable
Sheth M N Science College Patan Page no - 1
KARL - FISCHER Analysis Water Content Determination
2 Theory of Karl - Fischer Technique
A Karl Fischer titration determines the water content in a sample based on an
iodineiodide redox reaction The titration is based on the oxidation of sulphur dioxide by
iodine in the presence of water It is the same reaction as the iodometric titration of sulphur
dioxide in water Titration may be defined as ndash ldquoA technique to determine the concentration
of a substance in solution by adding to it a standard reagent of known concentration in
carefully measured amounts until a reaction of definite and known proportion is completed
as shown by a color change or by electrical measurement and then calculating the unknown
concentrationrdquo
In this method water reacts with iodine until the water is consumed and the
endpoint is reached Overall reaction can be written as ndash
H2O + SO2 + I2 harr 2HI + SO3
or
2H2O + SO2 + I2 harr2HI + H2SO4
In 1935 Karl Fischer published a description of ldquoa new procedure for the titration of waterrdquo
using the above reaction in an anhydrous nonaqueous solvent However in order to shift the
equilibrium to the right it was necessary to neutralise the acids produced Originally
pyridine was used as the neutralising base Later on diethanolamine followed by imidazole
were used as buffers Recent studies show that methanol which is the most commonly used
solvent contributes in the reaction Ethanol-based reagents have recently emerged These
have the advantage of being less toxic offering more stable endpoints and faster kinetics
Actually Karl Fischer reaction occurs in two stages
(1) SO2 + CH3OH + B harr CH3SO3- + HB+
(2) CH3SO3- + H2O + I2 + 2B harr CH3SO4
- + 2HB+ + 2I-
Where B = Base
Since water and I2 are consumed in equimolar amounts in reaction 2 if you know
the amount of I2 consumed you know the amount of water that was present in the sample
Sheth M N Science College Patan Page no - 2
KARL - FISCHER Analysis Water Content Determination
3 Factors to consider in Karl ndash Fischer Titration
A pH considerations
The Karl Fischer reaction can only take place in a certain pH range between 5 and 7
In this pH range the reaction remains constant If the pH drops too low end point
attainment becomes sluggish or an end point will not be reached at all If the pH is too high
side reactions occur making the titration non-stoichiometric We can therefore say that
errors occurring during a KF titration may be due to a change in the pH of the titration
solvent Use acid base or buffer if necessary
pH dependence of the Karl ndash Fischer reaction [K = Reaction kinetic constant]
The pH of the titration solvent can be tested using a combined pH electrode and a
pH meter The electrode is first calibrated with aqueous buffer solutions and afterwards the
pH of the titration solvent is measured
Note do not place the pH electrode directly into the KF cell because excessive moisture
will be introduced along with the electrode For further information consult the userrsquos
manuals of the main manufacturers of Karl Fischer reagents
B Aldehydes and ketones
Use special reagents to prevent side reactions which generate or consume water and prevent
good endpoints
Sheth M N Science College Patan Page no - 3
KARL - FISCHER Analysis Water Content Determination
C Water content
Consider when choosing auto-titrator
D Sample size
Choose sample size that is large enough for precise determination but not so large as to
prolong the titration exceed the capacity of the reagent or be insoluble
The sample size should be small so that as many samples as possible can be titrated in the
same electrolyte solution and the titration time kept short However take care that the
sample contains at least 50 μl water The following table provides guidelines for the sample
weight
Water content of Sample Sample size Water to be determined
100000 ppm = 10 50 mg 5000 microg
10000 ppm = 1 10 mg ndash 100 mg 100 microg ndash 1000 microg
1000 ppm = 01 100 mg ndash 1 g 100 microg ndash 1000 microg
100 ppm = 001 1 g 100 microg
10 ppm = 0001 5 g 50 microg
E Frequency of SolventWorking Medium Change
When the vessel is full when the methanol content is lt25 when the capacity of the
solvent in exceeded (generally not more than 1 mg water per 1 ml methanol)
F Exclusion of background water
Maintain desiccant tubes keep reagents dry good seals on instrument and reagent bottles
G Pre-titration conditioning of working solution
Solventworking solution must be dry but not over titrated before the sample titration to
avoid positive or negative bias to results
H Solubility of the sample
Sample Solubility is Extremely Important to Obtain Total Water Content Sample must be
completely dissolved prior to the start of the titration to obtain accurate and reproducible
results
Total Water = Surface Water + Bound Water
Sheth M N Science College Patan Page no - 4
KARL - FISCHER Analysis Water Content Determination
Different methods to improve sample solubility are
i Choose the right solvent
ii Allow sufficient time for dissolution (extract time)
iii Grind sample
iv Co-solvent additions
v Elevating solvent system temperature and
vi Adding homogenizer to system
I Conductivity
Conductivity of the solvent and added sample has to be sufficient to support proper function
of the electrodes in order to detect the endpoint andor generate the iodine titrant (by
coulometric type)
J Influence of the Base
Pyridine was the original Karl-Fischer base Pyridine because of its weak basicity cannot
totally neutralize the methylsulfurous acid The equilibrium in reaction at stage-1 is not
completely shifted to the right The reaction with pyridine is therefore slow and the
endpoint is not stable often making the repeatability using pyridine very poor
Pyridine Imidazole
Imidazole was chosen to replace pyridine as it shifts the reaction at stage-1
completely to the right reaction is swift and endpoints clear and stable Imidazole also does
not have the unpleasant odor and toxicity of pyridine
Sheth M N Science College Patan Page no - 5
KARL - FISCHER Analysis Water Content Determination
K Stability of the Reagent
The stability of the original Karl Fischer reagent initially prepared with an excess of
methanol was found to be fairly poor and hence evidently needed frequent standardization
However it was established subsequently that the stability could be improved significantly
by replacing the methanol by 2-methoxyethanol It has been observed that the titer of the
Karl Fischer reagent which stands at 35 mg of water per milliliter of reagent falls rapidly
upon standing with the passage of time Hence the following precautions must be observed
rigidly using the Karl Fischer reagent namely
(a) Always prepare the reagent a day or two before it is to be used
(b) Great care must be taken to prevent and check any possible contamination either of the
reagent or the sample by atmospheric moisture
(c) All glassware(s) must be thoroughly dried before use
(d) Standard solution should be stored out of contact with air and
(e) Essential to minimise contact between the atmosphere and the solution during the course
of titration
Sheth M N Science College Patan Page no - 6
KARL - FISCHER Analysis Water Content Determination
4 Types of Karl ndash Fischer titrations
There are two determination methods different in iodine-providing principle
(1) Volumetric titration method and
(2) Coulometric titration method
In the volumetric titration method iodine required for reaction with water is
previously dissolved in water determination TS and water content is determined by
measuring the amount of iodine consumed as a result of reaction with water in a sample It
is good for high level moisture determination
In the coulometric titration method first iodine is produced by electrolysis of the
reagent containing iodide ion and then the water content in a sample is determined by
measuring the quantity of electricity (Coulombs) which is required for the electrolysis (ie
for the production of iodine) based on the quantitative reaction of the generated iodine with
water It is good for low level moisture determination
1048707Volumetric titration ndashwater determination based on volumetric measurement of reagent
Good for high level moisture determinations
1048707Coulometrictitration ndashwater determination based on measurement of time and current (in
coulombs) used to generate the iodine Good for low level moisture determinations
Instrument Type Volumetric Coulometric
Best for Solids and Liquids Low moisture samples
Water content limit 50 ppm to 100 water 1 ppm to 100 water
Matrices Solid Liquid and Gas Liquid only
Advantages All around performance Low detection limit
Sheth M N Science College Patan Page no - 7
KARL - FISCHER Analysis Water Content Determination
(Two compartment system possess separate solvent and titrating solutions)
Sheth M N Science College Patan Page no - 8
KARL - FISCHER Analysis Water Content Determination
5 Advantages
A Karl Fischer moisture determination is advantageous as compared to a
determination based on weight loss because KF is not affected by volatile compounds The
popularity of the Karl Fischer titration is due in large part to several practical advantages
that it holds over other methods of moisture determination including
i High accuracy and precision
ii Selectivity for water
iii Small sample quantities required
iv Easy sample preparation
v Short analysis duration
vi Nearly unlimited measuring range (1ppm to 100)
vii Suitability for analyzing Solids Liquids and Gases
viii Independence of presence of other volatiles
ix Suitability for automation
In contrast loss on drying will detect the loss of any volatile substance
Sheth M N Science College Patan Page no - 9
KARL - FISCHER Analysis Water Content Determination
6 Instrumentation
Following Figure illustrates a simple dead-stop end-point assembly or a Karl Fischer
titration apparatus The titration vessel is fitted with a pair of identical platinum electrodes
a mechanical stirrer with adjustable speed and a burette It will be observed that absolutely
little or no current may flow unless and until the solution is totally free from any polarizing
substances this could perhaps be due to the absorbed layers of oxygen and hydrogen on the
anode and cathode respectively
However the current shall flow only when the two electrodes get depolarized The
Karl Fischer reagent is pumped into the burette by means of hand bellows the access of
moisture is usually prevented by employing an appropriate arrangement of desiccant tubes
Alternatively the stirring may also be accomplished either by using a magnetic stirrer or by
means of suitably dried nitrogen passed gently through the solution during the course of
titration
The end-point is achieved by employing an eiectrical circuit comprising of a
microammeter (A) platinum electrodes together with a 15 V to 20 V battery connected
across a variable resistance of about 25 kΩ First of all the resistance is adjusted in such a
manner that an initial current passes through the platinum electrodes in series with a
microammeter (A) After each addition of reagent the pointer of the microammeter gets
deflected but quickly returns to its original position At the end of the reaction a deflection
is obtained which persists for 10-15 seconds
7 Water Determination Methodology
Sheth M N Science College Patan Page no - 10
KARL - FISCHER Analysis Water Content Determination
71 Volumetric titration method
Apparatus
Generally the apparatus consists of an automatic burette a backtitration flask a stirrer and
equipment for amperometric titration at constant voltage or potentiometric titration at
constant current
Because water determination TS is extremely hygroscopic the titration apparatus
should be protected from atmospheric moisture Silica gel or calcium chloride for water
determination is usually used for moisture protection
Procedure
As a rule the titration of the sample with water determination TS should be performed at
the same temperature as that at the standardization of the TS while protecting from
moisture
The apparatus is equipped with a variable resistor in the circuit and the resistor is
adjusted to apply a definite voltage (mV) between a pair of platinum electrodes immersed in
the solution to be titrated The change in current (1048707A) is measured during the dropping of
water determination TS (Amperometric titration at constant voltage) As titration continues
the abrupt change in current in the circuit occurs but returns to the original state within
several seconds At the end of a titration the change in current persists for a certain time
(usually longer than 30 seconds) The end point of titration is determined at this electric
state
Otherwise by adjusting the resistor a definite current is passed between the two
platinum electrodes and the change in potential (mV) is measured during dropping water
determination TS (Potentiometric titration at constant current) With the progress of
titration the value indicated by the potentiometer in the circuit decreases suddenly from a
polarization state of several hundreds (mV) to the nonpolarization state but it returns to the
original state within several seconds At the end of titration the non-polarization state
persists for a certain time (usually longer than 30 seconds) The end point of titration is
determined when this electric state attains
In the case of back titration when the amperometric titration method is used at
constant voltage the needle of microammeter is out of scale while an excessive quantity of
water determination TS remains It returns rapidly to the original position when the titration
reaches the end point
Sheth M N Science College Patan Page no - 11
KARL - FISCHER Analysis Water Content Determination
Similarly when the Potentiometric titration method at constant current is used the
needle of the millivoltmeter is at the original position while an excessive quantity of water
determination TS remains A definite voltage is applied when the titration reaches the end
point Unless otherwise specified the titration of water with water determination TS is
performed by either of the methods below Usually the end point of the titration can be
observed more clearly in the back titration method than in the direct titration method
711 Direct titration
Unless otherwise specified proceed as directed below Take 25ml of methanol for water
determination in a dried titration flask and titrate with water determination TS to the end
point Unless otherwise specified weigh accurately a quantity of the sample containing 10
to 50 mg of water transfer it quickly into the titration flask and dissolve by stirring Titrate
the solution with water determination TS to the end point under vigorous stirring When the
sample is insoluble in the solvent powder the sample quickly weigh a suitable amount of
the sample accurately and transfer it quickly into the titration vessel stir the mixture for 30
minutes while protecting it from moisture
Perform a titration under vigorous stirring When the sample interferes with the Karl
Fisher reaction water in the sample can be removed by heating and under a stream of
nitrogen gas and introduced into the titration vessel by using a water-evaporation device
712 Back titration
Unless otherwise specified proceed as directed below Take 20ml of methanol for water
determination in the dried titration vessel and titrate with water determination TS Weigh
accurately a suitable quantity of the sample containing 10 to 50 mg of water transfer the
sample quickly into the titration vessel add an excessive and definite volume of water
determination TS stir for 30 min protecting from atmospheric moisture and then titrate the
solution with Water Methanol Standard Solution under vigorous stirring
Sheth M N Science College Patan Page no - 12
KARL - FISCHER Analysis Water Content Determination
Wheref = the number of mg of water (H2O) corresponding to 1 ml of water determination
TS
Where
f = the number of mg of water (H2O) corresponding to 1 ml of water determination TS
f rsquo = the number of mg of water (H2O) in 1 ml of Water Methanol Standard Solution
713 General Remarks
Volumetric Karl Fischer titration requires the determination of the titre (t) of the Karl
Fischer reagent It is usually quoted in mg of water per ml of Karl Fischer reagent Modern
reagents allow direct titration of water in the sample The sample may be introduced
directly into the KF cell or after an extraction or dissolution with a suitable solvent The
water concentration of the solvent must be determined previously in order to be subtracted
from the sample analysis In Radiometer Analytical titrators all these operations are
simplified and the different results are accounted for automatically The volumetric titration
of water allows the analysis of water concentrations between 01 and 100 If an aliquot
contains less than 1 mg of water coulometric determination will result in a more accurate
result For reasons of precision the titre of the titrant should be chosen so that the titration is
completed with a titrant demand between 1 and 10 ml
The working medium (ie the solvent required) can be freely chosen by the user
depending on the dissolution properties of the sample to be investigated For methanol-
based reagents the stoichiometry 11 of the Karl Fischer reaction is only fulfilled if there is
more than 25 methanol in the reaction mixture A methanol-free working medium can be
used however it is important to determine the titre of the KF reagent in the same working
medium The modern solvents available today present a high buffer and dissolution
capacity These solvents consist of sulphur dioxide a base and methanol or ethanol The
main advantages of these solvents are
bull A more rapid titration due to better reaction kinetics an advantage especially for the
titration of large amounts of water
bull A better reproducibility because the reaction environment is stable The pH and the
sulphur dioxide concentration remain constant
Sheth M N Science College Patan Page no - 13
KARL - FISCHER Analysis Water Content Determination
For the titration of samples producing side reactions (aldehydes ketones and
silanols) it is necessary to use an appropriate solvent Most reagent manufacturers include
the letter K in the commercial name of such solvents
72 Coulometric titration
Apparatus
Usually the apparatus is comprised of an electrolytic cell for iodine production a stirrer a
titration flask and a potentiometric titration system at constant current The iodine
production device is composed of an anode and a cathode separated by a diaphragm The
anode is immersed in the anolyte solution for water determination and the cathode is
immersed in the catholyte solution for water determination Both electrodes are usually
made of platinum-mesh
Because water determination TS is extremely hygroscopic the titration apparatus
should be protected from atmospheric moisture For this purpose silica gel or calcium
chloride for water determination is usually used Procedure Take a suitable volume of an
anolyte for water determination in a titration vessel immerse in this solution a pair of
platinum electrodes for potentiometric titration at constant current
Then immerse the iodide production system filled with a catholyte for water
determination in the anolyte solution Switch on the electrolytic system and make the
content of the titration vessel anhydrous Next take an accurately weighed amount of the
sample containing 15 mg of water add it quickly to the vessel and dissolve by stirring
Perform the titration to the end point under vigorous stirring When the sample is insoluble
in the anolyte powder it quickly and add an accurately weighed amount of the sample to
the vessel After stirring the mixture for 5 to 30 minutes while protecting from atmospheric
moisture perform the titration with vigorous stirring Determine the quantity of electricity
(C) [electric current (A) and time (s)] required for the production of iodine during the
titration and calculate the content () of the water in the sample by the formula
When the sample interferes with the Karl Fisher reaction water in the sample can be
removed by heating under a stream of nitrogen gas and introduced into the titration vessel
by using a water-evaporation device
Sheth M N Science College Patan Page no - 14
KARL - FISCHER Analysis Water Content Determination
8 Interference and Limitations of Karl Fischer Method
Interferences in the water determinations by Karl Fischer method are associated with
condensation or oxidation ndash reduction reactions with a number of substances and
compounds or by side reactions Other possible interferences to the KF reagent are certain
active metals metal oxides metal hydroxides chromates melamines etc
The Karl Fischer titration has a number of serious limitations due to possible
interferences tantamount to erroneous results namely
(i) Oxidizing agents for instance chromates Cu(II) Fe(III) Cr2O7 2ndash peroxides salts
higher oxides
Example MnO2 + 4C5H5NH+ + 2Indash rarr Mn2+ + 4C5H5N + I2 + H2O
(ii) Reducing agents such as Sn (II) salts sulphides and S2O32ndash and
(iii) Compounds that have a tendency to form water with the ingredients of the Karl Fischer
reagent for instance
(a) Basic oxides eg ZnO
Example ZnO + 2C5H5NH+ rarr Zn2+ + C5H5N + H2O
(b) Salts of weak oxy-acids eg NaHCO3
Example NaHCO3 + C5H5NH+ rarr Na+ + H2O + CO2 + C5H5N
[Note As H2CO3 carbonic acid is very unstable hence it splits up to yield a mole each of
water and CO2]
Use of certain reagents will minimize or eliminate the interferences of many classes
of compounds For example the use of non-methanol containing KF reagent and solvent will
reduce the interference from aldehyde and ketones Ammonia and amines can be eliminated
by the addition of salicylic acid to the solvent
Sheth M N Science College Patan Page no - 15
KARL - FISCHER Analysis Water Content Determination
9 Side reactions
Side Reactions are classified as
(1) Reactions Influenced by Methanol
(2) Reactions with Iodine
(1) Reactions Influenced by Methanol
It includes
bull Aldehydes react to form acetals also undergo Bisulfite addition
bull Ketones react to form ketals
bull Amines undergo methylation
bull Siloxanes undergo esterification
(2) Reactions with Iodine
It includes
bull Halogenated Hydrocarbons containing free halogens
bull Mercaptans (Thiols)
bull Phenols
bull Peroxides
Testing for Iodine Reactivity
bull Dissolve a few Iodine Crystals in Methanol and Introduce Sample
bull Watch for Solution to Change Color if Clear Iodine Reaction is Present
Sheth M N Science College Patan Page no - 16
KARL - FISCHER Analysis Water Content Determination
1 Mercaptans (Thiols) are oxidized by iodine Suppress this side reaction by treating
sample with N-ethylmaleimide in a buffered solvent system
2 Inhibition of Thiol Oxidation by Iodine
Most Phenols do not react with KF reagents higher molecular weight phenols and
Amino-phenols are most problematic Volumetric method is most suitable since the
Oxidation potential is high in coulometry or Acidify Solvent system to reduce interference
Sheth M N Science College Patan Page no - 17
KARL - FISCHER Analysis Water Content Determination
10 Applications of Karl Fischer method for determination of Water in
pharmaceutical analysis
101 PREDNISOLONE SODIUM PHOSPHATE
Materials Required Karl Fischer Reagent 100 ml prednisolone sodium phosphate
02gm anhydrous methanol 200 ml
Procedure Add about 20 ml of anhydrous methanol to the titration vessel and titrate to the
Amperometric End-point with the Karl Fischer reagent Quickly add 02 g of prednisolone
sodium phosphate sample stir for 1 minute and again titrate to the amperometric end-point
with the Karl Fischer reagent The difference between the two titrations gives the volume
(v) of Karl Fischer reagent consumed by the sample
The minimum water equivalent is 35 mg of water per ml of Karl Fischer reagent
Hence the percentage of water ww in the given sample may be calculated by the
following
Precautions
(1) The reagents and solutions used must be kept anhydrous and necessary care should be
taken throughout to prevent exposure to atmospheric moisture
(2) The Karl Fischer reagent should be protected from light and preferably stored in a bottle
fitted with an automatic burette and
(3) The water equivalent of Karl Fischer reagent should always be determined before use
102 COGNATE ASSAYS
A number of other official pharmaceutical substances may be assayed for their water
content by the Karl Fischer method as summarized in the following Table
Sheth M N Science College Patan Page no - 18
KARL - FISCHER Analysis Water Content Determination
11 Automated Electrochemical Karl Fischer Analysis
Commercially available Modern KF-Titrators are usually equipped with specifically
designed titration vessels that are exclusively meant to check and prevent the contact with
atmospheric moisture Quite a few such devices are armed with microprocessors that will
perform the requisite operations sequentially in a programmed manner automatically and
may also dish out a print-out of the desired results including the percentage moisture
content In fact these Modern KF-Titrators not only afford greater accuracy and precision
in results but also offer much ease and convenience in routine analysis as compared to the
classical techniques based on either caulometry or controlled current potentiometry using
two indicator electrodes
Thus the basis of the analysis rests upon the quantitative relationship existing
between charge passed and iodine produced by the reagent according to the above reaction
Therefore the generation of iodine is automatically stopped when an excess of it is detected
by the indicator electrode It essentially consists of two platinum electrodes across which an
AC is applied and subsequently a marked drop in voltage between the electrodes takes place
as soon as an excess of iodine is present Normally such automated instruments make use of
proprietory reagents exclusively
The major advantage of this approach to KF-analysis being that no calibration is
required as the method is absolute and is entirely based on the stoichiometry of the
aforesaid equation It is noteworthy that one may determine the amounts of water ranging
between 10 mcg and 10 mg in solid as well as liquid samples
Sheth M N Science College Patan Page no - 19
KARL - FISCHER Analysis Water Content Determination
12 Some of the innovative brands in Karl Fischer Auto ndash titrator
instruments
Sheth M N Science College Patan Page no - 20
KARL - FISCHER Analysis Water Content Determination
Sheth M N Science College Patan Page no - 21
KARL - FISCHER Analysis Water Content Determination
13 References
1 Chapter 14 Karl fishcer method for determination of water Page no 223-227
Pharmaceutical Drug Analysis Ashutosh Kar New age international limited publishers
2 Aqua Stars Karl fischer instruments wwwemdchemicalscom
3 Karl Fischer Volumetric Titration Theory and Practice by Radiometer analytical
SAS wwwradiometer-analyticalcom
4 MettlerToledo AG Analytical Switzerland wwwmtcom
5 Summary of Karl fischer titrations by Orion products Baverly MA Thermofischer
Scientific wwwthermocomwater
6 Moisture determination by Karl Fischer titration background of chemistry and
recent advances SupelcoSigma-Aldrich Bellefonte Pa wwwsigma-aldrichcom
7 Karl Fischer Titration Methods Methodology and water determination in 442
different substances European Pharmacopoeia (PharmEurope) 4th Edition plus
Supplements 2002 International Electrotechnical Commission Geneva
8 Karl ndash fischer titration wwwwikipediacom
Sheth M N Science College Patan Page no - 22
KARL - FISCHER Analysis Water Content Determination
2 Theory of Karl - Fischer Technique
A Karl Fischer titration determines the water content in a sample based on an
iodineiodide redox reaction The titration is based on the oxidation of sulphur dioxide by
iodine in the presence of water It is the same reaction as the iodometric titration of sulphur
dioxide in water Titration may be defined as ndash ldquoA technique to determine the concentration
of a substance in solution by adding to it a standard reagent of known concentration in
carefully measured amounts until a reaction of definite and known proportion is completed
as shown by a color change or by electrical measurement and then calculating the unknown
concentrationrdquo
In this method water reacts with iodine until the water is consumed and the
endpoint is reached Overall reaction can be written as ndash
H2O + SO2 + I2 harr 2HI + SO3
or
2H2O + SO2 + I2 harr2HI + H2SO4
In 1935 Karl Fischer published a description of ldquoa new procedure for the titration of waterrdquo
using the above reaction in an anhydrous nonaqueous solvent However in order to shift the
equilibrium to the right it was necessary to neutralise the acids produced Originally
pyridine was used as the neutralising base Later on diethanolamine followed by imidazole
were used as buffers Recent studies show that methanol which is the most commonly used
solvent contributes in the reaction Ethanol-based reagents have recently emerged These
have the advantage of being less toxic offering more stable endpoints and faster kinetics
Actually Karl Fischer reaction occurs in two stages
(1) SO2 + CH3OH + B harr CH3SO3- + HB+
(2) CH3SO3- + H2O + I2 + 2B harr CH3SO4
- + 2HB+ + 2I-
Where B = Base
Since water and I2 are consumed in equimolar amounts in reaction 2 if you know
the amount of I2 consumed you know the amount of water that was present in the sample
Sheth M N Science College Patan Page no - 2
KARL - FISCHER Analysis Water Content Determination
3 Factors to consider in Karl ndash Fischer Titration
A pH considerations
The Karl Fischer reaction can only take place in a certain pH range between 5 and 7
In this pH range the reaction remains constant If the pH drops too low end point
attainment becomes sluggish or an end point will not be reached at all If the pH is too high
side reactions occur making the titration non-stoichiometric We can therefore say that
errors occurring during a KF titration may be due to a change in the pH of the titration
solvent Use acid base or buffer if necessary
pH dependence of the Karl ndash Fischer reaction [K = Reaction kinetic constant]
The pH of the titration solvent can be tested using a combined pH electrode and a
pH meter The electrode is first calibrated with aqueous buffer solutions and afterwards the
pH of the titration solvent is measured
Note do not place the pH electrode directly into the KF cell because excessive moisture
will be introduced along with the electrode For further information consult the userrsquos
manuals of the main manufacturers of Karl Fischer reagents
B Aldehydes and ketones
Use special reagents to prevent side reactions which generate or consume water and prevent
good endpoints
Sheth M N Science College Patan Page no - 3
KARL - FISCHER Analysis Water Content Determination
C Water content
Consider when choosing auto-titrator
D Sample size
Choose sample size that is large enough for precise determination but not so large as to
prolong the titration exceed the capacity of the reagent or be insoluble
The sample size should be small so that as many samples as possible can be titrated in the
same electrolyte solution and the titration time kept short However take care that the
sample contains at least 50 μl water The following table provides guidelines for the sample
weight
Water content of Sample Sample size Water to be determined
100000 ppm = 10 50 mg 5000 microg
10000 ppm = 1 10 mg ndash 100 mg 100 microg ndash 1000 microg
1000 ppm = 01 100 mg ndash 1 g 100 microg ndash 1000 microg
100 ppm = 001 1 g 100 microg
10 ppm = 0001 5 g 50 microg
E Frequency of SolventWorking Medium Change
When the vessel is full when the methanol content is lt25 when the capacity of the
solvent in exceeded (generally not more than 1 mg water per 1 ml methanol)
F Exclusion of background water
Maintain desiccant tubes keep reagents dry good seals on instrument and reagent bottles
G Pre-titration conditioning of working solution
Solventworking solution must be dry but not over titrated before the sample titration to
avoid positive or negative bias to results
H Solubility of the sample
Sample Solubility is Extremely Important to Obtain Total Water Content Sample must be
completely dissolved prior to the start of the titration to obtain accurate and reproducible
results
Total Water = Surface Water + Bound Water
Sheth M N Science College Patan Page no - 4
KARL - FISCHER Analysis Water Content Determination
Different methods to improve sample solubility are
i Choose the right solvent
ii Allow sufficient time for dissolution (extract time)
iii Grind sample
iv Co-solvent additions
v Elevating solvent system temperature and
vi Adding homogenizer to system
I Conductivity
Conductivity of the solvent and added sample has to be sufficient to support proper function
of the electrodes in order to detect the endpoint andor generate the iodine titrant (by
coulometric type)
J Influence of the Base
Pyridine was the original Karl-Fischer base Pyridine because of its weak basicity cannot
totally neutralize the methylsulfurous acid The equilibrium in reaction at stage-1 is not
completely shifted to the right The reaction with pyridine is therefore slow and the
endpoint is not stable often making the repeatability using pyridine very poor
Pyridine Imidazole
Imidazole was chosen to replace pyridine as it shifts the reaction at stage-1
completely to the right reaction is swift and endpoints clear and stable Imidazole also does
not have the unpleasant odor and toxicity of pyridine
Sheth M N Science College Patan Page no - 5
KARL - FISCHER Analysis Water Content Determination
K Stability of the Reagent
The stability of the original Karl Fischer reagent initially prepared with an excess of
methanol was found to be fairly poor and hence evidently needed frequent standardization
However it was established subsequently that the stability could be improved significantly
by replacing the methanol by 2-methoxyethanol It has been observed that the titer of the
Karl Fischer reagent which stands at 35 mg of water per milliliter of reagent falls rapidly
upon standing with the passage of time Hence the following precautions must be observed
rigidly using the Karl Fischer reagent namely
(a) Always prepare the reagent a day or two before it is to be used
(b) Great care must be taken to prevent and check any possible contamination either of the
reagent or the sample by atmospheric moisture
(c) All glassware(s) must be thoroughly dried before use
(d) Standard solution should be stored out of contact with air and
(e) Essential to minimise contact between the atmosphere and the solution during the course
of titration
Sheth M N Science College Patan Page no - 6
KARL - FISCHER Analysis Water Content Determination
4 Types of Karl ndash Fischer titrations
There are two determination methods different in iodine-providing principle
(1) Volumetric titration method and
(2) Coulometric titration method
In the volumetric titration method iodine required for reaction with water is
previously dissolved in water determination TS and water content is determined by
measuring the amount of iodine consumed as a result of reaction with water in a sample It
is good for high level moisture determination
In the coulometric titration method first iodine is produced by electrolysis of the
reagent containing iodide ion and then the water content in a sample is determined by
measuring the quantity of electricity (Coulombs) which is required for the electrolysis (ie
for the production of iodine) based on the quantitative reaction of the generated iodine with
water It is good for low level moisture determination
1048707Volumetric titration ndashwater determination based on volumetric measurement of reagent
Good for high level moisture determinations
1048707Coulometrictitration ndashwater determination based on measurement of time and current (in
coulombs) used to generate the iodine Good for low level moisture determinations
Instrument Type Volumetric Coulometric
Best for Solids and Liquids Low moisture samples
Water content limit 50 ppm to 100 water 1 ppm to 100 water
Matrices Solid Liquid and Gas Liquid only
Advantages All around performance Low detection limit
Sheth M N Science College Patan Page no - 7
KARL - FISCHER Analysis Water Content Determination
(Two compartment system possess separate solvent and titrating solutions)
Sheth M N Science College Patan Page no - 8
KARL - FISCHER Analysis Water Content Determination
5 Advantages
A Karl Fischer moisture determination is advantageous as compared to a
determination based on weight loss because KF is not affected by volatile compounds The
popularity of the Karl Fischer titration is due in large part to several practical advantages
that it holds over other methods of moisture determination including
i High accuracy and precision
ii Selectivity for water
iii Small sample quantities required
iv Easy sample preparation
v Short analysis duration
vi Nearly unlimited measuring range (1ppm to 100)
vii Suitability for analyzing Solids Liquids and Gases
viii Independence of presence of other volatiles
ix Suitability for automation
In contrast loss on drying will detect the loss of any volatile substance
Sheth M N Science College Patan Page no - 9
KARL - FISCHER Analysis Water Content Determination
6 Instrumentation
Following Figure illustrates a simple dead-stop end-point assembly or a Karl Fischer
titration apparatus The titration vessel is fitted with a pair of identical platinum electrodes
a mechanical stirrer with adjustable speed and a burette It will be observed that absolutely
little or no current may flow unless and until the solution is totally free from any polarizing
substances this could perhaps be due to the absorbed layers of oxygen and hydrogen on the
anode and cathode respectively
However the current shall flow only when the two electrodes get depolarized The
Karl Fischer reagent is pumped into the burette by means of hand bellows the access of
moisture is usually prevented by employing an appropriate arrangement of desiccant tubes
Alternatively the stirring may also be accomplished either by using a magnetic stirrer or by
means of suitably dried nitrogen passed gently through the solution during the course of
titration
The end-point is achieved by employing an eiectrical circuit comprising of a
microammeter (A) platinum electrodes together with a 15 V to 20 V battery connected
across a variable resistance of about 25 kΩ First of all the resistance is adjusted in such a
manner that an initial current passes through the platinum electrodes in series with a
microammeter (A) After each addition of reagent the pointer of the microammeter gets
deflected but quickly returns to its original position At the end of the reaction a deflection
is obtained which persists for 10-15 seconds
7 Water Determination Methodology
Sheth M N Science College Patan Page no - 10
KARL - FISCHER Analysis Water Content Determination
71 Volumetric titration method
Apparatus
Generally the apparatus consists of an automatic burette a backtitration flask a stirrer and
equipment for amperometric titration at constant voltage or potentiometric titration at
constant current
Because water determination TS is extremely hygroscopic the titration apparatus
should be protected from atmospheric moisture Silica gel or calcium chloride for water
determination is usually used for moisture protection
Procedure
As a rule the titration of the sample with water determination TS should be performed at
the same temperature as that at the standardization of the TS while protecting from
moisture
The apparatus is equipped with a variable resistor in the circuit and the resistor is
adjusted to apply a definite voltage (mV) between a pair of platinum electrodes immersed in
the solution to be titrated The change in current (1048707A) is measured during the dropping of
water determination TS (Amperometric titration at constant voltage) As titration continues
the abrupt change in current in the circuit occurs but returns to the original state within
several seconds At the end of a titration the change in current persists for a certain time
(usually longer than 30 seconds) The end point of titration is determined at this electric
state
Otherwise by adjusting the resistor a definite current is passed between the two
platinum electrodes and the change in potential (mV) is measured during dropping water
determination TS (Potentiometric titration at constant current) With the progress of
titration the value indicated by the potentiometer in the circuit decreases suddenly from a
polarization state of several hundreds (mV) to the nonpolarization state but it returns to the
original state within several seconds At the end of titration the non-polarization state
persists for a certain time (usually longer than 30 seconds) The end point of titration is
determined when this electric state attains
In the case of back titration when the amperometric titration method is used at
constant voltage the needle of microammeter is out of scale while an excessive quantity of
water determination TS remains It returns rapidly to the original position when the titration
reaches the end point
Sheth M N Science College Patan Page no - 11
KARL - FISCHER Analysis Water Content Determination
Similarly when the Potentiometric titration method at constant current is used the
needle of the millivoltmeter is at the original position while an excessive quantity of water
determination TS remains A definite voltage is applied when the titration reaches the end
point Unless otherwise specified the titration of water with water determination TS is
performed by either of the methods below Usually the end point of the titration can be
observed more clearly in the back titration method than in the direct titration method
711 Direct titration
Unless otherwise specified proceed as directed below Take 25ml of methanol for water
determination in a dried titration flask and titrate with water determination TS to the end
point Unless otherwise specified weigh accurately a quantity of the sample containing 10
to 50 mg of water transfer it quickly into the titration flask and dissolve by stirring Titrate
the solution with water determination TS to the end point under vigorous stirring When the
sample is insoluble in the solvent powder the sample quickly weigh a suitable amount of
the sample accurately and transfer it quickly into the titration vessel stir the mixture for 30
minutes while protecting it from moisture
Perform a titration under vigorous stirring When the sample interferes with the Karl
Fisher reaction water in the sample can be removed by heating and under a stream of
nitrogen gas and introduced into the titration vessel by using a water-evaporation device
712 Back titration
Unless otherwise specified proceed as directed below Take 20ml of methanol for water
determination in the dried titration vessel and titrate with water determination TS Weigh
accurately a suitable quantity of the sample containing 10 to 50 mg of water transfer the
sample quickly into the titration vessel add an excessive and definite volume of water
determination TS stir for 30 min protecting from atmospheric moisture and then titrate the
solution with Water Methanol Standard Solution under vigorous stirring
Sheth M N Science College Patan Page no - 12
KARL - FISCHER Analysis Water Content Determination
Wheref = the number of mg of water (H2O) corresponding to 1 ml of water determination
TS
Where
f = the number of mg of water (H2O) corresponding to 1 ml of water determination TS
f rsquo = the number of mg of water (H2O) in 1 ml of Water Methanol Standard Solution
713 General Remarks
Volumetric Karl Fischer titration requires the determination of the titre (t) of the Karl
Fischer reagent It is usually quoted in mg of water per ml of Karl Fischer reagent Modern
reagents allow direct titration of water in the sample The sample may be introduced
directly into the KF cell or after an extraction or dissolution with a suitable solvent The
water concentration of the solvent must be determined previously in order to be subtracted
from the sample analysis In Radiometer Analytical titrators all these operations are
simplified and the different results are accounted for automatically The volumetric titration
of water allows the analysis of water concentrations between 01 and 100 If an aliquot
contains less than 1 mg of water coulometric determination will result in a more accurate
result For reasons of precision the titre of the titrant should be chosen so that the titration is
completed with a titrant demand between 1 and 10 ml
The working medium (ie the solvent required) can be freely chosen by the user
depending on the dissolution properties of the sample to be investigated For methanol-
based reagents the stoichiometry 11 of the Karl Fischer reaction is only fulfilled if there is
more than 25 methanol in the reaction mixture A methanol-free working medium can be
used however it is important to determine the titre of the KF reagent in the same working
medium The modern solvents available today present a high buffer and dissolution
capacity These solvents consist of sulphur dioxide a base and methanol or ethanol The
main advantages of these solvents are
bull A more rapid titration due to better reaction kinetics an advantage especially for the
titration of large amounts of water
bull A better reproducibility because the reaction environment is stable The pH and the
sulphur dioxide concentration remain constant
Sheth M N Science College Patan Page no - 13
KARL - FISCHER Analysis Water Content Determination
For the titration of samples producing side reactions (aldehydes ketones and
silanols) it is necessary to use an appropriate solvent Most reagent manufacturers include
the letter K in the commercial name of such solvents
72 Coulometric titration
Apparatus
Usually the apparatus is comprised of an electrolytic cell for iodine production a stirrer a
titration flask and a potentiometric titration system at constant current The iodine
production device is composed of an anode and a cathode separated by a diaphragm The
anode is immersed in the anolyte solution for water determination and the cathode is
immersed in the catholyte solution for water determination Both electrodes are usually
made of platinum-mesh
Because water determination TS is extremely hygroscopic the titration apparatus
should be protected from atmospheric moisture For this purpose silica gel or calcium
chloride for water determination is usually used Procedure Take a suitable volume of an
anolyte for water determination in a titration vessel immerse in this solution a pair of
platinum electrodes for potentiometric titration at constant current
Then immerse the iodide production system filled with a catholyte for water
determination in the anolyte solution Switch on the electrolytic system and make the
content of the titration vessel anhydrous Next take an accurately weighed amount of the
sample containing 15 mg of water add it quickly to the vessel and dissolve by stirring
Perform the titration to the end point under vigorous stirring When the sample is insoluble
in the anolyte powder it quickly and add an accurately weighed amount of the sample to
the vessel After stirring the mixture for 5 to 30 minutes while protecting from atmospheric
moisture perform the titration with vigorous stirring Determine the quantity of electricity
(C) [electric current (A) and time (s)] required for the production of iodine during the
titration and calculate the content () of the water in the sample by the formula
When the sample interferes with the Karl Fisher reaction water in the sample can be
removed by heating under a stream of nitrogen gas and introduced into the titration vessel
by using a water-evaporation device
Sheth M N Science College Patan Page no - 14
KARL - FISCHER Analysis Water Content Determination
8 Interference and Limitations of Karl Fischer Method
Interferences in the water determinations by Karl Fischer method are associated with
condensation or oxidation ndash reduction reactions with a number of substances and
compounds or by side reactions Other possible interferences to the KF reagent are certain
active metals metal oxides metal hydroxides chromates melamines etc
The Karl Fischer titration has a number of serious limitations due to possible
interferences tantamount to erroneous results namely
(i) Oxidizing agents for instance chromates Cu(II) Fe(III) Cr2O7 2ndash peroxides salts
higher oxides
Example MnO2 + 4C5H5NH+ + 2Indash rarr Mn2+ + 4C5H5N + I2 + H2O
(ii) Reducing agents such as Sn (II) salts sulphides and S2O32ndash and
(iii) Compounds that have a tendency to form water with the ingredients of the Karl Fischer
reagent for instance
(a) Basic oxides eg ZnO
Example ZnO + 2C5H5NH+ rarr Zn2+ + C5H5N + H2O
(b) Salts of weak oxy-acids eg NaHCO3
Example NaHCO3 + C5H5NH+ rarr Na+ + H2O + CO2 + C5H5N
[Note As H2CO3 carbonic acid is very unstable hence it splits up to yield a mole each of
water and CO2]
Use of certain reagents will minimize or eliminate the interferences of many classes
of compounds For example the use of non-methanol containing KF reagent and solvent will
reduce the interference from aldehyde and ketones Ammonia and amines can be eliminated
by the addition of salicylic acid to the solvent
Sheth M N Science College Patan Page no - 15
KARL - FISCHER Analysis Water Content Determination
9 Side reactions
Side Reactions are classified as
(1) Reactions Influenced by Methanol
(2) Reactions with Iodine
(1) Reactions Influenced by Methanol
It includes
bull Aldehydes react to form acetals also undergo Bisulfite addition
bull Ketones react to form ketals
bull Amines undergo methylation
bull Siloxanes undergo esterification
(2) Reactions with Iodine
It includes
bull Halogenated Hydrocarbons containing free halogens
bull Mercaptans (Thiols)
bull Phenols
bull Peroxides
Testing for Iodine Reactivity
bull Dissolve a few Iodine Crystals in Methanol and Introduce Sample
bull Watch for Solution to Change Color if Clear Iodine Reaction is Present
Sheth M N Science College Patan Page no - 16
KARL - FISCHER Analysis Water Content Determination
1 Mercaptans (Thiols) are oxidized by iodine Suppress this side reaction by treating
sample with N-ethylmaleimide in a buffered solvent system
2 Inhibition of Thiol Oxidation by Iodine
Most Phenols do not react with KF reagents higher molecular weight phenols and
Amino-phenols are most problematic Volumetric method is most suitable since the
Oxidation potential is high in coulometry or Acidify Solvent system to reduce interference
Sheth M N Science College Patan Page no - 17
KARL - FISCHER Analysis Water Content Determination
10 Applications of Karl Fischer method for determination of Water in
pharmaceutical analysis
101 PREDNISOLONE SODIUM PHOSPHATE
Materials Required Karl Fischer Reagent 100 ml prednisolone sodium phosphate
02gm anhydrous methanol 200 ml
Procedure Add about 20 ml of anhydrous methanol to the titration vessel and titrate to the
Amperometric End-point with the Karl Fischer reagent Quickly add 02 g of prednisolone
sodium phosphate sample stir for 1 minute and again titrate to the amperometric end-point
with the Karl Fischer reagent The difference between the two titrations gives the volume
(v) of Karl Fischer reagent consumed by the sample
The minimum water equivalent is 35 mg of water per ml of Karl Fischer reagent
Hence the percentage of water ww in the given sample may be calculated by the
following
Precautions
(1) The reagents and solutions used must be kept anhydrous and necessary care should be
taken throughout to prevent exposure to atmospheric moisture
(2) The Karl Fischer reagent should be protected from light and preferably stored in a bottle
fitted with an automatic burette and
(3) The water equivalent of Karl Fischer reagent should always be determined before use
102 COGNATE ASSAYS
A number of other official pharmaceutical substances may be assayed for their water
content by the Karl Fischer method as summarized in the following Table
Sheth M N Science College Patan Page no - 18
KARL - FISCHER Analysis Water Content Determination
11 Automated Electrochemical Karl Fischer Analysis
Commercially available Modern KF-Titrators are usually equipped with specifically
designed titration vessels that are exclusively meant to check and prevent the contact with
atmospheric moisture Quite a few such devices are armed with microprocessors that will
perform the requisite operations sequentially in a programmed manner automatically and
may also dish out a print-out of the desired results including the percentage moisture
content In fact these Modern KF-Titrators not only afford greater accuracy and precision
in results but also offer much ease and convenience in routine analysis as compared to the
classical techniques based on either caulometry or controlled current potentiometry using
two indicator electrodes
Thus the basis of the analysis rests upon the quantitative relationship existing
between charge passed and iodine produced by the reagent according to the above reaction
Therefore the generation of iodine is automatically stopped when an excess of it is detected
by the indicator electrode It essentially consists of two platinum electrodes across which an
AC is applied and subsequently a marked drop in voltage between the electrodes takes place
as soon as an excess of iodine is present Normally such automated instruments make use of
proprietory reagents exclusively
The major advantage of this approach to KF-analysis being that no calibration is
required as the method is absolute and is entirely based on the stoichiometry of the
aforesaid equation It is noteworthy that one may determine the amounts of water ranging
between 10 mcg and 10 mg in solid as well as liquid samples
Sheth M N Science College Patan Page no - 19
KARL - FISCHER Analysis Water Content Determination
12 Some of the innovative brands in Karl Fischer Auto ndash titrator
instruments
Sheth M N Science College Patan Page no - 20
KARL - FISCHER Analysis Water Content Determination
Sheth M N Science College Patan Page no - 21
KARL - FISCHER Analysis Water Content Determination
13 References
1 Chapter 14 Karl fishcer method for determination of water Page no 223-227
Pharmaceutical Drug Analysis Ashutosh Kar New age international limited publishers
2 Aqua Stars Karl fischer instruments wwwemdchemicalscom
3 Karl Fischer Volumetric Titration Theory and Practice by Radiometer analytical
SAS wwwradiometer-analyticalcom
4 MettlerToledo AG Analytical Switzerland wwwmtcom
5 Summary of Karl fischer titrations by Orion products Baverly MA Thermofischer
Scientific wwwthermocomwater
6 Moisture determination by Karl Fischer titration background of chemistry and
recent advances SupelcoSigma-Aldrich Bellefonte Pa wwwsigma-aldrichcom
7 Karl Fischer Titration Methods Methodology and water determination in 442
different substances European Pharmacopoeia (PharmEurope) 4th Edition plus
Supplements 2002 International Electrotechnical Commission Geneva
8 Karl ndash fischer titration wwwwikipediacom
Sheth M N Science College Patan Page no - 22
KARL - FISCHER Analysis Water Content Determination
3 Factors to consider in Karl ndash Fischer Titration
A pH considerations
The Karl Fischer reaction can only take place in a certain pH range between 5 and 7
In this pH range the reaction remains constant If the pH drops too low end point
attainment becomes sluggish or an end point will not be reached at all If the pH is too high
side reactions occur making the titration non-stoichiometric We can therefore say that
errors occurring during a KF titration may be due to a change in the pH of the titration
solvent Use acid base or buffer if necessary
pH dependence of the Karl ndash Fischer reaction [K = Reaction kinetic constant]
The pH of the titration solvent can be tested using a combined pH electrode and a
pH meter The electrode is first calibrated with aqueous buffer solutions and afterwards the
pH of the titration solvent is measured
Note do not place the pH electrode directly into the KF cell because excessive moisture
will be introduced along with the electrode For further information consult the userrsquos
manuals of the main manufacturers of Karl Fischer reagents
B Aldehydes and ketones
Use special reagents to prevent side reactions which generate or consume water and prevent
good endpoints
Sheth M N Science College Patan Page no - 3
KARL - FISCHER Analysis Water Content Determination
C Water content
Consider when choosing auto-titrator
D Sample size
Choose sample size that is large enough for precise determination but not so large as to
prolong the titration exceed the capacity of the reagent or be insoluble
The sample size should be small so that as many samples as possible can be titrated in the
same electrolyte solution and the titration time kept short However take care that the
sample contains at least 50 μl water The following table provides guidelines for the sample
weight
Water content of Sample Sample size Water to be determined
100000 ppm = 10 50 mg 5000 microg
10000 ppm = 1 10 mg ndash 100 mg 100 microg ndash 1000 microg
1000 ppm = 01 100 mg ndash 1 g 100 microg ndash 1000 microg
100 ppm = 001 1 g 100 microg
10 ppm = 0001 5 g 50 microg
E Frequency of SolventWorking Medium Change
When the vessel is full when the methanol content is lt25 when the capacity of the
solvent in exceeded (generally not more than 1 mg water per 1 ml methanol)
F Exclusion of background water
Maintain desiccant tubes keep reagents dry good seals on instrument and reagent bottles
G Pre-titration conditioning of working solution
Solventworking solution must be dry but not over titrated before the sample titration to
avoid positive or negative bias to results
H Solubility of the sample
Sample Solubility is Extremely Important to Obtain Total Water Content Sample must be
completely dissolved prior to the start of the titration to obtain accurate and reproducible
results
Total Water = Surface Water + Bound Water
Sheth M N Science College Patan Page no - 4
KARL - FISCHER Analysis Water Content Determination
Different methods to improve sample solubility are
i Choose the right solvent
ii Allow sufficient time for dissolution (extract time)
iii Grind sample
iv Co-solvent additions
v Elevating solvent system temperature and
vi Adding homogenizer to system
I Conductivity
Conductivity of the solvent and added sample has to be sufficient to support proper function
of the electrodes in order to detect the endpoint andor generate the iodine titrant (by
coulometric type)
J Influence of the Base
Pyridine was the original Karl-Fischer base Pyridine because of its weak basicity cannot
totally neutralize the methylsulfurous acid The equilibrium in reaction at stage-1 is not
completely shifted to the right The reaction with pyridine is therefore slow and the
endpoint is not stable often making the repeatability using pyridine very poor
Pyridine Imidazole
Imidazole was chosen to replace pyridine as it shifts the reaction at stage-1
completely to the right reaction is swift and endpoints clear and stable Imidazole also does
not have the unpleasant odor and toxicity of pyridine
Sheth M N Science College Patan Page no - 5
KARL - FISCHER Analysis Water Content Determination
K Stability of the Reagent
The stability of the original Karl Fischer reagent initially prepared with an excess of
methanol was found to be fairly poor and hence evidently needed frequent standardization
However it was established subsequently that the stability could be improved significantly
by replacing the methanol by 2-methoxyethanol It has been observed that the titer of the
Karl Fischer reagent which stands at 35 mg of water per milliliter of reagent falls rapidly
upon standing with the passage of time Hence the following precautions must be observed
rigidly using the Karl Fischer reagent namely
(a) Always prepare the reagent a day or two before it is to be used
(b) Great care must be taken to prevent and check any possible contamination either of the
reagent or the sample by atmospheric moisture
(c) All glassware(s) must be thoroughly dried before use
(d) Standard solution should be stored out of contact with air and
(e) Essential to minimise contact between the atmosphere and the solution during the course
of titration
Sheth M N Science College Patan Page no - 6
KARL - FISCHER Analysis Water Content Determination
4 Types of Karl ndash Fischer titrations
There are two determination methods different in iodine-providing principle
(1) Volumetric titration method and
(2) Coulometric titration method
In the volumetric titration method iodine required for reaction with water is
previously dissolved in water determination TS and water content is determined by
measuring the amount of iodine consumed as a result of reaction with water in a sample It
is good for high level moisture determination
In the coulometric titration method first iodine is produced by electrolysis of the
reagent containing iodide ion and then the water content in a sample is determined by
measuring the quantity of electricity (Coulombs) which is required for the electrolysis (ie
for the production of iodine) based on the quantitative reaction of the generated iodine with
water It is good for low level moisture determination
1048707Volumetric titration ndashwater determination based on volumetric measurement of reagent
Good for high level moisture determinations
1048707Coulometrictitration ndashwater determination based on measurement of time and current (in
coulombs) used to generate the iodine Good for low level moisture determinations
Instrument Type Volumetric Coulometric
Best for Solids and Liquids Low moisture samples
Water content limit 50 ppm to 100 water 1 ppm to 100 water
Matrices Solid Liquid and Gas Liquid only
Advantages All around performance Low detection limit
Sheth M N Science College Patan Page no - 7
KARL - FISCHER Analysis Water Content Determination
(Two compartment system possess separate solvent and titrating solutions)
Sheth M N Science College Patan Page no - 8
KARL - FISCHER Analysis Water Content Determination
5 Advantages
A Karl Fischer moisture determination is advantageous as compared to a
determination based on weight loss because KF is not affected by volatile compounds The
popularity of the Karl Fischer titration is due in large part to several practical advantages
that it holds over other methods of moisture determination including
i High accuracy and precision
ii Selectivity for water
iii Small sample quantities required
iv Easy sample preparation
v Short analysis duration
vi Nearly unlimited measuring range (1ppm to 100)
vii Suitability for analyzing Solids Liquids and Gases
viii Independence of presence of other volatiles
ix Suitability for automation
In contrast loss on drying will detect the loss of any volatile substance
Sheth M N Science College Patan Page no - 9
KARL - FISCHER Analysis Water Content Determination
6 Instrumentation
Following Figure illustrates a simple dead-stop end-point assembly or a Karl Fischer
titration apparatus The titration vessel is fitted with a pair of identical platinum electrodes
a mechanical stirrer with adjustable speed and a burette It will be observed that absolutely
little or no current may flow unless and until the solution is totally free from any polarizing
substances this could perhaps be due to the absorbed layers of oxygen and hydrogen on the
anode and cathode respectively
However the current shall flow only when the two electrodes get depolarized The
Karl Fischer reagent is pumped into the burette by means of hand bellows the access of
moisture is usually prevented by employing an appropriate arrangement of desiccant tubes
Alternatively the stirring may also be accomplished either by using a magnetic stirrer or by
means of suitably dried nitrogen passed gently through the solution during the course of
titration
The end-point is achieved by employing an eiectrical circuit comprising of a
microammeter (A) platinum electrodes together with a 15 V to 20 V battery connected
across a variable resistance of about 25 kΩ First of all the resistance is adjusted in such a
manner that an initial current passes through the platinum electrodes in series with a
microammeter (A) After each addition of reagent the pointer of the microammeter gets
deflected but quickly returns to its original position At the end of the reaction a deflection
is obtained which persists for 10-15 seconds
7 Water Determination Methodology
Sheth M N Science College Patan Page no - 10
KARL - FISCHER Analysis Water Content Determination
71 Volumetric titration method
Apparatus
Generally the apparatus consists of an automatic burette a backtitration flask a stirrer and
equipment for amperometric titration at constant voltage or potentiometric titration at
constant current
Because water determination TS is extremely hygroscopic the titration apparatus
should be protected from atmospheric moisture Silica gel or calcium chloride for water
determination is usually used for moisture protection
Procedure
As a rule the titration of the sample with water determination TS should be performed at
the same temperature as that at the standardization of the TS while protecting from
moisture
The apparatus is equipped with a variable resistor in the circuit and the resistor is
adjusted to apply a definite voltage (mV) between a pair of platinum electrodes immersed in
the solution to be titrated The change in current (1048707A) is measured during the dropping of
water determination TS (Amperometric titration at constant voltage) As titration continues
the abrupt change in current in the circuit occurs but returns to the original state within
several seconds At the end of a titration the change in current persists for a certain time
(usually longer than 30 seconds) The end point of titration is determined at this electric
state
Otherwise by adjusting the resistor a definite current is passed between the two
platinum electrodes and the change in potential (mV) is measured during dropping water
determination TS (Potentiometric titration at constant current) With the progress of
titration the value indicated by the potentiometer in the circuit decreases suddenly from a
polarization state of several hundreds (mV) to the nonpolarization state but it returns to the
original state within several seconds At the end of titration the non-polarization state
persists for a certain time (usually longer than 30 seconds) The end point of titration is
determined when this electric state attains
In the case of back titration when the amperometric titration method is used at
constant voltage the needle of microammeter is out of scale while an excessive quantity of
water determination TS remains It returns rapidly to the original position when the titration
reaches the end point
Sheth M N Science College Patan Page no - 11
KARL - FISCHER Analysis Water Content Determination
Similarly when the Potentiometric titration method at constant current is used the
needle of the millivoltmeter is at the original position while an excessive quantity of water
determination TS remains A definite voltage is applied when the titration reaches the end
point Unless otherwise specified the titration of water with water determination TS is
performed by either of the methods below Usually the end point of the titration can be
observed more clearly in the back titration method than in the direct titration method
711 Direct titration
Unless otherwise specified proceed as directed below Take 25ml of methanol for water
determination in a dried titration flask and titrate with water determination TS to the end
point Unless otherwise specified weigh accurately a quantity of the sample containing 10
to 50 mg of water transfer it quickly into the titration flask and dissolve by stirring Titrate
the solution with water determination TS to the end point under vigorous stirring When the
sample is insoluble in the solvent powder the sample quickly weigh a suitable amount of
the sample accurately and transfer it quickly into the titration vessel stir the mixture for 30
minutes while protecting it from moisture
Perform a titration under vigorous stirring When the sample interferes with the Karl
Fisher reaction water in the sample can be removed by heating and under a stream of
nitrogen gas and introduced into the titration vessel by using a water-evaporation device
712 Back titration
Unless otherwise specified proceed as directed below Take 20ml of methanol for water
determination in the dried titration vessel and titrate with water determination TS Weigh
accurately a suitable quantity of the sample containing 10 to 50 mg of water transfer the
sample quickly into the titration vessel add an excessive and definite volume of water
determination TS stir for 30 min protecting from atmospheric moisture and then titrate the
solution with Water Methanol Standard Solution under vigorous stirring
Sheth M N Science College Patan Page no - 12
KARL - FISCHER Analysis Water Content Determination
Wheref = the number of mg of water (H2O) corresponding to 1 ml of water determination
TS
Where
f = the number of mg of water (H2O) corresponding to 1 ml of water determination TS
f rsquo = the number of mg of water (H2O) in 1 ml of Water Methanol Standard Solution
713 General Remarks
Volumetric Karl Fischer titration requires the determination of the titre (t) of the Karl
Fischer reagent It is usually quoted in mg of water per ml of Karl Fischer reagent Modern
reagents allow direct titration of water in the sample The sample may be introduced
directly into the KF cell or after an extraction or dissolution with a suitable solvent The
water concentration of the solvent must be determined previously in order to be subtracted
from the sample analysis In Radiometer Analytical titrators all these operations are
simplified and the different results are accounted for automatically The volumetric titration
of water allows the analysis of water concentrations between 01 and 100 If an aliquot
contains less than 1 mg of water coulometric determination will result in a more accurate
result For reasons of precision the titre of the titrant should be chosen so that the titration is
completed with a titrant demand between 1 and 10 ml
The working medium (ie the solvent required) can be freely chosen by the user
depending on the dissolution properties of the sample to be investigated For methanol-
based reagents the stoichiometry 11 of the Karl Fischer reaction is only fulfilled if there is
more than 25 methanol in the reaction mixture A methanol-free working medium can be
used however it is important to determine the titre of the KF reagent in the same working
medium The modern solvents available today present a high buffer and dissolution
capacity These solvents consist of sulphur dioxide a base and methanol or ethanol The
main advantages of these solvents are
bull A more rapid titration due to better reaction kinetics an advantage especially for the
titration of large amounts of water
bull A better reproducibility because the reaction environment is stable The pH and the
sulphur dioxide concentration remain constant
Sheth M N Science College Patan Page no - 13
KARL - FISCHER Analysis Water Content Determination
For the titration of samples producing side reactions (aldehydes ketones and
silanols) it is necessary to use an appropriate solvent Most reagent manufacturers include
the letter K in the commercial name of such solvents
72 Coulometric titration
Apparatus
Usually the apparatus is comprised of an electrolytic cell for iodine production a stirrer a
titration flask and a potentiometric titration system at constant current The iodine
production device is composed of an anode and a cathode separated by a diaphragm The
anode is immersed in the anolyte solution for water determination and the cathode is
immersed in the catholyte solution for water determination Both electrodes are usually
made of platinum-mesh
Because water determination TS is extremely hygroscopic the titration apparatus
should be protected from atmospheric moisture For this purpose silica gel or calcium
chloride for water determination is usually used Procedure Take a suitable volume of an
anolyte for water determination in a titration vessel immerse in this solution a pair of
platinum electrodes for potentiometric titration at constant current
Then immerse the iodide production system filled with a catholyte for water
determination in the anolyte solution Switch on the electrolytic system and make the
content of the titration vessel anhydrous Next take an accurately weighed amount of the
sample containing 15 mg of water add it quickly to the vessel and dissolve by stirring
Perform the titration to the end point under vigorous stirring When the sample is insoluble
in the anolyte powder it quickly and add an accurately weighed amount of the sample to
the vessel After stirring the mixture for 5 to 30 minutes while protecting from atmospheric
moisture perform the titration with vigorous stirring Determine the quantity of electricity
(C) [electric current (A) and time (s)] required for the production of iodine during the
titration and calculate the content () of the water in the sample by the formula
When the sample interferes with the Karl Fisher reaction water in the sample can be
removed by heating under a stream of nitrogen gas and introduced into the titration vessel
by using a water-evaporation device
Sheth M N Science College Patan Page no - 14
KARL - FISCHER Analysis Water Content Determination
8 Interference and Limitations of Karl Fischer Method
Interferences in the water determinations by Karl Fischer method are associated with
condensation or oxidation ndash reduction reactions with a number of substances and
compounds or by side reactions Other possible interferences to the KF reagent are certain
active metals metal oxides metal hydroxides chromates melamines etc
The Karl Fischer titration has a number of serious limitations due to possible
interferences tantamount to erroneous results namely
(i) Oxidizing agents for instance chromates Cu(II) Fe(III) Cr2O7 2ndash peroxides salts
higher oxides
Example MnO2 + 4C5H5NH+ + 2Indash rarr Mn2+ + 4C5H5N + I2 + H2O
(ii) Reducing agents such as Sn (II) salts sulphides and S2O32ndash and
(iii) Compounds that have a tendency to form water with the ingredients of the Karl Fischer
reagent for instance
(a) Basic oxides eg ZnO
Example ZnO + 2C5H5NH+ rarr Zn2+ + C5H5N + H2O
(b) Salts of weak oxy-acids eg NaHCO3
Example NaHCO3 + C5H5NH+ rarr Na+ + H2O + CO2 + C5H5N
[Note As H2CO3 carbonic acid is very unstable hence it splits up to yield a mole each of
water and CO2]
Use of certain reagents will minimize or eliminate the interferences of many classes
of compounds For example the use of non-methanol containing KF reagent and solvent will
reduce the interference from aldehyde and ketones Ammonia and amines can be eliminated
by the addition of salicylic acid to the solvent
Sheth M N Science College Patan Page no - 15
KARL - FISCHER Analysis Water Content Determination
9 Side reactions
Side Reactions are classified as
(1) Reactions Influenced by Methanol
(2) Reactions with Iodine
(1) Reactions Influenced by Methanol
It includes
bull Aldehydes react to form acetals also undergo Bisulfite addition
bull Ketones react to form ketals
bull Amines undergo methylation
bull Siloxanes undergo esterification
(2) Reactions with Iodine
It includes
bull Halogenated Hydrocarbons containing free halogens
bull Mercaptans (Thiols)
bull Phenols
bull Peroxides
Testing for Iodine Reactivity
bull Dissolve a few Iodine Crystals in Methanol and Introduce Sample
bull Watch for Solution to Change Color if Clear Iodine Reaction is Present
Sheth M N Science College Patan Page no - 16
KARL - FISCHER Analysis Water Content Determination
1 Mercaptans (Thiols) are oxidized by iodine Suppress this side reaction by treating
sample with N-ethylmaleimide in a buffered solvent system
2 Inhibition of Thiol Oxidation by Iodine
Most Phenols do not react with KF reagents higher molecular weight phenols and
Amino-phenols are most problematic Volumetric method is most suitable since the
Oxidation potential is high in coulometry or Acidify Solvent system to reduce interference
Sheth M N Science College Patan Page no - 17
KARL - FISCHER Analysis Water Content Determination
10 Applications of Karl Fischer method for determination of Water in
pharmaceutical analysis
101 PREDNISOLONE SODIUM PHOSPHATE
Materials Required Karl Fischer Reagent 100 ml prednisolone sodium phosphate
02gm anhydrous methanol 200 ml
Procedure Add about 20 ml of anhydrous methanol to the titration vessel and titrate to the
Amperometric End-point with the Karl Fischer reagent Quickly add 02 g of prednisolone
sodium phosphate sample stir for 1 minute and again titrate to the amperometric end-point
with the Karl Fischer reagent The difference between the two titrations gives the volume
(v) of Karl Fischer reagent consumed by the sample
The minimum water equivalent is 35 mg of water per ml of Karl Fischer reagent
Hence the percentage of water ww in the given sample may be calculated by the
following
Precautions
(1) The reagents and solutions used must be kept anhydrous and necessary care should be
taken throughout to prevent exposure to atmospheric moisture
(2) The Karl Fischer reagent should be protected from light and preferably stored in a bottle
fitted with an automatic burette and
(3) The water equivalent of Karl Fischer reagent should always be determined before use
102 COGNATE ASSAYS
A number of other official pharmaceutical substances may be assayed for their water
content by the Karl Fischer method as summarized in the following Table
Sheth M N Science College Patan Page no - 18
KARL - FISCHER Analysis Water Content Determination
11 Automated Electrochemical Karl Fischer Analysis
Commercially available Modern KF-Titrators are usually equipped with specifically
designed titration vessels that are exclusively meant to check and prevent the contact with
atmospheric moisture Quite a few such devices are armed with microprocessors that will
perform the requisite operations sequentially in a programmed manner automatically and
may also dish out a print-out of the desired results including the percentage moisture
content In fact these Modern KF-Titrators not only afford greater accuracy and precision
in results but also offer much ease and convenience in routine analysis as compared to the
classical techniques based on either caulometry or controlled current potentiometry using
two indicator electrodes
Thus the basis of the analysis rests upon the quantitative relationship existing
between charge passed and iodine produced by the reagent according to the above reaction
Therefore the generation of iodine is automatically stopped when an excess of it is detected
by the indicator electrode It essentially consists of two platinum electrodes across which an
AC is applied and subsequently a marked drop in voltage between the electrodes takes place
as soon as an excess of iodine is present Normally such automated instruments make use of
proprietory reagents exclusively
The major advantage of this approach to KF-analysis being that no calibration is
required as the method is absolute and is entirely based on the stoichiometry of the
aforesaid equation It is noteworthy that one may determine the amounts of water ranging
between 10 mcg and 10 mg in solid as well as liquid samples
Sheth M N Science College Patan Page no - 19
KARL - FISCHER Analysis Water Content Determination
12 Some of the innovative brands in Karl Fischer Auto ndash titrator
instruments
Sheth M N Science College Patan Page no - 20
KARL - FISCHER Analysis Water Content Determination
Sheth M N Science College Patan Page no - 21
KARL - FISCHER Analysis Water Content Determination
13 References
1 Chapter 14 Karl fishcer method for determination of water Page no 223-227
Pharmaceutical Drug Analysis Ashutosh Kar New age international limited publishers
2 Aqua Stars Karl fischer instruments wwwemdchemicalscom
3 Karl Fischer Volumetric Titration Theory and Practice by Radiometer analytical
SAS wwwradiometer-analyticalcom
4 MettlerToledo AG Analytical Switzerland wwwmtcom
5 Summary of Karl fischer titrations by Orion products Baverly MA Thermofischer
Scientific wwwthermocomwater
6 Moisture determination by Karl Fischer titration background of chemistry and
recent advances SupelcoSigma-Aldrich Bellefonte Pa wwwsigma-aldrichcom
7 Karl Fischer Titration Methods Methodology and water determination in 442
different substances European Pharmacopoeia (PharmEurope) 4th Edition plus
Supplements 2002 International Electrotechnical Commission Geneva
8 Karl ndash fischer titration wwwwikipediacom
Sheth M N Science College Patan Page no - 22
KARL - FISCHER Analysis Water Content Determination
C Water content
Consider when choosing auto-titrator
D Sample size
Choose sample size that is large enough for precise determination but not so large as to
prolong the titration exceed the capacity of the reagent or be insoluble
The sample size should be small so that as many samples as possible can be titrated in the
same electrolyte solution and the titration time kept short However take care that the
sample contains at least 50 μl water The following table provides guidelines for the sample
weight
Water content of Sample Sample size Water to be determined
100000 ppm = 10 50 mg 5000 microg
10000 ppm = 1 10 mg ndash 100 mg 100 microg ndash 1000 microg
1000 ppm = 01 100 mg ndash 1 g 100 microg ndash 1000 microg
100 ppm = 001 1 g 100 microg
10 ppm = 0001 5 g 50 microg
E Frequency of SolventWorking Medium Change
When the vessel is full when the methanol content is lt25 when the capacity of the
solvent in exceeded (generally not more than 1 mg water per 1 ml methanol)
F Exclusion of background water
Maintain desiccant tubes keep reagents dry good seals on instrument and reagent bottles
G Pre-titration conditioning of working solution
Solventworking solution must be dry but not over titrated before the sample titration to
avoid positive or negative bias to results
H Solubility of the sample
Sample Solubility is Extremely Important to Obtain Total Water Content Sample must be
completely dissolved prior to the start of the titration to obtain accurate and reproducible
results
Total Water = Surface Water + Bound Water
Sheth M N Science College Patan Page no - 4
KARL - FISCHER Analysis Water Content Determination
Different methods to improve sample solubility are
i Choose the right solvent
ii Allow sufficient time for dissolution (extract time)
iii Grind sample
iv Co-solvent additions
v Elevating solvent system temperature and
vi Adding homogenizer to system
I Conductivity
Conductivity of the solvent and added sample has to be sufficient to support proper function
of the electrodes in order to detect the endpoint andor generate the iodine titrant (by
coulometric type)
J Influence of the Base
Pyridine was the original Karl-Fischer base Pyridine because of its weak basicity cannot
totally neutralize the methylsulfurous acid The equilibrium in reaction at stage-1 is not
completely shifted to the right The reaction with pyridine is therefore slow and the
endpoint is not stable often making the repeatability using pyridine very poor
Pyridine Imidazole
Imidazole was chosen to replace pyridine as it shifts the reaction at stage-1
completely to the right reaction is swift and endpoints clear and stable Imidazole also does
not have the unpleasant odor and toxicity of pyridine
Sheth M N Science College Patan Page no - 5
KARL - FISCHER Analysis Water Content Determination
K Stability of the Reagent
The stability of the original Karl Fischer reagent initially prepared with an excess of
methanol was found to be fairly poor and hence evidently needed frequent standardization
However it was established subsequently that the stability could be improved significantly
by replacing the methanol by 2-methoxyethanol It has been observed that the titer of the
Karl Fischer reagent which stands at 35 mg of water per milliliter of reagent falls rapidly
upon standing with the passage of time Hence the following precautions must be observed
rigidly using the Karl Fischer reagent namely
(a) Always prepare the reagent a day or two before it is to be used
(b) Great care must be taken to prevent and check any possible contamination either of the
reagent or the sample by atmospheric moisture
(c) All glassware(s) must be thoroughly dried before use
(d) Standard solution should be stored out of contact with air and
(e) Essential to minimise contact between the atmosphere and the solution during the course
of titration
Sheth M N Science College Patan Page no - 6
KARL - FISCHER Analysis Water Content Determination
4 Types of Karl ndash Fischer titrations
There are two determination methods different in iodine-providing principle
(1) Volumetric titration method and
(2) Coulometric titration method
In the volumetric titration method iodine required for reaction with water is
previously dissolved in water determination TS and water content is determined by
measuring the amount of iodine consumed as a result of reaction with water in a sample It
is good for high level moisture determination
In the coulometric titration method first iodine is produced by electrolysis of the
reagent containing iodide ion and then the water content in a sample is determined by
measuring the quantity of electricity (Coulombs) which is required for the electrolysis (ie
for the production of iodine) based on the quantitative reaction of the generated iodine with
water It is good for low level moisture determination
1048707Volumetric titration ndashwater determination based on volumetric measurement of reagent
Good for high level moisture determinations
1048707Coulometrictitration ndashwater determination based on measurement of time and current (in
coulombs) used to generate the iodine Good for low level moisture determinations
Instrument Type Volumetric Coulometric
Best for Solids and Liquids Low moisture samples
Water content limit 50 ppm to 100 water 1 ppm to 100 water
Matrices Solid Liquid and Gas Liquid only
Advantages All around performance Low detection limit
Sheth M N Science College Patan Page no - 7
KARL - FISCHER Analysis Water Content Determination
(Two compartment system possess separate solvent and titrating solutions)
Sheth M N Science College Patan Page no - 8
KARL - FISCHER Analysis Water Content Determination
5 Advantages
A Karl Fischer moisture determination is advantageous as compared to a
determination based on weight loss because KF is not affected by volatile compounds The
popularity of the Karl Fischer titration is due in large part to several practical advantages
that it holds over other methods of moisture determination including
i High accuracy and precision
ii Selectivity for water
iii Small sample quantities required
iv Easy sample preparation
v Short analysis duration
vi Nearly unlimited measuring range (1ppm to 100)
vii Suitability for analyzing Solids Liquids and Gases
viii Independence of presence of other volatiles
ix Suitability for automation
In contrast loss on drying will detect the loss of any volatile substance
Sheth M N Science College Patan Page no - 9
KARL - FISCHER Analysis Water Content Determination
6 Instrumentation
Following Figure illustrates a simple dead-stop end-point assembly or a Karl Fischer
titration apparatus The titration vessel is fitted with a pair of identical platinum electrodes
a mechanical stirrer with adjustable speed and a burette It will be observed that absolutely
little or no current may flow unless and until the solution is totally free from any polarizing
substances this could perhaps be due to the absorbed layers of oxygen and hydrogen on the
anode and cathode respectively
However the current shall flow only when the two electrodes get depolarized The
Karl Fischer reagent is pumped into the burette by means of hand bellows the access of
moisture is usually prevented by employing an appropriate arrangement of desiccant tubes
Alternatively the stirring may also be accomplished either by using a magnetic stirrer or by
means of suitably dried nitrogen passed gently through the solution during the course of
titration
The end-point is achieved by employing an eiectrical circuit comprising of a
microammeter (A) platinum electrodes together with a 15 V to 20 V battery connected
across a variable resistance of about 25 kΩ First of all the resistance is adjusted in such a
manner that an initial current passes through the platinum electrodes in series with a
microammeter (A) After each addition of reagent the pointer of the microammeter gets
deflected but quickly returns to its original position At the end of the reaction a deflection
is obtained which persists for 10-15 seconds
7 Water Determination Methodology
Sheth M N Science College Patan Page no - 10
KARL - FISCHER Analysis Water Content Determination
71 Volumetric titration method
Apparatus
Generally the apparatus consists of an automatic burette a backtitration flask a stirrer and
equipment for amperometric titration at constant voltage or potentiometric titration at
constant current
Because water determination TS is extremely hygroscopic the titration apparatus
should be protected from atmospheric moisture Silica gel or calcium chloride for water
determination is usually used for moisture protection
Procedure
As a rule the titration of the sample with water determination TS should be performed at
the same temperature as that at the standardization of the TS while protecting from
moisture
The apparatus is equipped with a variable resistor in the circuit and the resistor is
adjusted to apply a definite voltage (mV) between a pair of platinum electrodes immersed in
the solution to be titrated The change in current (1048707A) is measured during the dropping of
water determination TS (Amperometric titration at constant voltage) As titration continues
the abrupt change in current in the circuit occurs but returns to the original state within
several seconds At the end of a titration the change in current persists for a certain time
(usually longer than 30 seconds) The end point of titration is determined at this electric
state
Otherwise by adjusting the resistor a definite current is passed between the two
platinum electrodes and the change in potential (mV) is measured during dropping water
determination TS (Potentiometric titration at constant current) With the progress of
titration the value indicated by the potentiometer in the circuit decreases suddenly from a
polarization state of several hundreds (mV) to the nonpolarization state but it returns to the
original state within several seconds At the end of titration the non-polarization state
persists for a certain time (usually longer than 30 seconds) The end point of titration is
determined when this electric state attains
In the case of back titration when the amperometric titration method is used at
constant voltage the needle of microammeter is out of scale while an excessive quantity of
water determination TS remains It returns rapidly to the original position when the titration
reaches the end point
Sheth M N Science College Patan Page no - 11
KARL - FISCHER Analysis Water Content Determination
Similarly when the Potentiometric titration method at constant current is used the
needle of the millivoltmeter is at the original position while an excessive quantity of water
determination TS remains A definite voltage is applied when the titration reaches the end
point Unless otherwise specified the titration of water with water determination TS is
performed by either of the methods below Usually the end point of the titration can be
observed more clearly in the back titration method than in the direct titration method
711 Direct titration
Unless otherwise specified proceed as directed below Take 25ml of methanol for water
determination in a dried titration flask and titrate with water determination TS to the end
point Unless otherwise specified weigh accurately a quantity of the sample containing 10
to 50 mg of water transfer it quickly into the titration flask and dissolve by stirring Titrate
the solution with water determination TS to the end point under vigorous stirring When the
sample is insoluble in the solvent powder the sample quickly weigh a suitable amount of
the sample accurately and transfer it quickly into the titration vessel stir the mixture for 30
minutes while protecting it from moisture
Perform a titration under vigorous stirring When the sample interferes with the Karl
Fisher reaction water in the sample can be removed by heating and under a stream of
nitrogen gas and introduced into the titration vessel by using a water-evaporation device
712 Back titration
Unless otherwise specified proceed as directed below Take 20ml of methanol for water
determination in the dried titration vessel and titrate with water determination TS Weigh
accurately a suitable quantity of the sample containing 10 to 50 mg of water transfer the
sample quickly into the titration vessel add an excessive and definite volume of water
determination TS stir for 30 min protecting from atmospheric moisture and then titrate the
solution with Water Methanol Standard Solution under vigorous stirring
Sheth M N Science College Patan Page no - 12
KARL - FISCHER Analysis Water Content Determination
Wheref = the number of mg of water (H2O) corresponding to 1 ml of water determination
TS
Where
f = the number of mg of water (H2O) corresponding to 1 ml of water determination TS
f rsquo = the number of mg of water (H2O) in 1 ml of Water Methanol Standard Solution
713 General Remarks
Volumetric Karl Fischer titration requires the determination of the titre (t) of the Karl
Fischer reagent It is usually quoted in mg of water per ml of Karl Fischer reagent Modern
reagents allow direct titration of water in the sample The sample may be introduced
directly into the KF cell or after an extraction or dissolution with a suitable solvent The
water concentration of the solvent must be determined previously in order to be subtracted
from the sample analysis In Radiometer Analytical titrators all these operations are
simplified and the different results are accounted for automatically The volumetric titration
of water allows the analysis of water concentrations between 01 and 100 If an aliquot
contains less than 1 mg of water coulometric determination will result in a more accurate
result For reasons of precision the titre of the titrant should be chosen so that the titration is
completed with a titrant demand between 1 and 10 ml
The working medium (ie the solvent required) can be freely chosen by the user
depending on the dissolution properties of the sample to be investigated For methanol-
based reagents the stoichiometry 11 of the Karl Fischer reaction is only fulfilled if there is
more than 25 methanol in the reaction mixture A methanol-free working medium can be
used however it is important to determine the titre of the KF reagent in the same working
medium The modern solvents available today present a high buffer and dissolution
capacity These solvents consist of sulphur dioxide a base and methanol or ethanol The
main advantages of these solvents are
bull A more rapid titration due to better reaction kinetics an advantage especially for the
titration of large amounts of water
bull A better reproducibility because the reaction environment is stable The pH and the
sulphur dioxide concentration remain constant
Sheth M N Science College Patan Page no - 13
KARL - FISCHER Analysis Water Content Determination
For the titration of samples producing side reactions (aldehydes ketones and
silanols) it is necessary to use an appropriate solvent Most reagent manufacturers include
the letter K in the commercial name of such solvents
72 Coulometric titration
Apparatus
Usually the apparatus is comprised of an electrolytic cell for iodine production a stirrer a
titration flask and a potentiometric titration system at constant current The iodine
production device is composed of an anode and a cathode separated by a diaphragm The
anode is immersed in the anolyte solution for water determination and the cathode is
immersed in the catholyte solution for water determination Both electrodes are usually
made of platinum-mesh
Because water determination TS is extremely hygroscopic the titration apparatus
should be protected from atmospheric moisture For this purpose silica gel or calcium
chloride for water determination is usually used Procedure Take a suitable volume of an
anolyte for water determination in a titration vessel immerse in this solution a pair of
platinum electrodes for potentiometric titration at constant current
Then immerse the iodide production system filled with a catholyte for water
determination in the anolyte solution Switch on the electrolytic system and make the
content of the titration vessel anhydrous Next take an accurately weighed amount of the
sample containing 15 mg of water add it quickly to the vessel and dissolve by stirring
Perform the titration to the end point under vigorous stirring When the sample is insoluble
in the anolyte powder it quickly and add an accurately weighed amount of the sample to
the vessel After stirring the mixture for 5 to 30 minutes while protecting from atmospheric
moisture perform the titration with vigorous stirring Determine the quantity of electricity
(C) [electric current (A) and time (s)] required for the production of iodine during the
titration and calculate the content () of the water in the sample by the formula
When the sample interferes with the Karl Fisher reaction water in the sample can be
removed by heating under a stream of nitrogen gas and introduced into the titration vessel
by using a water-evaporation device
Sheth M N Science College Patan Page no - 14
KARL - FISCHER Analysis Water Content Determination
8 Interference and Limitations of Karl Fischer Method
Interferences in the water determinations by Karl Fischer method are associated with
condensation or oxidation ndash reduction reactions with a number of substances and
compounds or by side reactions Other possible interferences to the KF reagent are certain
active metals metal oxides metal hydroxides chromates melamines etc
The Karl Fischer titration has a number of serious limitations due to possible
interferences tantamount to erroneous results namely
(i) Oxidizing agents for instance chromates Cu(II) Fe(III) Cr2O7 2ndash peroxides salts
higher oxides
Example MnO2 + 4C5H5NH+ + 2Indash rarr Mn2+ + 4C5H5N + I2 + H2O
(ii) Reducing agents such as Sn (II) salts sulphides and S2O32ndash and
(iii) Compounds that have a tendency to form water with the ingredients of the Karl Fischer
reagent for instance
(a) Basic oxides eg ZnO
Example ZnO + 2C5H5NH+ rarr Zn2+ + C5H5N + H2O
(b) Salts of weak oxy-acids eg NaHCO3
Example NaHCO3 + C5H5NH+ rarr Na+ + H2O + CO2 + C5H5N
[Note As H2CO3 carbonic acid is very unstable hence it splits up to yield a mole each of
water and CO2]
Use of certain reagents will minimize or eliminate the interferences of many classes
of compounds For example the use of non-methanol containing KF reagent and solvent will
reduce the interference from aldehyde and ketones Ammonia and amines can be eliminated
by the addition of salicylic acid to the solvent
Sheth M N Science College Patan Page no - 15
KARL - FISCHER Analysis Water Content Determination
9 Side reactions
Side Reactions are classified as
(1) Reactions Influenced by Methanol
(2) Reactions with Iodine
(1) Reactions Influenced by Methanol
It includes
bull Aldehydes react to form acetals also undergo Bisulfite addition
bull Ketones react to form ketals
bull Amines undergo methylation
bull Siloxanes undergo esterification
(2) Reactions with Iodine
It includes
bull Halogenated Hydrocarbons containing free halogens
bull Mercaptans (Thiols)
bull Phenols
bull Peroxides
Testing for Iodine Reactivity
bull Dissolve a few Iodine Crystals in Methanol and Introduce Sample
bull Watch for Solution to Change Color if Clear Iodine Reaction is Present
Sheth M N Science College Patan Page no - 16
KARL - FISCHER Analysis Water Content Determination
1 Mercaptans (Thiols) are oxidized by iodine Suppress this side reaction by treating
sample with N-ethylmaleimide in a buffered solvent system
2 Inhibition of Thiol Oxidation by Iodine
Most Phenols do not react with KF reagents higher molecular weight phenols and
Amino-phenols are most problematic Volumetric method is most suitable since the
Oxidation potential is high in coulometry or Acidify Solvent system to reduce interference
Sheth M N Science College Patan Page no - 17
KARL - FISCHER Analysis Water Content Determination
10 Applications of Karl Fischer method for determination of Water in
pharmaceutical analysis
101 PREDNISOLONE SODIUM PHOSPHATE
Materials Required Karl Fischer Reagent 100 ml prednisolone sodium phosphate
02gm anhydrous methanol 200 ml
Procedure Add about 20 ml of anhydrous methanol to the titration vessel and titrate to the
Amperometric End-point with the Karl Fischer reagent Quickly add 02 g of prednisolone
sodium phosphate sample stir for 1 minute and again titrate to the amperometric end-point
with the Karl Fischer reagent The difference between the two titrations gives the volume
(v) of Karl Fischer reagent consumed by the sample
The minimum water equivalent is 35 mg of water per ml of Karl Fischer reagent
Hence the percentage of water ww in the given sample may be calculated by the
following
Precautions
(1) The reagents and solutions used must be kept anhydrous and necessary care should be
taken throughout to prevent exposure to atmospheric moisture
(2) The Karl Fischer reagent should be protected from light and preferably stored in a bottle
fitted with an automatic burette and
(3) The water equivalent of Karl Fischer reagent should always be determined before use
102 COGNATE ASSAYS
A number of other official pharmaceutical substances may be assayed for their water
content by the Karl Fischer method as summarized in the following Table
Sheth M N Science College Patan Page no - 18
KARL - FISCHER Analysis Water Content Determination
11 Automated Electrochemical Karl Fischer Analysis
Commercially available Modern KF-Titrators are usually equipped with specifically
designed titration vessels that are exclusively meant to check and prevent the contact with
atmospheric moisture Quite a few such devices are armed with microprocessors that will
perform the requisite operations sequentially in a programmed manner automatically and
may also dish out a print-out of the desired results including the percentage moisture
content In fact these Modern KF-Titrators not only afford greater accuracy and precision
in results but also offer much ease and convenience in routine analysis as compared to the
classical techniques based on either caulometry or controlled current potentiometry using
two indicator electrodes
Thus the basis of the analysis rests upon the quantitative relationship existing
between charge passed and iodine produced by the reagent according to the above reaction
Therefore the generation of iodine is automatically stopped when an excess of it is detected
by the indicator electrode It essentially consists of two platinum electrodes across which an
AC is applied and subsequently a marked drop in voltage between the electrodes takes place
as soon as an excess of iodine is present Normally such automated instruments make use of
proprietory reagents exclusively
The major advantage of this approach to KF-analysis being that no calibration is
required as the method is absolute and is entirely based on the stoichiometry of the
aforesaid equation It is noteworthy that one may determine the amounts of water ranging
between 10 mcg and 10 mg in solid as well as liquid samples
Sheth M N Science College Patan Page no - 19
KARL - FISCHER Analysis Water Content Determination
12 Some of the innovative brands in Karl Fischer Auto ndash titrator
instruments
Sheth M N Science College Patan Page no - 20
KARL - FISCHER Analysis Water Content Determination
Sheth M N Science College Patan Page no - 21
KARL - FISCHER Analysis Water Content Determination
13 References
1 Chapter 14 Karl fishcer method for determination of water Page no 223-227
Pharmaceutical Drug Analysis Ashutosh Kar New age international limited publishers
2 Aqua Stars Karl fischer instruments wwwemdchemicalscom
3 Karl Fischer Volumetric Titration Theory and Practice by Radiometer analytical
SAS wwwradiometer-analyticalcom
4 MettlerToledo AG Analytical Switzerland wwwmtcom
5 Summary of Karl fischer titrations by Orion products Baverly MA Thermofischer
Scientific wwwthermocomwater
6 Moisture determination by Karl Fischer titration background of chemistry and
recent advances SupelcoSigma-Aldrich Bellefonte Pa wwwsigma-aldrichcom
7 Karl Fischer Titration Methods Methodology and water determination in 442
different substances European Pharmacopoeia (PharmEurope) 4th Edition plus
Supplements 2002 International Electrotechnical Commission Geneva
8 Karl ndash fischer titration wwwwikipediacom
Sheth M N Science College Patan Page no - 22
KARL - FISCHER Analysis Water Content Determination
Different methods to improve sample solubility are
i Choose the right solvent
ii Allow sufficient time for dissolution (extract time)
iii Grind sample
iv Co-solvent additions
v Elevating solvent system temperature and
vi Adding homogenizer to system
I Conductivity
Conductivity of the solvent and added sample has to be sufficient to support proper function
of the electrodes in order to detect the endpoint andor generate the iodine titrant (by
coulometric type)
J Influence of the Base
Pyridine was the original Karl-Fischer base Pyridine because of its weak basicity cannot
totally neutralize the methylsulfurous acid The equilibrium in reaction at stage-1 is not
completely shifted to the right The reaction with pyridine is therefore slow and the
endpoint is not stable often making the repeatability using pyridine very poor
Pyridine Imidazole
Imidazole was chosen to replace pyridine as it shifts the reaction at stage-1
completely to the right reaction is swift and endpoints clear and stable Imidazole also does
not have the unpleasant odor and toxicity of pyridine
Sheth M N Science College Patan Page no - 5
KARL - FISCHER Analysis Water Content Determination
K Stability of the Reagent
The stability of the original Karl Fischer reagent initially prepared with an excess of
methanol was found to be fairly poor and hence evidently needed frequent standardization
However it was established subsequently that the stability could be improved significantly
by replacing the methanol by 2-methoxyethanol It has been observed that the titer of the
Karl Fischer reagent which stands at 35 mg of water per milliliter of reagent falls rapidly
upon standing with the passage of time Hence the following precautions must be observed
rigidly using the Karl Fischer reagent namely
(a) Always prepare the reagent a day or two before it is to be used
(b) Great care must be taken to prevent and check any possible contamination either of the
reagent or the sample by atmospheric moisture
(c) All glassware(s) must be thoroughly dried before use
(d) Standard solution should be stored out of contact with air and
(e) Essential to minimise contact between the atmosphere and the solution during the course
of titration
Sheth M N Science College Patan Page no - 6
KARL - FISCHER Analysis Water Content Determination
4 Types of Karl ndash Fischer titrations
There are two determination methods different in iodine-providing principle
(1) Volumetric titration method and
(2) Coulometric titration method
In the volumetric titration method iodine required for reaction with water is
previously dissolved in water determination TS and water content is determined by
measuring the amount of iodine consumed as a result of reaction with water in a sample It
is good for high level moisture determination
In the coulometric titration method first iodine is produced by electrolysis of the
reagent containing iodide ion and then the water content in a sample is determined by
measuring the quantity of electricity (Coulombs) which is required for the electrolysis (ie
for the production of iodine) based on the quantitative reaction of the generated iodine with
water It is good for low level moisture determination
1048707Volumetric titration ndashwater determination based on volumetric measurement of reagent
Good for high level moisture determinations
1048707Coulometrictitration ndashwater determination based on measurement of time and current (in
coulombs) used to generate the iodine Good for low level moisture determinations
Instrument Type Volumetric Coulometric
Best for Solids and Liquids Low moisture samples
Water content limit 50 ppm to 100 water 1 ppm to 100 water
Matrices Solid Liquid and Gas Liquid only
Advantages All around performance Low detection limit
Sheth M N Science College Patan Page no - 7
KARL - FISCHER Analysis Water Content Determination
(Two compartment system possess separate solvent and titrating solutions)
Sheth M N Science College Patan Page no - 8
KARL - FISCHER Analysis Water Content Determination
5 Advantages
A Karl Fischer moisture determination is advantageous as compared to a
determination based on weight loss because KF is not affected by volatile compounds The
popularity of the Karl Fischer titration is due in large part to several practical advantages
that it holds over other methods of moisture determination including
i High accuracy and precision
ii Selectivity for water
iii Small sample quantities required
iv Easy sample preparation
v Short analysis duration
vi Nearly unlimited measuring range (1ppm to 100)
vii Suitability for analyzing Solids Liquids and Gases
viii Independence of presence of other volatiles
ix Suitability for automation
In contrast loss on drying will detect the loss of any volatile substance
Sheth M N Science College Patan Page no - 9
KARL - FISCHER Analysis Water Content Determination
6 Instrumentation
Following Figure illustrates a simple dead-stop end-point assembly or a Karl Fischer
titration apparatus The titration vessel is fitted with a pair of identical platinum electrodes
a mechanical stirrer with adjustable speed and a burette It will be observed that absolutely
little or no current may flow unless and until the solution is totally free from any polarizing
substances this could perhaps be due to the absorbed layers of oxygen and hydrogen on the
anode and cathode respectively
However the current shall flow only when the two electrodes get depolarized The
Karl Fischer reagent is pumped into the burette by means of hand bellows the access of
moisture is usually prevented by employing an appropriate arrangement of desiccant tubes
Alternatively the stirring may also be accomplished either by using a magnetic stirrer or by
means of suitably dried nitrogen passed gently through the solution during the course of
titration
The end-point is achieved by employing an eiectrical circuit comprising of a
microammeter (A) platinum electrodes together with a 15 V to 20 V battery connected
across a variable resistance of about 25 kΩ First of all the resistance is adjusted in such a
manner that an initial current passes through the platinum electrodes in series with a
microammeter (A) After each addition of reagent the pointer of the microammeter gets
deflected but quickly returns to its original position At the end of the reaction a deflection
is obtained which persists for 10-15 seconds
7 Water Determination Methodology
Sheth M N Science College Patan Page no - 10
KARL - FISCHER Analysis Water Content Determination
71 Volumetric titration method
Apparatus
Generally the apparatus consists of an automatic burette a backtitration flask a stirrer and
equipment for amperometric titration at constant voltage or potentiometric titration at
constant current
Because water determination TS is extremely hygroscopic the titration apparatus
should be protected from atmospheric moisture Silica gel or calcium chloride for water
determination is usually used for moisture protection
Procedure
As a rule the titration of the sample with water determination TS should be performed at
the same temperature as that at the standardization of the TS while protecting from
moisture
The apparatus is equipped with a variable resistor in the circuit and the resistor is
adjusted to apply a definite voltage (mV) between a pair of platinum electrodes immersed in
the solution to be titrated The change in current (1048707A) is measured during the dropping of
water determination TS (Amperometric titration at constant voltage) As titration continues
the abrupt change in current in the circuit occurs but returns to the original state within
several seconds At the end of a titration the change in current persists for a certain time
(usually longer than 30 seconds) The end point of titration is determined at this electric
state
Otherwise by adjusting the resistor a definite current is passed between the two
platinum electrodes and the change in potential (mV) is measured during dropping water
determination TS (Potentiometric titration at constant current) With the progress of
titration the value indicated by the potentiometer in the circuit decreases suddenly from a
polarization state of several hundreds (mV) to the nonpolarization state but it returns to the
original state within several seconds At the end of titration the non-polarization state
persists for a certain time (usually longer than 30 seconds) The end point of titration is
determined when this electric state attains
In the case of back titration when the amperometric titration method is used at
constant voltage the needle of microammeter is out of scale while an excessive quantity of
water determination TS remains It returns rapidly to the original position when the titration
reaches the end point
Sheth M N Science College Patan Page no - 11
KARL - FISCHER Analysis Water Content Determination
Similarly when the Potentiometric titration method at constant current is used the
needle of the millivoltmeter is at the original position while an excessive quantity of water
determination TS remains A definite voltage is applied when the titration reaches the end
point Unless otherwise specified the titration of water with water determination TS is
performed by either of the methods below Usually the end point of the titration can be
observed more clearly in the back titration method than in the direct titration method
711 Direct titration
Unless otherwise specified proceed as directed below Take 25ml of methanol for water
determination in a dried titration flask and titrate with water determination TS to the end
point Unless otherwise specified weigh accurately a quantity of the sample containing 10
to 50 mg of water transfer it quickly into the titration flask and dissolve by stirring Titrate
the solution with water determination TS to the end point under vigorous stirring When the
sample is insoluble in the solvent powder the sample quickly weigh a suitable amount of
the sample accurately and transfer it quickly into the titration vessel stir the mixture for 30
minutes while protecting it from moisture
Perform a titration under vigorous stirring When the sample interferes with the Karl
Fisher reaction water in the sample can be removed by heating and under a stream of
nitrogen gas and introduced into the titration vessel by using a water-evaporation device
712 Back titration
Unless otherwise specified proceed as directed below Take 20ml of methanol for water
determination in the dried titration vessel and titrate with water determination TS Weigh
accurately a suitable quantity of the sample containing 10 to 50 mg of water transfer the
sample quickly into the titration vessel add an excessive and definite volume of water
determination TS stir for 30 min protecting from atmospheric moisture and then titrate the
solution with Water Methanol Standard Solution under vigorous stirring
Sheth M N Science College Patan Page no - 12
KARL - FISCHER Analysis Water Content Determination
Wheref = the number of mg of water (H2O) corresponding to 1 ml of water determination
TS
Where
f = the number of mg of water (H2O) corresponding to 1 ml of water determination TS
f rsquo = the number of mg of water (H2O) in 1 ml of Water Methanol Standard Solution
713 General Remarks
Volumetric Karl Fischer titration requires the determination of the titre (t) of the Karl
Fischer reagent It is usually quoted in mg of water per ml of Karl Fischer reagent Modern
reagents allow direct titration of water in the sample The sample may be introduced
directly into the KF cell or after an extraction or dissolution with a suitable solvent The
water concentration of the solvent must be determined previously in order to be subtracted
from the sample analysis In Radiometer Analytical titrators all these operations are
simplified and the different results are accounted for automatically The volumetric titration
of water allows the analysis of water concentrations between 01 and 100 If an aliquot
contains less than 1 mg of water coulometric determination will result in a more accurate
result For reasons of precision the titre of the titrant should be chosen so that the titration is
completed with a titrant demand between 1 and 10 ml
The working medium (ie the solvent required) can be freely chosen by the user
depending on the dissolution properties of the sample to be investigated For methanol-
based reagents the stoichiometry 11 of the Karl Fischer reaction is only fulfilled if there is
more than 25 methanol in the reaction mixture A methanol-free working medium can be
used however it is important to determine the titre of the KF reagent in the same working
medium The modern solvents available today present a high buffer and dissolution
capacity These solvents consist of sulphur dioxide a base and methanol or ethanol The
main advantages of these solvents are
bull A more rapid titration due to better reaction kinetics an advantage especially for the
titration of large amounts of water
bull A better reproducibility because the reaction environment is stable The pH and the
sulphur dioxide concentration remain constant
Sheth M N Science College Patan Page no - 13
KARL - FISCHER Analysis Water Content Determination
For the titration of samples producing side reactions (aldehydes ketones and
silanols) it is necessary to use an appropriate solvent Most reagent manufacturers include
the letter K in the commercial name of such solvents
72 Coulometric titration
Apparatus
Usually the apparatus is comprised of an electrolytic cell for iodine production a stirrer a
titration flask and a potentiometric titration system at constant current The iodine
production device is composed of an anode and a cathode separated by a diaphragm The
anode is immersed in the anolyte solution for water determination and the cathode is
immersed in the catholyte solution for water determination Both electrodes are usually
made of platinum-mesh
Because water determination TS is extremely hygroscopic the titration apparatus
should be protected from atmospheric moisture For this purpose silica gel or calcium
chloride for water determination is usually used Procedure Take a suitable volume of an
anolyte for water determination in a titration vessel immerse in this solution a pair of
platinum electrodes for potentiometric titration at constant current
Then immerse the iodide production system filled with a catholyte for water
determination in the anolyte solution Switch on the electrolytic system and make the
content of the titration vessel anhydrous Next take an accurately weighed amount of the
sample containing 15 mg of water add it quickly to the vessel and dissolve by stirring
Perform the titration to the end point under vigorous stirring When the sample is insoluble
in the anolyte powder it quickly and add an accurately weighed amount of the sample to
the vessel After stirring the mixture for 5 to 30 minutes while protecting from atmospheric
moisture perform the titration with vigorous stirring Determine the quantity of electricity
(C) [electric current (A) and time (s)] required for the production of iodine during the
titration and calculate the content () of the water in the sample by the formula
When the sample interferes with the Karl Fisher reaction water in the sample can be
removed by heating under a stream of nitrogen gas and introduced into the titration vessel
by using a water-evaporation device
Sheth M N Science College Patan Page no - 14
KARL - FISCHER Analysis Water Content Determination
8 Interference and Limitations of Karl Fischer Method
Interferences in the water determinations by Karl Fischer method are associated with
condensation or oxidation ndash reduction reactions with a number of substances and
compounds or by side reactions Other possible interferences to the KF reagent are certain
active metals metal oxides metal hydroxides chromates melamines etc
The Karl Fischer titration has a number of serious limitations due to possible
interferences tantamount to erroneous results namely
(i) Oxidizing agents for instance chromates Cu(II) Fe(III) Cr2O7 2ndash peroxides salts
higher oxides
Example MnO2 + 4C5H5NH+ + 2Indash rarr Mn2+ + 4C5H5N + I2 + H2O
(ii) Reducing agents such as Sn (II) salts sulphides and S2O32ndash and
(iii) Compounds that have a tendency to form water with the ingredients of the Karl Fischer
reagent for instance
(a) Basic oxides eg ZnO
Example ZnO + 2C5H5NH+ rarr Zn2+ + C5H5N + H2O
(b) Salts of weak oxy-acids eg NaHCO3
Example NaHCO3 + C5H5NH+ rarr Na+ + H2O + CO2 + C5H5N
[Note As H2CO3 carbonic acid is very unstable hence it splits up to yield a mole each of
water and CO2]
Use of certain reagents will minimize or eliminate the interferences of many classes
of compounds For example the use of non-methanol containing KF reagent and solvent will
reduce the interference from aldehyde and ketones Ammonia and amines can be eliminated
by the addition of salicylic acid to the solvent
Sheth M N Science College Patan Page no - 15
KARL - FISCHER Analysis Water Content Determination
9 Side reactions
Side Reactions are classified as
(1) Reactions Influenced by Methanol
(2) Reactions with Iodine
(1) Reactions Influenced by Methanol
It includes
bull Aldehydes react to form acetals also undergo Bisulfite addition
bull Ketones react to form ketals
bull Amines undergo methylation
bull Siloxanes undergo esterification
(2) Reactions with Iodine
It includes
bull Halogenated Hydrocarbons containing free halogens
bull Mercaptans (Thiols)
bull Phenols
bull Peroxides
Testing for Iodine Reactivity
bull Dissolve a few Iodine Crystals in Methanol and Introduce Sample
bull Watch for Solution to Change Color if Clear Iodine Reaction is Present
Sheth M N Science College Patan Page no - 16
KARL - FISCHER Analysis Water Content Determination
1 Mercaptans (Thiols) are oxidized by iodine Suppress this side reaction by treating
sample with N-ethylmaleimide in a buffered solvent system
2 Inhibition of Thiol Oxidation by Iodine
Most Phenols do not react with KF reagents higher molecular weight phenols and
Amino-phenols are most problematic Volumetric method is most suitable since the
Oxidation potential is high in coulometry or Acidify Solvent system to reduce interference
Sheth M N Science College Patan Page no - 17
KARL - FISCHER Analysis Water Content Determination
10 Applications of Karl Fischer method for determination of Water in
pharmaceutical analysis
101 PREDNISOLONE SODIUM PHOSPHATE
Materials Required Karl Fischer Reagent 100 ml prednisolone sodium phosphate
02gm anhydrous methanol 200 ml
Procedure Add about 20 ml of anhydrous methanol to the titration vessel and titrate to the
Amperometric End-point with the Karl Fischer reagent Quickly add 02 g of prednisolone
sodium phosphate sample stir for 1 minute and again titrate to the amperometric end-point
with the Karl Fischer reagent The difference between the two titrations gives the volume
(v) of Karl Fischer reagent consumed by the sample
The minimum water equivalent is 35 mg of water per ml of Karl Fischer reagent
Hence the percentage of water ww in the given sample may be calculated by the
following
Precautions
(1) The reagents and solutions used must be kept anhydrous and necessary care should be
taken throughout to prevent exposure to atmospheric moisture
(2) The Karl Fischer reagent should be protected from light and preferably stored in a bottle
fitted with an automatic burette and
(3) The water equivalent of Karl Fischer reagent should always be determined before use
102 COGNATE ASSAYS
A number of other official pharmaceutical substances may be assayed for their water
content by the Karl Fischer method as summarized in the following Table
Sheth M N Science College Patan Page no - 18
KARL - FISCHER Analysis Water Content Determination
11 Automated Electrochemical Karl Fischer Analysis
Commercially available Modern KF-Titrators are usually equipped with specifically
designed titration vessels that are exclusively meant to check and prevent the contact with
atmospheric moisture Quite a few such devices are armed with microprocessors that will
perform the requisite operations sequentially in a programmed manner automatically and
may also dish out a print-out of the desired results including the percentage moisture
content In fact these Modern KF-Titrators not only afford greater accuracy and precision
in results but also offer much ease and convenience in routine analysis as compared to the
classical techniques based on either caulometry or controlled current potentiometry using
two indicator electrodes
Thus the basis of the analysis rests upon the quantitative relationship existing
between charge passed and iodine produced by the reagent according to the above reaction
Therefore the generation of iodine is automatically stopped when an excess of it is detected
by the indicator electrode It essentially consists of two platinum electrodes across which an
AC is applied and subsequently a marked drop in voltage between the electrodes takes place
as soon as an excess of iodine is present Normally such automated instruments make use of
proprietory reagents exclusively
The major advantage of this approach to KF-analysis being that no calibration is
required as the method is absolute and is entirely based on the stoichiometry of the
aforesaid equation It is noteworthy that one may determine the amounts of water ranging
between 10 mcg and 10 mg in solid as well as liquid samples
Sheth M N Science College Patan Page no - 19
KARL - FISCHER Analysis Water Content Determination
12 Some of the innovative brands in Karl Fischer Auto ndash titrator
instruments
Sheth M N Science College Patan Page no - 20
KARL - FISCHER Analysis Water Content Determination
Sheth M N Science College Patan Page no - 21
KARL - FISCHER Analysis Water Content Determination
13 References
1 Chapter 14 Karl fishcer method for determination of water Page no 223-227
Pharmaceutical Drug Analysis Ashutosh Kar New age international limited publishers
2 Aqua Stars Karl fischer instruments wwwemdchemicalscom
3 Karl Fischer Volumetric Titration Theory and Practice by Radiometer analytical
SAS wwwradiometer-analyticalcom
4 MettlerToledo AG Analytical Switzerland wwwmtcom
5 Summary of Karl fischer titrations by Orion products Baverly MA Thermofischer
Scientific wwwthermocomwater
6 Moisture determination by Karl Fischer titration background of chemistry and
recent advances SupelcoSigma-Aldrich Bellefonte Pa wwwsigma-aldrichcom
7 Karl Fischer Titration Methods Methodology and water determination in 442
different substances European Pharmacopoeia (PharmEurope) 4th Edition plus
Supplements 2002 International Electrotechnical Commission Geneva
8 Karl ndash fischer titration wwwwikipediacom
Sheth M N Science College Patan Page no - 22
KARL - FISCHER Analysis Water Content Determination
K Stability of the Reagent
The stability of the original Karl Fischer reagent initially prepared with an excess of
methanol was found to be fairly poor and hence evidently needed frequent standardization
However it was established subsequently that the stability could be improved significantly
by replacing the methanol by 2-methoxyethanol It has been observed that the titer of the
Karl Fischer reagent which stands at 35 mg of water per milliliter of reagent falls rapidly
upon standing with the passage of time Hence the following precautions must be observed
rigidly using the Karl Fischer reagent namely
(a) Always prepare the reagent a day or two before it is to be used
(b) Great care must be taken to prevent and check any possible contamination either of the
reagent or the sample by atmospheric moisture
(c) All glassware(s) must be thoroughly dried before use
(d) Standard solution should be stored out of contact with air and
(e) Essential to minimise contact between the atmosphere and the solution during the course
of titration
Sheth M N Science College Patan Page no - 6
KARL - FISCHER Analysis Water Content Determination
4 Types of Karl ndash Fischer titrations
There are two determination methods different in iodine-providing principle
(1) Volumetric titration method and
(2) Coulometric titration method
In the volumetric titration method iodine required for reaction with water is
previously dissolved in water determination TS and water content is determined by
measuring the amount of iodine consumed as a result of reaction with water in a sample It
is good for high level moisture determination
In the coulometric titration method first iodine is produced by electrolysis of the
reagent containing iodide ion and then the water content in a sample is determined by
measuring the quantity of electricity (Coulombs) which is required for the electrolysis (ie
for the production of iodine) based on the quantitative reaction of the generated iodine with
water It is good for low level moisture determination
1048707Volumetric titration ndashwater determination based on volumetric measurement of reagent
Good for high level moisture determinations
1048707Coulometrictitration ndashwater determination based on measurement of time and current (in
coulombs) used to generate the iodine Good for low level moisture determinations
Instrument Type Volumetric Coulometric
Best for Solids and Liquids Low moisture samples
Water content limit 50 ppm to 100 water 1 ppm to 100 water
Matrices Solid Liquid and Gas Liquid only
Advantages All around performance Low detection limit
Sheth M N Science College Patan Page no - 7
KARL - FISCHER Analysis Water Content Determination
(Two compartment system possess separate solvent and titrating solutions)
Sheth M N Science College Patan Page no - 8
KARL - FISCHER Analysis Water Content Determination
5 Advantages
A Karl Fischer moisture determination is advantageous as compared to a
determination based on weight loss because KF is not affected by volatile compounds The
popularity of the Karl Fischer titration is due in large part to several practical advantages
that it holds over other methods of moisture determination including
i High accuracy and precision
ii Selectivity for water
iii Small sample quantities required
iv Easy sample preparation
v Short analysis duration
vi Nearly unlimited measuring range (1ppm to 100)
vii Suitability for analyzing Solids Liquids and Gases
viii Independence of presence of other volatiles
ix Suitability for automation
In contrast loss on drying will detect the loss of any volatile substance
Sheth M N Science College Patan Page no - 9
KARL - FISCHER Analysis Water Content Determination
6 Instrumentation
Following Figure illustrates a simple dead-stop end-point assembly or a Karl Fischer
titration apparatus The titration vessel is fitted with a pair of identical platinum electrodes
a mechanical stirrer with adjustable speed and a burette It will be observed that absolutely
little or no current may flow unless and until the solution is totally free from any polarizing
substances this could perhaps be due to the absorbed layers of oxygen and hydrogen on the
anode and cathode respectively
However the current shall flow only when the two electrodes get depolarized The
Karl Fischer reagent is pumped into the burette by means of hand bellows the access of
moisture is usually prevented by employing an appropriate arrangement of desiccant tubes
Alternatively the stirring may also be accomplished either by using a magnetic stirrer or by
means of suitably dried nitrogen passed gently through the solution during the course of
titration
The end-point is achieved by employing an eiectrical circuit comprising of a
microammeter (A) platinum electrodes together with a 15 V to 20 V battery connected
across a variable resistance of about 25 kΩ First of all the resistance is adjusted in such a
manner that an initial current passes through the platinum electrodes in series with a
microammeter (A) After each addition of reagent the pointer of the microammeter gets
deflected but quickly returns to its original position At the end of the reaction a deflection
is obtained which persists for 10-15 seconds
7 Water Determination Methodology
Sheth M N Science College Patan Page no - 10
KARL - FISCHER Analysis Water Content Determination
71 Volumetric titration method
Apparatus
Generally the apparatus consists of an automatic burette a backtitration flask a stirrer and
equipment for amperometric titration at constant voltage or potentiometric titration at
constant current
Because water determination TS is extremely hygroscopic the titration apparatus
should be protected from atmospheric moisture Silica gel or calcium chloride for water
determination is usually used for moisture protection
Procedure
As a rule the titration of the sample with water determination TS should be performed at
the same temperature as that at the standardization of the TS while protecting from
moisture
The apparatus is equipped with a variable resistor in the circuit and the resistor is
adjusted to apply a definite voltage (mV) between a pair of platinum electrodes immersed in
the solution to be titrated The change in current (1048707A) is measured during the dropping of
water determination TS (Amperometric titration at constant voltage) As titration continues
the abrupt change in current in the circuit occurs but returns to the original state within
several seconds At the end of a titration the change in current persists for a certain time
(usually longer than 30 seconds) The end point of titration is determined at this electric
state
Otherwise by adjusting the resistor a definite current is passed between the two
platinum electrodes and the change in potential (mV) is measured during dropping water
determination TS (Potentiometric titration at constant current) With the progress of
titration the value indicated by the potentiometer in the circuit decreases suddenly from a
polarization state of several hundreds (mV) to the nonpolarization state but it returns to the
original state within several seconds At the end of titration the non-polarization state
persists for a certain time (usually longer than 30 seconds) The end point of titration is
determined when this electric state attains
In the case of back titration when the amperometric titration method is used at
constant voltage the needle of microammeter is out of scale while an excessive quantity of
water determination TS remains It returns rapidly to the original position when the titration
reaches the end point
Sheth M N Science College Patan Page no - 11
KARL - FISCHER Analysis Water Content Determination
Similarly when the Potentiometric titration method at constant current is used the
needle of the millivoltmeter is at the original position while an excessive quantity of water
determination TS remains A definite voltage is applied when the titration reaches the end
point Unless otherwise specified the titration of water with water determination TS is
performed by either of the methods below Usually the end point of the titration can be
observed more clearly in the back titration method than in the direct titration method
711 Direct titration
Unless otherwise specified proceed as directed below Take 25ml of methanol for water
determination in a dried titration flask and titrate with water determination TS to the end
point Unless otherwise specified weigh accurately a quantity of the sample containing 10
to 50 mg of water transfer it quickly into the titration flask and dissolve by stirring Titrate
the solution with water determination TS to the end point under vigorous stirring When the
sample is insoluble in the solvent powder the sample quickly weigh a suitable amount of
the sample accurately and transfer it quickly into the titration vessel stir the mixture for 30
minutes while protecting it from moisture
Perform a titration under vigorous stirring When the sample interferes with the Karl
Fisher reaction water in the sample can be removed by heating and under a stream of
nitrogen gas and introduced into the titration vessel by using a water-evaporation device
712 Back titration
Unless otherwise specified proceed as directed below Take 20ml of methanol for water
determination in the dried titration vessel and titrate with water determination TS Weigh
accurately a suitable quantity of the sample containing 10 to 50 mg of water transfer the
sample quickly into the titration vessel add an excessive and definite volume of water
determination TS stir for 30 min protecting from atmospheric moisture and then titrate the
solution with Water Methanol Standard Solution under vigorous stirring
Sheth M N Science College Patan Page no - 12
KARL - FISCHER Analysis Water Content Determination
Wheref = the number of mg of water (H2O) corresponding to 1 ml of water determination
TS
Where
f = the number of mg of water (H2O) corresponding to 1 ml of water determination TS
f rsquo = the number of mg of water (H2O) in 1 ml of Water Methanol Standard Solution
713 General Remarks
Volumetric Karl Fischer titration requires the determination of the titre (t) of the Karl
Fischer reagent It is usually quoted in mg of water per ml of Karl Fischer reagent Modern
reagents allow direct titration of water in the sample The sample may be introduced
directly into the KF cell or after an extraction or dissolution with a suitable solvent The
water concentration of the solvent must be determined previously in order to be subtracted
from the sample analysis In Radiometer Analytical titrators all these operations are
simplified and the different results are accounted for automatically The volumetric titration
of water allows the analysis of water concentrations between 01 and 100 If an aliquot
contains less than 1 mg of water coulometric determination will result in a more accurate
result For reasons of precision the titre of the titrant should be chosen so that the titration is
completed with a titrant demand between 1 and 10 ml
The working medium (ie the solvent required) can be freely chosen by the user
depending on the dissolution properties of the sample to be investigated For methanol-
based reagents the stoichiometry 11 of the Karl Fischer reaction is only fulfilled if there is
more than 25 methanol in the reaction mixture A methanol-free working medium can be
used however it is important to determine the titre of the KF reagent in the same working
medium The modern solvents available today present a high buffer and dissolution
capacity These solvents consist of sulphur dioxide a base and methanol or ethanol The
main advantages of these solvents are
bull A more rapid titration due to better reaction kinetics an advantage especially for the
titration of large amounts of water
bull A better reproducibility because the reaction environment is stable The pH and the
sulphur dioxide concentration remain constant
Sheth M N Science College Patan Page no - 13
KARL - FISCHER Analysis Water Content Determination
For the titration of samples producing side reactions (aldehydes ketones and
silanols) it is necessary to use an appropriate solvent Most reagent manufacturers include
the letter K in the commercial name of such solvents
72 Coulometric titration
Apparatus
Usually the apparatus is comprised of an electrolytic cell for iodine production a stirrer a
titration flask and a potentiometric titration system at constant current The iodine
production device is composed of an anode and a cathode separated by a diaphragm The
anode is immersed in the anolyte solution for water determination and the cathode is
immersed in the catholyte solution for water determination Both electrodes are usually
made of platinum-mesh
Because water determination TS is extremely hygroscopic the titration apparatus
should be protected from atmospheric moisture For this purpose silica gel or calcium
chloride for water determination is usually used Procedure Take a suitable volume of an
anolyte for water determination in a titration vessel immerse in this solution a pair of
platinum electrodes for potentiometric titration at constant current
Then immerse the iodide production system filled with a catholyte for water
determination in the anolyte solution Switch on the electrolytic system and make the
content of the titration vessel anhydrous Next take an accurately weighed amount of the
sample containing 15 mg of water add it quickly to the vessel and dissolve by stirring
Perform the titration to the end point under vigorous stirring When the sample is insoluble
in the anolyte powder it quickly and add an accurately weighed amount of the sample to
the vessel After stirring the mixture for 5 to 30 minutes while protecting from atmospheric
moisture perform the titration with vigorous stirring Determine the quantity of electricity
(C) [electric current (A) and time (s)] required for the production of iodine during the
titration and calculate the content () of the water in the sample by the formula
When the sample interferes with the Karl Fisher reaction water in the sample can be
removed by heating under a stream of nitrogen gas and introduced into the titration vessel
by using a water-evaporation device
Sheth M N Science College Patan Page no - 14
KARL - FISCHER Analysis Water Content Determination
8 Interference and Limitations of Karl Fischer Method
Interferences in the water determinations by Karl Fischer method are associated with
condensation or oxidation ndash reduction reactions with a number of substances and
compounds or by side reactions Other possible interferences to the KF reagent are certain
active metals metal oxides metal hydroxides chromates melamines etc
The Karl Fischer titration has a number of serious limitations due to possible
interferences tantamount to erroneous results namely
(i) Oxidizing agents for instance chromates Cu(II) Fe(III) Cr2O7 2ndash peroxides salts
higher oxides
Example MnO2 + 4C5H5NH+ + 2Indash rarr Mn2+ + 4C5H5N + I2 + H2O
(ii) Reducing agents such as Sn (II) salts sulphides and S2O32ndash and
(iii) Compounds that have a tendency to form water with the ingredients of the Karl Fischer
reagent for instance
(a) Basic oxides eg ZnO
Example ZnO + 2C5H5NH+ rarr Zn2+ + C5H5N + H2O
(b) Salts of weak oxy-acids eg NaHCO3
Example NaHCO3 + C5H5NH+ rarr Na+ + H2O + CO2 + C5H5N
[Note As H2CO3 carbonic acid is very unstable hence it splits up to yield a mole each of
water and CO2]
Use of certain reagents will minimize or eliminate the interferences of many classes
of compounds For example the use of non-methanol containing KF reagent and solvent will
reduce the interference from aldehyde and ketones Ammonia and amines can be eliminated
by the addition of salicylic acid to the solvent
Sheth M N Science College Patan Page no - 15
KARL - FISCHER Analysis Water Content Determination
9 Side reactions
Side Reactions are classified as
(1) Reactions Influenced by Methanol
(2) Reactions with Iodine
(1) Reactions Influenced by Methanol
It includes
bull Aldehydes react to form acetals also undergo Bisulfite addition
bull Ketones react to form ketals
bull Amines undergo methylation
bull Siloxanes undergo esterification
(2) Reactions with Iodine
It includes
bull Halogenated Hydrocarbons containing free halogens
bull Mercaptans (Thiols)
bull Phenols
bull Peroxides
Testing for Iodine Reactivity
bull Dissolve a few Iodine Crystals in Methanol and Introduce Sample
bull Watch for Solution to Change Color if Clear Iodine Reaction is Present
Sheth M N Science College Patan Page no - 16
KARL - FISCHER Analysis Water Content Determination
1 Mercaptans (Thiols) are oxidized by iodine Suppress this side reaction by treating
sample with N-ethylmaleimide in a buffered solvent system
2 Inhibition of Thiol Oxidation by Iodine
Most Phenols do not react with KF reagents higher molecular weight phenols and
Amino-phenols are most problematic Volumetric method is most suitable since the
Oxidation potential is high in coulometry or Acidify Solvent system to reduce interference
Sheth M N Science College Patan Page no - 17
KARL - FISCHER Analysis Water Content Determination
10 Applications of Karl Fischer method for determination of Water in
pharmaceutical analysis
101 PREDNISOLONE SODIUM PHOSPHATE
Materials Required Karl Fischer Reagent 100 ml prednisolone sodium phosphate
02gm anhydrous methanol 200 ml
Procedure Add about 20 ml of anhydrous methanol to the titration vessel and titrate to the
Amperometric End-point with the Karl Fischer reagent Quickly add 02 g of prednisolone
sodium phosphate sample stir for 1 minute and again titrate to the amperometric end-point
with the Karl Fischer reagent The difference between the two titrations gives the volume
(v) of Karl Fischer reagent consumed by the sample
The minimum water equivalent is 35 mg of water per ml of Karl Fischer reagent
Hence the percentage of water ww in the given sample may be calculated by the
following
Precautions
(1) The reagents and solutions used must be kept anhydrous and necessary care should be
taken throughout to prevent exposure to atmospheric moisture
(2) The Karl Fischer reagent should be protected from light and preferably stored in a bottle
fitted with an automatic burette and
(3) The water equivalent of Karl Fischer reagent should always be determined before use
102 COGNATE ASSAYS
A number of other official pharmaceutical substances may be assayed for their water
content by the Karl Fischer method as summarized in the following Table
Sheth M N Science College Patan Page no - 18
KARL - FISCHER Analysis Water Content Determination
11 Automated Electrochemical Karl Fischer Analysis
Commercially available Modern KF-Titrators are usually equipped with specifically
designed titration vessels that are exclusively meant to check and prevent the contact with
atmospheric moisture Quite a few such devices are armed with microprocessors that will
perform the requisite operations sequentially in a programmed manner automatically and
may also dish out a print-out of the desired results including the percentage moisture
content In fact these Modern KF-Titrators not only afford greater accuracy and precision
in results but also offer much ease and convenience in routine analysis as compared to the
classical techniques based on either caulometry or controlled current potentiometry using
two indicator electrodes
Thus the basis of the analysis rests upon the quantitative relationship existing
between charge passed and iodine produced by the reagent according to the above reaction
Therefore the generation of iodine is automatically stopped when an excess of it is detected
by the indicator electrode It essentially consists of two platinum electrodes across which an
AC is applied and subsequently a marked drop in voltage between the electrodes takes place
as soon as an excess of iodine is present Normally such automated instruments make use of
proprietory reagents exclusively
The major advantage of this approach to KF-analysis being that no calibration is
required as the method is absolute and is entirely based on the stoichiometry of the
aforesaid equation It is noteworthy that one may determine the amounts of water ranging
between 10 mcg and 10 mg in solid as well as liquid samples
Sheth M N Science College Patan Page no - 19
KARL - FISCHER Analysis Water Content Determination
12 Some of the innovative brands in Karl Fischer Auto ndash titrator
instruments
Sheth M N Science College Patan Page no - 20
KARL - FISCHER Analysis Water Content Determination
Sheth M N Science College Patan Page no - 21
KARL - FISCHER Analysis Water Content Determination
13 References
1 Chapter 14 Karl fishcer method for determination of water Page no 223-227
Pharmaceutical Drug Analysis Ashutosh Kar New age international limited publishers
2 Aqua Stars Karl fischer instruments wwwemdchemicalscom
3 Karl Fischer Volumetric Titration Theory and Practice by Radiometer analytical
SAS wwwradiometer-analyticalcom
4 MettlerToledo AG Analytical Switzerland wwwmtcom
5 Summary of Karl fischer titrations by Orion products Baverly MA Thermofischer
Scientific wwwthermocomwater
6 Moisture determination by Karl Fischer titration background of chemistry and
recent advances SupelcoSigma-Aldrich Bellefonte Pa wwwsigma-aldrichcom
7 Karl Fischer Titration Methods Methodology and water determination in 442
different substances European Pharmacopoeia (PharmEurope) 4th Edition plus
Supplements 2002 International Electrotechnical Commission Geneva
8 Karl ndash fischer titration wwwwikipediacom
Sheth M N Science College Patan Page no - 22
KARL - FISCHER Analysis Water Content Determination
4 Types of Karl ndash Fischer titrations
There are two determination methods different in iodine-providing principle
(1) Volumetric titration method and
(2) Coulometric titration method
In the volumetric titration method iodine required for reaction with water is
previously dissolved in water determination TS and water content is determined by
measuring the amount of iodine consumed as a result of reaction with water in a sample It
is good for high level moisture determination
In the coulometric titration method first iodine is produced by electrolysis of the
reagent containing iodide ion and then the water content in a sample is determined by
measuring the quantity of electricity (Coulombs) which is required for the electrolysis (ie
for the production of iodine) based on the quantitative reaction of the generated iodine with
water It is good for low level moisture determination
1048707Volumetric titration ndashwater determination based on volumetric measurement of reagent
Good for high level moisture determinations
1048707Coulometrictitration ndashwater determination based on measurement of time and current (in
coulombs) used to generate the iodine Good for low level moisture determinations
Instrument Type Volumetric Coulometric
Best for Solids and Liquids Low moisture samples
Water content limit 50 ppm to 100 water 1 ppm to 100 water
Matrices Solid Liquid and Gas Liquid only
Advantages All around performance Low detection limit
Sheth M N Science College Patan Page no - 7
KARL - FISCHER Analysis Water Content Determination
(Two compartment system possess separate solvent and titrating solutions)
Sheth M N Science College Patan Page no - 8
KARL - FISCHER Analysis Water Content Determination
5 Advantages
A Karl Fischer moisture determination is advantageous as compared to a
determination based on weight loss because KF is not affected by volatile compounds The
popularity of the Karl Fischer titration is due in large part to several practical advantages
that it holds over other methods of moisture determination including
i High accuracy and precision
ii Selectivity for water
iii Small sample quantities required
iv Easy sample preparation
v Short analysis duration
vi Nearly unlimited measuring range (1ppm to 100)
vii Suitability for analyzing Solids Liquids and Gases
viii Independence of presence of other volatiles
ix Suitability for automation
In contrast loss on drying will detect the loss of any volatile substance
Sheth M N Science College Patan Page no - 9
KARL - FISCHER Analysis Water Content Determination
6 Instrumentation
Following Figure illustrates a simple dead-stop end-point assembly or a Karl Fischer
titration apparatus The titration vessel is fitted with a pair of identical platinum electrodes
a mechanical stirrer with adjustable speed and a burette It will be observed that absolutely
little or no current may flow unless and until the solution is totally free from any polarizing
substances this could perhaps be due to the absorbed layers of oxygen and hydrogen on the
anode and cathode respectively
However the current shall flow only when the two electrodes get depolarized The
Karl Fischer reagent is pumped into the burette by means of hand bellows the access of
moisture is usually prevented by employing an appropriate arrangement of desiccant tubes
Alternatively the stirring may also be accomplished either by using a magnetic stirrer or by
means of suitably dried nitrogen passed gently through the solution during the course of
titration
The end-point is achieved by employing an eiectrical circuit comprising of a
microammeter (A) platinum electrodes together with a 15 V to 20 V battery connected
across a variable resistance of about 25 kΩ First of all the resistance is adjusted in such a
manner that an initial current passes through the platinum electrodes in series with a
microammeter (A) After each addition of reagent the pointer of the microammeter gets
deflected but quickly returns to its original position At the end of the reaction a deflection
is obtained which persists for 10-15 seconds
7 Water Determination Methodology
Sheth M N Science College Patan Page no - 10
KARL - FISCHER Analysis Water Content Determination
71 Volumetric titration method
Apparatus
Generally the apparatus consists of an automatic burette a backtitration flask a stirrer and
equipment for amperometric titration at constant voltage or potentiometric titration at
constant current
Because water determination TS is extremely hygroscopic the titration apparatus
should be protected from atmospheric moisture Silica gel or calcium chloride for water
determination is usually used for moisture protection
Procedure
As a rule the titration of the sample with water determination TS should be performed at
the same temperature as that at the standardization of the TS while protecting from
moisture
The apparatus is equipped with a variable resistor in the circuit and the resistor is
adjusted to apply a definite voltage (mV) between a pair of platinum electrodes immersed in
the solution to be titrated The change in current (1048707A) is measured during the dropping of
water determination TS (Amperometric titration at constant voltage) As titration continues
the abrupt change in current in the circuit occurs but returns to the original state within
several seconds At the end of a titration the change in current persists for a certain time
(usually longer than 30 seconds) The end point of titration is determined at this electric
state
Otherwise by adjusting the resistor a definite current is passed between the two
platinum electrodes and the change in potential (mV) is measured during dropping water
determination TS (Potentiometric titration at constant current) With the progress of
titration the value indicated by the potentiometer in the circuit decreases suddenly from a
polarization state of several hundreds (mV) to the nonpolarization state but it returns to the
original state within several seconds At the end of titration the non-polarization state
persists for a certain time (usually longer than 30 seconds) The end point of titration is
determined when this electric state attains
In the case of back titration when the amperometric titration method is used at
constant voltage the needle of microammeter is out of scale while an excessive quantity of
water determination TS remains It returns rapidly to the original position when the titration
reaches the end point
Sheth M N Science College Patan Page no - 11
KARL - FISCHER Analysis Water Content Determination
Similarly when the Potentiometric titration method at constant current is used the
needle of the millivoltmeter is at the original position while an excessive quantity of water
determination TS remains A definite voltage is applied when the titration reaches the end
point Unless otherwise specified the titration of water with water determination TS is
performed by either of the methods below Usually the end point of the titration can be
observed more clearly in the back titration method than in the direct titration method
711 Direct titration
Unless otherwise specified proceed as directed below Take 25ml of methanol for water
determination in a dried titration flask and titrate with water determination TS to the end
point Unless otherwise specified weigh accurately a quantity of the sample containing 10
to 50 mg of water transfer it quickly into the titration flask and dissolve by stirring Titrate
the solution with water determination TS to the end point under vigorous stirring When the
sample is insoluble in the solvent powder the sample quickly weigh a suitable amount of
the sample accurately and transfer it quickly into the titration vessel stir the mixture for 30
minutes while protecting it from moisture
Perform a titration under vigorous stirring When the sample interferes with the Karl
Fisher reaction water in the sample can be removed by heating and under a stream of
nitrogen gas and introduced into the titration vessel by using a water-evaporation device
712 Back titration
Unless otherwise specified proceed as directed below Take 20ml of methanol for water
determination in the dried titration vessel and titrate with water determination TS Weigh
accurately a suitable quantity of the sample containing 10 to 50 mg of water transfer the
sample quickly into the titration vessel add an excessive and definite volume of water
determination TS stir for 30 min protecting from atmospheric moisture and then titrate the
solution with Water Methanol Standard Solution under vigorous stirring
Sheth M N Science College Patan Page no - 12
KARL - FISCHER Analysis Water Content Determination
Wheref = the number of mg of water (H2O) corresponding to 1 ml of water determination
TS
Where
f = the number of mg of water (H2O) corresponding to 1 ml of water determination TS
f rsquo = the number of mg of water (H2O) in 1 ml of Water Methanol Standard Solution
713 General Remarks
Volumetric Karl Fischer titration requires the determination of the titre (t) of the Karl
Fischer reagent It is usually quoted in mg of water per ml of Karl Fischer reagent Modern
reagents allow direct titration of water in the sample The sample may be introduced
directly into the KF cell or after an extraction or dissolution with a suitable solvent The
water concentration of the solvent must be determined previously in order to be subtracted
from the sample analysis In Radiometer Analytical titrators all these operations are
simplified and the different results are accounted for automatically The volumetric titration
of water allows the analysis of water concentrations between 01 and 100 If an aliquot
contains less than 1 mg of water coulometric determination will result in a more accurate
result For reasons of precision the titre of the titrant should be chosen so that the titration is
completed with a titrant demand between 1 and 10 ml
The working medium (ie the solvent required) can be freely chosen by the user
depending on the dissolution properties of the sample to be investigated For methanol-
based reagents the stoichiometry 11 of the Karl Fischer reaction is only fulfilled if there is
more than 25 methanol in the reaction mixture A methanol-free working medium can be
used however it is important to determine the titre of the KF reagent in the same working
medium The modern solvents available today present a high buffer and dissolution
capacity These solvents consist of sulphur dioxide a base and methanol or ethanol The
main advantages of these solvents are
bull A more rapid titration due to better reaction kinetics an advantage especially for the
titration of large amounts of water
bull A better reproducibility because the reaction environment is stable The pH and the
sulphur dioxide concentration remain constant
Sheth M N Science College Patan Page no - 13
KARL - FISCHER Analysis Water Content Determination
For the titration of samples producing side reactions (aldehydes ketones and
silanols) it is necessary to use an appropriate solvent Most reagent manufacturers include
the letter K in the commercial name of such solvents
72 Coulometric titration
Apparatus
Usually the apparatus is comprised of an electrolytic cell for iodine production a stirrer a
titration flask and a potentiometric titration system at constant current The iodine
production device is composed of an anode and a cathode separated by a diaphragm The
anode is immersed in the anolyte solution for water determination and the cathode is
immersed in the catholyte solution for water determination Both electrodes are usually
made of platinum-mesh
Because water determination TS is extremely hygroscopic the titration apparatus
should be protected from atmospheric moisture For this purpose silica gel or calcium
chloride for water determination is usually used Procedure Take a suitable volume of an
anolyte for water determination in a titration vessel immerse in this solution a pair of
platinum electrodes for potentiometric titration at constant current
Then immerse the iodide production system filled with a catholyte for water
determination in the anolyte solution Switch on the electrolytic system and make the
content of the titration vessel anhydrous Next take an accurately weighed amount of the
sample containing 15 mg of water add it quickly to the vessel and dissolve by stirring
Perform the titration to the end point under vigorous stirring When the sample is insoluble
in the anolyte powder it quickly and add an accurately weighed amount of the sample to
the vessel After stirring the mixture for 5 to 30 minutes while protecting from atmospheric
moisture perform the titration with vigorous stirring Determine the quantity of electricity
(C) [electric current (A) and time (s)] required for the production of iodine during the
titration and calculate the content () of the water in the sample by the formula
When the sample interferes with the Karl Fisher reaction water in the sample can be
removed by heating under a stream of nitrogen gas and introduced into the titration vessel
by using a water-evaporation device
Sheth M N Science College Patan Page no - 14
KARL - FISCHER Analysis Water Content Determination
8 Interference and Limitations of Karl Fischer Method
Interferences in the water determinations by Karl Fischer method are associated with
condensation or oxidation ndash reduction reactions with a number of substances and
compounds or by side reactions Other possible interferences to the KF reagent are certain
active metals metal oxides metal hydroxides chromates melamines etc
The Karl Fischer titration has a number of serious limitations due to possible
interferences tantamount to erroneous results namely
(i) Oxidizing agents for instance chromates Cu(II) Fe(III) Cr2O7 2ndash peroxides salts
higher oxides
Example MnO2 + 4C5H5NH+ + 2Indash rarr Mn2+ + 4C5H5N + I2 + H2O
(ii) Reducing agents such as Sn (II) salts sulphides and S2O32ndash and
(iii) Compounds that have a tendency to form water with the ingredients of the Karl Fischer
reagent for instance
(a) Basic oxides eg ZnO
Example ZnO + 2C5H5NH+ rarr Zn2+ + C5H5N + H2O
(b) Salts of weak oxy-acids eg NaHCO3
Example NaHCO3 + C5H5NH+ rarr Na+ + H2O + CO2 + C5H5N
[Note As H2CO3 carbonic acid is very unstable hence it splits up to yield a mole each of
water and CO2]
Use of certain reagents will minimize or eliminate the interferences of many classes
of compounds For example the use of non-methanol containing KF reagent and solvent will
reduce the interference from aldehyde and ketones Ammonia and amines can be eliminated
by the addition of salicylic acid to the solvent
Sheth M N Science College Patan Page no - 15
KARL - FISCHER Analysis Water Content Determination
9 Side reactions
Side Reactions are classified as
(1) Reactions Influenced by Methanol
(2) Reactions with Iodine
(1) Reactions Influenced by Methanol
It includes
bull Aldehydes react to form acetals also undergo Bisulfite addition
bull Ketones react to form ketals
bull Amines undergo methylation
bull Siloxanes undergo esterification
(2) Reactions with Iodine
It includes
bull Halogenated Hydrocarbons containing free halogens
bull Mercaptans (Thiols)
bull Phenols
bull Peroxides
Testing for Iodine Reactivity
bull Dissolve a few Iodine Crystals in Methanol and Introduce Sample
bull Watch for Solution to Change Color if Clear Iodine Reaction is Present
Sheth M N Science College Patan Page no - 16
KARL - FISCHER Analysis Water Content Determination
1 Mercaptans (Thiols) are oxidized by iodine Suppress this side reaction by treating
sample with N-ethylmaleimide in a buffered solvent system
2 Inhibition of Thiol Oxidation by Iodine
Most Phenols do not react with KF reagents higher molecular weight phenols and
Amino-phenols are most problematic Volumetric method is most suitable since the
Oxidation potential is high in coulometry or Acidify Solvent system to reduce interference
Sheth M N Science College Patan Page no - 17
KARL - FISCHER Analysis Water Content Determination
10 Applications of Karl Fischer method for determination of Water in
pharmaceutical analysis
101 PREDNISOLONE SODIUM PHOSPHATE
Materials Required Karl Fischer Reagent 100 ml prednisolone sodium phosphate
02gm anhydrous methanol 200 ml
Procedure Add about 20 ml of anhydrous methanol to the titration vessel and titrate to the
Amperometric End-point with the Karl Fischer reagent Quickly add 02 g of prednisolone
sodium phosphate sample stir for 1 minute and again titrate to the amperometric end-point
with the Karl Fischer reagent The difference between the two titrations gives the volume
(v) of Karl Fischer reagent consumed by the sample
The minimum water equivalent is 35 mg of water per ml of Karl Fischer reagent
Hence the percentage of water ww in the given sample may be calculated by the
following
Precautions
(1) The reagents and solutions used must be kept anhydrous and necessary care should be
taken throughout to prevent exposure to atmospheric moisture
(2) The Karl Fischer reagent should be protected from light and preferably stored in a bottle
fitted with an automatic burette and
(3) The water equivalent of Karl Fischer reagent should always be determined before use
102 COGNATE ASSAYS
A number of other official pharmaceutical substances may be assayed for their water
content by the Karl Fischer method as summarized in the following Table
Sheth M N Science College Patan Page no - 18
KARL - FISCHER Analysis Water Content Determination
11 Automated Electrochemical Karl Fischer Analysis
Commercially available Modern KF-Titrators are usually equipped with specifically
designed titration vessels that are exclusively meant to check and prevent the contact with
atmospheric moisture Quite a few such devices are armed with microprocessors that will
perform the requisite operations sequentially in a programmed manner automatically and
may also dish out a print-out of the desired results including the percentage moisture
content In fact these Modern KF-Titrators not only afford greater accuracy and precision
in results but also offer much ease and convenience in routine analysis as compared to the
classical techniques based on either caulometry or controlled current potentiometry using
two indicator electrodes
Thus the basis of the analysis rests upon the quantitative relationship existing
between charge passed and iodine produced by the reagent according to the above reaction
Therefore the generation of iodine is automatically stopped when an excess of it is detected
by the indicator electrode It essentially consists of two platinum electrodes across which an
AC is applied and subsequently a marked drop in voltage between the electrodes takes place
as soon as an excess of iodine is present Normally such automated instruments make use of
proprietory reagents exclusively
The major advantage of this approach to KF-analysis being that no calibration is
required as the method is absolute and is entirely based on the stoichiometry of the
aforesaid equation It is noteworthy that one may determine the amounts of water ranging
between 10 mcg and 10 mg in solid as well as liquid samples
Sheth M N Science College Patan Page no - 19
KARL - FISCHER Analysis Water Content Determination
12 Some of the innovative brands in Karl Fischer Auto ndash titrator
instruments
Sheth M N Science College Patan Page no - 20
KARL - FISCHER Analysis Water Content Determination
Sheth M N Science College Patan Page no - 21
KARL - FISCHER Analysis Water Content Determination
13 References
1 Chapter 14 Karl fishcer method for determination of water Page no 223-227
Pharmaceutical Drug Analysis Ashutosh Kar New age international limited publishers
2 Aqua Stars Karl fischer instruments wwwemdchemicalscom
3 Karl Fischer Volumetric Titration Theory and Practice by Radiometer analytical
SAS wwwradiometer-analyticalcom
4 MettlerToledo AG Analytical Switzerland wwwmtcom
5 Summary of Karl fischer titrations by Orion products Baverly MA Thermofischer
Scientific wwwthermocomwater
6 Moisture determination by Karl Fischer titration background of chemistry and
recent advances SupelcoSigma-Aldrich Bellefonte Pa wwwsigma-aldrichcom
7 Karl Fischer Titration Methods Methodology and water determination in 442
different substances European Pharmacopoeia (PharmEurope) 4th Edition plus
Supplements 2002 International Electrotechnical Commission Geneva
8 Karl ndash fischer titration wwwwikipediacom
Sheth M N Science College Patan Page no - 22
KARL - FISCHER Analysis Water Content Determination
(Two compartment system possess separate solvent and titrating solutions)
Sheth M N Science College Patan Page no - 8
KARL - FISCHER Analysis Water Content Determination
5 Advantages
A Karl Fischer moisture determination is advantageous as compared to a
determination based on weight loss because KF is not affected by volatile compounds The
popularity of the Karl Fischer titration is due in large part to several practical advantages
that it holds over other methods of moisture determination including
i High accuracy and precision
ii Selectivity for water
iii Small sample quantities required
iv Easy sample preparation
v Short analysis duration
vi Nearly unlimited measuring range (1ppm to 100)
vii Suitability for analyzing Solids Liquids and Gases
viii Independence of presence of other volatiles
ix Suitability for automation
In contrast loss on drying will detect the loss of any volatile substance
Sheth M N Science College Patan Page no - 9
KARL - FISCHER Analysis Water Content Determination
6 Instrumentation
Following Figure illustrates a simple dead-stop end-point assembly or a Karl Fischer
titration apparatus The titration vessel is fitted with a pair of identical platinum electrodes
a mechanical stirrer with adjustable speed and a burette It will be observed that absolutely
little or no current may flow unless and until the solution is totally free from any polarizing
substances this could perhaps be due to the absorbed layers of oxygen and hydrogen on the
anode and cathode respectively
However the current shall flow only when the two electrodes get depolarized The
Karl Fischer reagent is pumped into the burette by means of hand bellows the access of
moisture is usually prevented by employing an appropriate arrangement of desiccant tubes
Alternatively the stirring may also be accomplished either by using a magnetic stirrer or by
means of suitably dried nitrogen passed gently through the solution during the course of
titration
The end-point is achieved by employing an eiectrical circuit comprising of a
microammeter (A) platinum electrodes together with a 15 V to 20 V battery connected
across a variable resistance of about 25 kΩ First of all the resistance is adjusted in such a
manner that an initial current passes through the platinum electrodes in series with a
microammeter (A) After each addition of reagent the pointer of the microammeter gets
deflected but quickly returns to its original position At the end of the reaction a deflection
is obtained which persists for 10-15 seconds
7 Water Determination Methodology
Sheth M N Science College Patan Page no - 10
KARL - FISCHER Analysis Water Content Determination
71 Volumetric titration method
Apparatus
Generally the apparatus consists of an automatic burette a backtitration flask a stirrer and
equipment for amperometric titration at constant voltage or potentiometric titration at
constant current
Because water determination TS is extremely hygroscopic the titration apparatus
should be protected from atmospheric moisture Silica gel or calcium chloride for water
determination is usually used for moisture protection
Procedure
As a rule the titration of the sample with water determination TS should be performed at
the same temperature as that at the standardization of the TS while protecting from
moisture
The apparatus is equipped with a variable resistor in the circuit and the resistor is
adjusted to apply a definite voltage (mV) between a pair of platinum electrodes immersed in
the solution to be titrated The change in current (1048707A) is measured during the dropping of
water determination TS (Amperometric titration at constant voltage) As titration continues
the abrupt change in current in the circuit occurs but returns to the original state within
several seconds At the end of a titration the change in current persists for a certain time
(usually longer than 30 seconds) The end point of titration is determined at this electric
state
Otherwise by adjusting the resistor a definite current is passed between the two
platinum electrodes and the change in potential (mV) is measured during dropping water
determination TS (Potentiometric titration at constant current) With the progress of
titration the value indicated by the potentiometer in the circuit decreases suddenly from a
polarization state of several hundreds (mV) to the nonpolarization state but it returns to the
original state within several seconds At the end of titration the non-polarization state
persists for a certain time (usually longer than 30 seconds) The end point of titration is
determined when this electric state attains
In the case of back titration when the amperometric titration method is used at
constant voltage the needle of microammeter is out of scale while an excessive quantity of
water determination TS remains It returns rapidly to the original position when the titration
reaches the end point
Sheth M N Science College Patan Page no - 11
KARL - FISCHER Analysis Water Content Determination
Similarly when the Potentiometric titration method at constant current is used the
needle of the millivoltmeter is at the original position while an excessive quantity of water
determination TS remains A definite voltage is applied when the titration reaches the end
point Unless otherwise specified the titration of water with water determination TS is
performed by either of the methods below Usually the end point of the titration can be
observed more clearly in the back titration method than in the direct titration method
711 Direct titration
Unless otherwise specified proceed as directed below Take 25ml of methanol for water
determination in a dried titration flask and titrate with water determination TS to the end
point Unless otherwise specified weigh accurately a quantity of the sample containing 10
to 50 mg of water transfer it quickly into the titration flask and dissolve by stirring Titrate
the solution with water determination TS to the end point under vigorous stirring When the
sample is insoluble in the solvent powder the sample quickly weigh a suitable amount of
the sample accurately and transfer it quickly into the titration vessel stir the mixture for 30
minutes while protecting it from moisture
Perform a titration under vigorous stirring When the sample interferes with the Karl
Fisher reaction water in the sample can be removed by heating and under a stream of
nitrogen gas and introduced into the titration vessel by using a water-evaporation device
712 Back titration
Unless otherwise specified proceed as directed below Take 20ml of methanol for water
determination in the dried titration vessel and titrate with water determination TS Weigh
accurately a suitable quantity of the sample containing 10 to 50 mg of water transfer the
sample quickly into the titration vessel add an excessive and definite volume of water
determination TS stir for 30 min protecting from atmospheric moisture and then titrate the
solution with Water Methanol Standard Solution under vigorous stirring
Sheth M N Science College Patan Page no - 12
KARL - FISCHER Analysis Water Content Determination
Wheref = the number of mg of water (H2O) corresponding to 1 ml of water determination
TS
Where
f = the number of mg of water (H2O) corresponding to 1 ml of water determination TS
f rsquo = the number of mg of water (H2O) in 1 ml of Water Methanol Standard Solution
713 General Remarks
Volumetric Karl Fischer titration requires the determination of the titre (t) of the Karl
Fischer reagent It is usually quoted in mg of water per ml of Karl Fischer reagent Modern
reagents allow direct titration of water in the sample The sample may be introduced
directly into the KF cell or after an extraction or dissolution with a suitable solvent The
water concentration of the solvent must be determined previously in order to be subtracted
from the sample analysis In Radiometer Analytical titrators all these operations are
simplified and the different results are accounted for automatically The volumetric titration
of water allows the analysis of water concentrations between 01 and 100 If an aliquot
contains less than 1 mg of water coulometric determination will result in a more accurate
result For reasons of precision the titre of the titrant should be chosen so that the titration is
completed with a titrant demand between 1 and 10 ml
The working medium (ie the solvent required) can be freely chosen by the user
depending on the dissolution properties of the sample to be investigated For methanol-
based reagents the stoichiometry 11 of the Karl Fischer reaction is only fulfilled if there is
more than 25 methanol in the reaction mixture A methanol-free working medium can be
used however it is important to determine the titre of the KF reagent in the same working
medium The modern solvents available today present a high buffer and dissolution
capacity These solvents consist of sulphur dioxide a base and methanol or ethanol The
main advantages of these solvents are
bull A more rapid titration due to better reaction kinetics an advantage especially for the
titration of large amounts of water
bull A better reproducibility because the reaction environment is stable The pH and the
sulphur dioxide concentration remain constant
Sheth M N Science College Patan Page no - 13
KARL - FISCHER Analysis Water Content Determination
For the titration of samples producing side reactions (aldehydes ketones and
silanols) it is necessary to use an appropriate solvent Most reagent manufacturers include
the letter K in the commercial name of such solvents
72 Coulometric titration
Apparatus
Usually the apparatus is comprised of an electrolytic cell for iodine production a stirrer a
titration flask and a potentiometric titration system at constant current The iodine
production device is composed of an anode and a cathode separated by a diaphragm The
anode is immersed in the anolyte solution for water determination and the cathode is
immersed in the catholyte solution for water determination Both electrodes are usually
made of platinum-mesh
Because water determination TS is extremely hygroscopic the titration apparatus
should be protected from atmospheric moisture For this purpose silica gel or calcium
chloride for water determination is usually used Procedure Take a suitable volume of an
anolyte for water determination in a titration vessel immerse in this solution a pair of
platinum electrodes for potentiometric titration at constant current
Then immerse the iodide production system filled with a catholyte for water
determination in the anolyte solution Switch on the electrolytic system and make the
content of the titration vessel anhydrous Next take an accurately weighed amount of the
sample containing 15 mg of water add it quickly to the vessel and dissolve by stirring
Perform the titration to the end point under vigorous stirring When the sample is insoluble
in the anolyte powder it quickly and add an accurately weighed amount of the sample to
the vessel After stirring the mixture for 5 to 30 minutes while protecting from atmospheric
moisture perform the titration with vigorous stirring Determine the quantity of electricity
(C) [electric current (A) and time (s)] required for the production of iodine during the
titration and calculate the content () of the water in the sample by the formula
When the sample interferes with the Karl Fisher reaction water in the sample can be
removed by heating under a stream of nitrogen gas and introduced into the titration vessel
by using a water-evaporation device
Sheth M N Science College Patan Page no - 14
KARL - FISCHER Analysis Water Content Determination
8 Interference and Limitations of Karl Fischer Method
Interferences in the water determinations by Karl Fischer method are associated with
condensation or oxidation ndash reduction reactions with a number of substances and
compounds or by side reactions Other possible interferences to the KF reagent are certain
active metals metal oxides metal hydroxides chromates melamines etc
The Karl Fischer titration has a number of serious limitations due to possible
interferences tantamount to erroneous results namely
(i) Oxidizing agents for instance chromates Cu(II) Fe(III) Cr2O7 2ndash peroxides salts
higher oxides
Example MnO2 + 4C5H5NH+ + 2Indash rarr Mn2+ + 4C5H5N + I2 + H2O
(ii) Reducing agents such as Sn (II) salts sulphides and S2O32ndash and
(iii) Compounds that have a tendency to form water with the ingredients of the Karl Fischer
reagent for instance
(a) Basic oxides eg ZnO
Example ZnO + 2C5H5NH+ rarr Zn2+ + C5H5N + H2O
(b) Salts of weak oxy-acids eg NaHCO3
Example NaHCO3 + C5H5NH+ rarr Na+ + H2O + CO2 + C5H5N
[Note As H2CO3 carbonic acid is very unstable hence it splits up to yield a mole each of
water and CO2]
Use of certain reagents will minimize or eliminate the interferences of many classes
of compounds For example the use of non-methanol containing KF reagent and solvent will
reduce the interference from aldehyde and ketones Ammonia and amines can be eliminated
by the addition of salicylic acid to the solvent
Sheth M N Science College Patan Page no - 15
KARL - FISCHER Analysis Water Content Determination
9 Side reactions
Side Reactions are classified as
(1) Reactions Influenced by Methanol
(2) Reactions with Iodine
(1) Reactions Influenced by Methanol
It includes
bull Aldehydes react to form acetals also undergo Bisulfite addition
bull Ketones react to form ketals
bull Amines undergo methylation
bull Siloxanes undergo esterification
(2) Reactions with Iodine
It includes
bull Halogenated Hydrocarbons containing free halogens
bull Mercaptans (Thiols)
bull Phenols
bull Peroxides
Testing for Iodine Reactivity
bull Dissolve a few Iodine Crystals in Methanol and Introduce Sample
bull Watch for Solution to Change Color if Clear Iodine Reaction is Present
Sheth M N Science College Patan Page no - 16
KARL - FISCHER Analysis Water Content Determination
1 Mercaptans (Thiols) are oxidized by iodine Suppress this side reaction by treating
sample with N-ethylmaleimide in a buffered solvent system
2 Inhibition of Thiol Oxidation by Iodine
Most Phenols do not react with KF reagents higher molecular weight phenols and
Amino-phenols are most problematic Volumetric method is most suitable since the
Oxidation potential is high in coulometry or Acidify Solvent system to reduce interference
Sheth M N Science College Patan Page no - 17
KARL - FISCHER Analysis Water Content Determination
10 Applications of Karl Fischer method for determination of Water in
pharmaceutical analysis
101 PREDNISOLONE SODIUM PHOSPHATE
Materials Required Karl Fischer Reagent 100 ml prednisolone sodium phosphate
02gm anhydrous methanol 200 ml
Procedure Add about 20 ml of anhydrous methanol to the titration vessel and titrate to the
Amperometric End-point with the Karl Fischer reagent Quickly add 02 g of prednisolone
sodium phosphate sample stir for 1 minute and again titrate to the amperometric end-point
with the Karl Fischer reagent The difference between the two titrations gives the volume
(v) of Karl Fischer reagent consumed by the sample
The minimum water equivalent is 35 mg of water per ml of Karl Fischer reagent
Hence the percentage of water ww in the given sample may be calculated by the
following
Precautions
(1) The reagents and solutions used must be kept anhydrous and necessary care should be
taken throughout to prevent exposure to atmospheric moisture
(2) The Karl Fischer reagent should be protected from light and preferably stored in a bottle
fitted with an automatic burette and
(3) The water equivalent of Karl Fischer reagent should always be determined before use
102 COGNATE ASSAYS
A number of other official pharmaceutical substances may be assayed for their water
content by the Karl Fischer method as summarized in the following Table
Sheth M N Science College Patan Page no - 18
KARL - FISCHER Analysis Water Content Determination
11 Automated Electrochemical Karl Fischer Analysis
Commercially available Modern KF-Titrators are usually equipped with specifically
designed titration vessels that are exclusively meant to check and prevent the contact with
atmospheric moisture Quite a few such devices are armed with microprocessors that will
perform the requisite operations sequentially in a programmed manner automatically and
may also dish out a print-out of the desired results including the percentage moisture
content In fact these Modern KF-Titrators not only afford greater accuracy and precision
in results but also offer much ease and convenience in routine analysis as compared to the
classical techniques based on either caulometry or controlled current potentiometry using
two indicator electrodes
Thus the basis of the analysis rests upon the quantitative relationship existing
between charge passed and iodine produced by the reagent according to the above reaction
Therefore the generation of iodine is automatically stopped when an excess of it is detected
by the indicator electrode It essentially consists of two platinum electrodes across which an
AC is applied and subsequently a marked drop in voltage between the electrodes takes place
as soon as an excess of iodine is present Normally such automated instruments make use of
proprietory reagents exclusively
The major advantage of this approach to KF-analysis being that no calibration is
required as the method is absolute and is entirely based on the stoichiometry of the
aforesaid equation It is noteworthy that one may determine the amounts of water ranging
between 10 mcg and 10 mg in solid as well as liquid samples
Sheth M N Science College Patan Page no - 19
KARL - FISCHER Analysis Water Content Determination
12 Some of the innovative brands in Karl Fischer Auto ndash titrator
instruments
Sheth M N Science College Patan Page no - 20
KARL - FISCHER Analysis Water Content Determination
Sheth M N Science College Patan Page no - 21
KARL - FISCHER Analysis Water Content Determination
13 References
1 Chapter 14 Karl fishcer method for determination of water Page no 223-227
Pharmaceutical Drug Analysis Ashutosh Kar New age international limited publishers
2 Aqua Stars Karl fischer instruments wwwemdchemicalscom
3 Karl Fischer Volumetric Titration Theory and Practice by Radiometer analytical
SAS wwwradiometer-analyticalcom
4 MettlerToledo AG Analytical Switzerland wwwmtcom
5 Summary of Karl fischer titrations by Orion products Baverly MA Thermofischer
Scientific wwwthermocomwater
6 Moisture determination by Karl Fischer titration background of chemistry and
recent advances SupelcoSigma-Aldrich Bellefonte Pa wwwsigma-aldrichcom
7 Karl Fischer Titration Methods Methodology and water determination in 442
different substances European Pharmacopoeia (PharmEurope) 4th Edition plus
Supplements 2002 International Electrotechnical Commission Geneva
8 Karl ndash fischer titration wwwwikipediacom
Sheth M N Science College Patan Page no - 22
KARL - FISCHER Analysis Water Content Determination
5 Advantages
A Karl Fischer moisture determination is advantageous as compared to a
determination based on weight loss because KF is not affected by volatile compounds The
popularity of the Karl Fischer titration is due in large part to several practical advantages
that it holds over other methods of moisture determination including
i High accuracy and precision
ii Selectivity for water
iii Small sample quantities required
iv Easy sample preparation
v Short analysis duration
vi Nearly unlimited measuring range (1ppm to 100)
vii Suitability for analyzing Solids Liquids and Gases
viii Independence of presence of other volatiles
ix Suitability for automation
In contrast loss on drying will detect the loss of any volatile substance
Sheth M N Science College Patan Page no - 9
KARL - FISCHER Analysis Water Content Determination
6 Instrumentation
Following Figure illustrates a simple dead-stop end-point assembly or a Karl Fischer
titration apparatus The titration vessel is fitted with a pair of identical platinum electrodes
a mechanical stirrer with adjustable speed and a burette It will be observed that absolutely
little or no current may flow unless and until the solution is totally free from any polarizing
substances this could perhaps be due to the absorbed layers of oxygen and hydrogen on the
anode and cathode respectively
However the current shall flow only when the two electrodes get depolarized The
Karl Fischer reagent is pumped into the burette by means of hand bellows the access of
moisture is usually prevented by employing an appropriate arrangement of desiccant tubes
Alternatively the stirring may also be accomplished either by using a magnetic stirrer or by
means of suitably dried nitrogen passed gently through the solution during the course of
titration
The end-point is achieved by employing an eiectrical circuit comprising of a
microammeter (A) platinum electrodes together with a 15 V to 20 V battery connected
across a variable resistance of about 25 kΩ First of all the resistance is adjusted in such a
manner that an initial current passes through the platinum electrodes in series with a
microammeter (A) After each addition of reagent the pointer of the microammeter gets
deflected but quickly returns to its original position At the end of the reaction a deflection
is obtained which persists for 10-15 seconds
7 Water Determination Methodology
Sheth M N Science College Patan Page no - 10
KARL - FISCHER Analysis Water Content Determination
71 Volumetric titration method
Apparatus
Generally the apparatus consists of an automatic burette a backtitration flask a stirrer and
equipment for amperometric titration at constant voltage or potentiometric titration at
constant current
Because water determination TS is extremely hygroscopic the titration apparatus
should be protected from atmospheric moisture Silica gel or calcium chloride for water
determination is usually used for moisture protection
Procedure
As a rule the titration of the sample with water determination TS should be performed at
the same temperature as that at the standardization of the TS while protecting from
moisture
The apparatus is equipped with a variable resistor in the circuit and the resistor is
adjusted to apply a definite voltage (mV) between a pair of platinum electrodes immersed in
the solution to be titrated The change in current (1048707A) is measured during the dropping of
water determination TS (Amperometric titration at constant voltage) As titration continues
the abrupt change in current in the circuit occurs but returns to the original state within
several seconds At the end of a titration the change in current persists for a certain time
(usually longer than 30 seconds) The end point of titration is determined at this electric
state
Otherwise by adjusting the resistor a definite current is passed between the two
platinum electrodes and the change in potential (mV) is measured during dropping water
determination TS (Potentiometric titration at constant current) With the progress of
titration the value indicated by the potentiometer in the circuit decreases suddenly from a
polarization state of several hundreds (mV) to the nonpolarization state but it returns to the
original state within several seconds At the end of titration the non-polarization state
persists for a certain time (usually longer than 30 seconds) The end point of titration is
determined when this electric state attains
In the case of back titration when the amperometric titration method is used at
constant voltage the needle of microammeter is out of scale while an excessive quantity of
water determination TS remains It returns rapidly to the original position when the titration
reaches the end point
Sheth M N Science College Patan Page no - 11
KARL - FISCHER Analysis Water Content Determination
Similarly when the Potentiometric titration method at constant current is used the
needle of the millivoltmeter is at the original position while an excessive quantity of water
determination TS remains A definite voltage is applied when the titration reaches the end
point Unless otherwise specified the titration of water with water determination TS is
performed by either of the methods below Usually the end point of the titration can be
observed more clearly in the back titration method than in the direct titration method
711 Direct titration
Unless otherwise specified proceed as directed below Take 25ml of methanol for water
determination in a dried titration flask and titrate with water determination TS to the end
point Unless otherwise specified weigh accurately a quantity of the sample containing 10
to 50 mg of water transfer it quickly into the titration flask and dissolve by stirring Titrate
the solution with water determination TS to the end point under vigorous stirring When the
sample is insoluble in the solvent powder the sample quickly weigh a suitable amount of
the sample accurately and transfer it quickly into the titration vessel stir the mixture for 30
minutes while protecting it from moisture
Perform a titration under vigorous stirring When the sample interferes with the Karl
Fisher reaction water in the sample can be removed by heating and under a stream of
nitrogen gas and introduced into the titration vessel by using a water-evaporation device
712 Back titration
Unless otherwise specified proceed as directed below Take 20ml of methanol for water
determination in the dried titration vessel and titrate with water determination TS Weigh
accurately a suitable quantity of the sample containing 10 to 50 mg of water transfer the
sample quickly into the titration vessel add an excessive and definite volume of water
determination TS stir for 30 min protecting from atmospheric moisture and then titrate the
solution with Water Methanol Standard Solution under vigorous stirring
Sheth M N Science College Patan Page no - 12
KARL - FISCHER Analysis Water Content Determination
Wheref = the number of mg of water (H2O) corresponding to 1 ml of water determination
TS
Where
f = the number of mg of water (H2O) corresponding to 1 ml of water determination TS
f rsquo = the number of mg of water (H2O) in 1 ml of Water Methanol Standard Solution
713 General Remarks
Volumetric Karl Fischer titration requires the determination of the titre (t) of the Karl
Fischer reagent It is usually quoted in mg of water per ml of Karl Fischer reagent Modern
reagents allow direct titration of water in the sample The sample may be introduced
directly into the KF cell or after an extraction or dissolution with a suitable solvent The
water concentration of the solvent must be determined previously in order to be subtracted
from the sample analysis In Radiometer Analytical titrators all these operations are
simplified and the different results are accounted for automatically The volumetric titration
of water allows the analysis of water concentrations between 01 and 100 If an aliquot
contains less than 1 mg of water coulometric determination will result in a more accurate
result For reasons of precision the titre of the titrant should be chosen so that the titration is
completed with a titrant demand between 1 and 10 ml
The working medium (ie the solvent required) can be freely chosen by the user
depending on the dissolution properties of the sample to be investigated For methanol-
based reagents the stoichiometry 11 of the Karl Fischer reaction is only fulfilled if there is
more than 25 methanol in the reaction mixture A methanol-free working medium can be
used however it is important to determine the titre of the KF reagent in the same working
medium The modern solvents available today present a high buffer and dissolution
capacity These solvents consist of sulphur dioxide a base and methanol or ethanol The
main advantages of these solvents are
bull A more rapid titration due to better reaction kinetics an advantage especially for the
titration of large amounts of water
bull A better reproducibility because the reaction environment is stable The pH and the
sulphur dioxide concentration remain constant
Sheth M N Science College Patan Page no - 13
KARL - FISCHER Analysis Water Content Determination
For the titration of samples producing side reactions (aldehydes ketones and
silanols) it is necessary to use an appropriate solvent Most reagent manufacturers include
the letter K in the commercial name of such solvents
72 Coulometric titration
Apparatus
Usually the apparatus is comprised of an electrolytic cell for iodine production a stirrer a
titration flask and a potentiometric titration system at constant current The iodine
production device is composed of an anode and a cathode separated by a diaphragm The
anode is immersed in the anolyte solution for water determination and the cathode is
immersed in the catholyte solution for water determination Both electrodes are usually
made of platinum-mesh
Because water determination TS is extremely hygroscopic the titration apparatus
should be protected from atmospheric moisture For this purpose silica gel or calcium
chloride for water determination is usually used Procedure Take a suitable volume of an
anolyte for water determination in a titration vessel immerse in this solution a pair of
platinum electrodes for potentiometric titration at constant current
Then immerse the iodide production system filled with a catholyte for water
determination in the anolyte solution Switch on the electrolytic system and make the
content of the titration vessel anhydrous Next take an accurately weighed amount of the
sample containing 15 mg of water add it quickly to the vessel and dissolve by stirring
Perform the titration to the end point under vigorous stirring When the sample is insoluble
in the anolyte powder it quickly and add an accurately weighed amount of the sample to
the vessel After stirring the mixture for 5 to 30 minutes while protecting from atmospheric
moisture perform the titration with vigorous stirring Determine the quantity of electricity
(C) [electric current (A) and time (s)] required for the production of iodine during the
titration and calculate the content () of the water in the sample by the formula
When the sample interferes with the Karl Fisher reaction water in the sample can be
removed by heating under a stream of nitrogen gas and introduced into the titration vessel
by using a water-evaporation device
Sheth M N Science College Patan Page no - 14
KARL - FISCHER Analysis Water Content Determination
8 Interference and Limitations of Karl Fischer Method
Interferences in the water determinations by Karl Fischer method are associated with
condensation or oxidation ndash reduction reactions with a number of substances and
compounds or by side reactions Other possible interferences to the KF reagent are certain
active metals metal oxides metal hydroxides chromates melamines etc
The Karl Fischer titration has a number of serious limitations due to possible
interferences tantamount to erroneous results namely
(i) Oxidizing agents for instance chromates Cu(II) Fe(III) Cr2O7 2ndash peroxides salts
higher oxides
Example MnO2 + 4C5H5NH+ + 2Indash rarr Mn2+ + 4C5H5N + I2 + H2O
(ii) Reducing agents such as Sn (II) salts sulphides and S2O32ndash and
(iii) Compounds that have a tendency to form water with the ingredients of the Karl Fischer
reagent for instance
(a) Basic oxides eg ZnO
Example ZnO + 2C5H5NH+ rarr Zn2+ + C5H5N + H2O
(b) Salts of weak oxy-acids eg NaHCO3
Example NaHCO3 + C5H5NH+ rarr Na+ + H2O + CO2 + C5H5N
[Note As H2CO3 carbonic acid is very unstable hence it splits up to yield a mole each of
water and CO2]
Use of certain reagents will minimize or eliminate the interferences of many classes
of compounds For example the use of non-methanol containing KF reagent and solvent will
reduce the interference from aldehyde and ketones Ammonia and amines can be eliminated
by the addition of salicylic acid to the solvent
Sheth M N Science College Patan Page no - 15
KARL - FISCHER Analysis Water Content Determination
9 Side reactions
Side Reactions are classified as
(1) Reactions Influenced by Methanol
(2) Reactions with Iodine
(1) Reactions Influenced by Methanol
It includes
bull Aldehydes react to form acetals also undergo Bisulfite addition
bull Ketones react to form ketals
bull Amines undergo methylation
bull Siloxanes undergo esterification
(2) Reactions with Iodine
It includes
bull Halogenated Hydrocarbons containing free halogens
bull Mercaptans (Thiols)
bull Phenols
bull Peroxides
Testing for Iodine Reactivity
bull Dissolve a few Iodine Crystals in Methanol and Introduce Sample
bull Watch for Solution to Change Color if Clear Iodine Reaction is Present
Sheth M N Science College Patan Page no - 16
KARL - FISCHER Analysis Water Content Determination
1 Mercaptans (Thiols) are oxidized by iodine Suppress this side reaction by treating
sample with N-ethylmaleimide in a buffered solvent system
2 Inhibition of Thiol Oxidation by Iodine
Most Phenols do not react with KF reagents higher molecular weight phenols and
Amino-phenols are most problematic Volumetric method is most suitable since the
Oxidation potential is high in coulometry or Acidify Solvent system to reduce interference
Sheth M N Science College Patan Page no - 17
KARL - FISCHER Analysis Water Content Determination
10 Applications of Karl Fischer method for determination of Water in
pharmaceutical analysis
101 PREDNISOLONE SODIUM PHOSPHATE
Materials Required Karl Fischer Reagent 100 ml prednisolone sodium phosphate
02gm anhydrous methanol 200 ml
Procedure Add about 20 ml of anhydrous methanol to the titration vessel and titrate to the
Amperometric End-point with the Karl Fischer reagent Quickly add 02 g of prednisolone
sodium phosphate sample stir for 1 minute and again titrate to the amperometric end-point
with the Karl Fischer reagent The difference between the two titrations gives the volume
(v) of Karl Fischer reagent consumed by the sample
The minimum water equivalent is 35 mg of water per ml of Karl Fischer reagent
Hence the percentage of water ww in the given sample may be calculated by the
following
Precautions
(1) The reagents and solutions used must be kept anhydrous and necessary care should be
taken throughout to prevent exposure to atmospheric moisture
(2) The Karl Fischer reagent should be protected from light and preferably stored in a bottle
fitted with an automatic burette and
(3) The water equivalent of Karl Fischer reagent should always be determined before use
102 COGNATE ASSAYS
A number of other official pharmaceutical substances may be assayed for their water
content by the Karl Fischer method as summarized in the following Table
Sheth M N Science College Patan Page no - 18
KARL - FISCHER Analysis Water Content Determination
11 Automated Electrochemical Karl Fischer Analysis
Commercially available Modern KF-Titrators are usually equipped with specifically
designed titration vessels that are exclusively meant to check and prevent the contact with
atmospheric moisture Quite a few such devices are armed with microprocessors that will
perform the requisite operations sequentially in a programmed manner automatically and
may also dish out a print-out of the desired results including the percentage moisture
content In fact these Modern KF-Titrators not only afford greater accuracy and precision
in results but also offer much ease and convenience in routine analysis as compared to the
classical techniques based on either caulometry or controlled current potentiometry using
two indicator electrodes
Thus the basis of the analysis rests upon the quantitative relationship existing
between charge passed and iodine produced by the reagent according to the above reaction
Therefore the generation of iodine is automatically stopped when an excess of it is detected
by the indicator electrode It essentially consists of two platinum electrodes across which an
AC is applied and subsequently a marked drop in voltage between the electrodes takes place
as soon as an excess of iodine is present Normally such automated instruments make use of
proprietory reagents exclusively
The major advantage of this approach to KF-analysis being that no calibration is
required as the method is absolute and is entirely based on the stoichiometry of the
aforesaid equation It is noteworthy that one may determine the amounts of water ranging
between 10 mcg and 10 mg in solid as well as liquid samples
Sheth M N Science College Patan Page no - 19
KARL - FISCHER Analysis Water Content Determination
12 Some of the innovative brands in Karl Fischer Auto ndash titrator
instruments
Sheth M N Science College Patan Page no - 20
KARL - FISCHER Analysis Water Content Determination
Sheth M N Science College Patan Page no - 21
KARL - FISCHER Analysis Water Content Determination
13 References
1 Chapter 14 Karl fishcer method for determination of water Page no 223-227
Pharmaceutical Drug Analysis Ashutosh Kar New age international limited publishers
2 Aqua Stars Karl fischer instruments wwwemdchemicalscom
3 Karl Fischer Volumetric Titration Theory and Practice by Radiometer analytical
SAS wwwradiometer-analyticalcom
4 MettlerToledo AG Analytical Switzerland wwwmtcom
5 Summary of Karl fischer titrations by Orion products Baverly MA Thermofischer
Scientific wwwthermocomwater
6 Moisture determination by Karl Fischer titration background of chemistry and
recent advances SupelcoSigma-Aldrich Bellefonte Pa wwwsigma-aldrichcom
7 Karl Fischer Titration Methods Methodology and water determination in 442
different substances European Pharmacopoeia (PharmEurope) 4th Edition plus
Supplements 2002 International Electrotechnical Commission Geneva
8 Karl ndash fischer titration wwwwikipediacom
Sheth M N Science College Patan Page no - 22
KARL - FISCHER Analysis Water Content Determination
6 Instrumentation
Following Figure illustrates a simple dead-stop end-point assembly or a Karl Fischer
titration apparatus The titration vessel is fitted with a pair of identical platinum electrodes
a mechanical stirrer with adjustable speed and a burette It will be observed that absolutely
little or no current may flow unless and until the solution is totally free from any polarizing
substances this could perhaps be due to the absorbed layers of oxygen and hydrogen on the
anode and cathode respectively
However the current shall flow only when the two electrodes get depolarized The
Karl Fischer reagent is pumped into the burette by means of hand bellows the access of
moisture is usually prevented by employing an appropriate arrangement of desiccant tubes
Alternatively the stirring may also be accomplished either by using a magnetic stirrer or by
means of suitably dried nitrogen passed gently through the solution during the course of
titration
The end-point is achieved by employing an eiectrical circuit comprising of a
microammeter (A) platinum electrodes together with a 15 V to 20 V battery connected
across a variable resistance of about 25 kΩ First of all the resistance is adjusted in such a
manner that an initial current passes through the platinum electrodes in series with a
microammeter (A) After each addition of reagent the pointer of the microammeter gets
deflected but quickly returns to its original position At the end of the reaction a deflection
is obtained which persists for 10-15 seconds
7 Water Determination Methodology
Sheth M N Science College Patan Page no - 10
KARL - FISCHER Analysis Water Content Determination
71 Volumetric titration method
Apparatus
Generally the apparatus consists of an automatic burette a backtitration flask a stirrer and
equipment for amperometric titration at constant voltage or potentiometric titration at
constant current
Because water determination TS is extremely hygroscopic the titration apparatus
should be protected from atmospheric moisture Silica gel or calcium chloride for water
determination is usually used for moisture protection
Procedure
As a rule the titration of the sample with water determination TS should be performed at
the same temperature as that at the standardization of the TS while protecting from
moisture
The apparatus is equipped with a variable resistor in the circuit and the resistor is
adjusted to apply a definite voltage (mV) between a pair of platinum electrodes immersed in
the solution to be titrated The change in current (1048707A) is measured during the dropping of
water determination TS (Amperometric titration at constant voltage) As titration continues
the abrupt change in current in the circuit occurs but returns to the original state within
several seconds At the end of a titration the change in current persists for a certain time
(usually longer than 30 seconds) The end point of titration is determined at this electric
state
Otherwise by adjusting the resistor a definite current is passed between the two
platinum electrodes and the change in potential (mV) is measured during dropping water
determination TS (Potentiometric titration at constant current) With the progress of
titration the value indicated by the potentiometer in the circuit decreases suddenly from a
polarization state of several hundreds (mV) to the nonpolarization state but it returns to the
original state within several seconds At the end of titration the non-polarization state
persists for a certain time (usually longer than 30 seconds) The end point of titration is
determined when this electric state attains
In the case of back titration when the amperometric titration method is used at
constant voltage the needle of microammeter is out of scale while an excessive quantity of
water determination TS remains It returns rapidly to the original position when the titration
reaches the end point
Sheth M N Science College Patan Page no - 11
KARL - FISCHER Analysis Water Content Determination
Similarly when the Potentiometric titration method at constant current is used the
needle of the millivoltmeter is at the original position while an excessive quantity of water
determination TS remains A definite voltage is applied when the titration reaches the end
point Unless otherwise specified the titration of water with water determination TS is
performed by either of the methods below Usually the end point of the titration can be
observed more clearly in the back titration method than in the direct titration method
711 Direct titration
Unless otherwise specified proceed as directed below Take 25ml of methanol for water
determination in a dried titration flask and titrate with water determination TS to the end
point Unless otherwise specified weigh accurately a quantity of the sample containing 10
to 50 mg of water transfer it quickly into the titration flask and dissolve by stirring Titrate
the solution with water determination TS to the end point under vigorous stirring When the
sample is insoluble in the solvent powder the sample quickly weigh a suitable amount of
the sample accurately and transfer it quickly into the titration vessel stir the mixture for 30
minutes while protecting it from moisture
Perform a titration under vigorous stirring When the sample interferes with the Karl
Fisher reaction water in the sample can be removed by heating and under a stream of
nitrogen gas and introduced into the titration vessel by using a water-evaporation device
712 Back titration
Unless otherwise specified proceed as directed below Take 20ml of methanol for water
determination in the dried titration vessel and titrate with water determination TS Weigh
accurately a suitable quantity of the sample containing 10 to 50 mg of water transfer the
sample quickly into the titration vessel add an excessive and definite volume of water
determination TS stir for 30 min protecting from atmospheric moisture and then titrate the
solution with Water Methanol Standard Solution under vigorous stirring
Sheth M N Science College Patan Page no - 12
KARL - FISCHER Analysis Water Content Determination
Wheref = the number of mg of water (H2O) corresponding to 1 ml of water determination
TS
Where
f = the number of mg of water (H2O) corresponding to 1 ml of water determination TS
f rsquo = the number of mg of water (H2O) in 1 ml of Water Methanol Standard Solution
713 General Remarks
Volumetric Karl Fischer titration requires the determination of the titre (t) of the Karl
Fischer reagent It is usually quoted in mg of water per ml of Karl Fischer reagent Modern
reagents allow direct titration of water in the sample The sample may be introduced
directly into the KF cell or after an extraction or dissolution with a suitable solvent The
water concentration of the solvent must be determined previously in order to be subtracted
from the sample analysis In Radiometer Analytical titrators all these operations are
simplified and the different results are accounted for automatically The volumetric titration
of water allows the analysis of water concentrations between 01 and 100 If an aliquot
contains less than 1 mg of water coulometric determination will result in a more accurate
result For reasons of precision the titre of the titrant should be chosen so that the titration is
completed with a titrant demand between 1 and 10 ml
The working medium (ie the solvent required) can be freely chosen by the user
depending on the dissolution properties of the sample to be investigated For methanol-
based reagents the stoichiometry 11 of the Karl Fischer reaction is only fulfilled if there is
more than 25 methanol in the reaction mixture A methanol-free working medium can be
used however it is important to determine the titre of the KF reagent in the same working
medium The modern solvents available today present a high buffer and dissolution
capacity These solvents consist of sulphur dioxide a base and methanol or ethanol The
main advantages of these solvents are
bull A more rapid titration due to better reaction kinetics an advantage especially for the
titration of large amounts of water
bull A better reproducibility because the reaction environment is stable The pH and the
sulphur dioxide concentration remain constant
Sheth M N Science College Patan Page no - 13
KARL - FISCHER Analysis Water Content Determination
For the titration of samples producing side reactions (aldehydes ketones and
silanols) it is necessary to use an appropriate solvent Most reagent manufacturers include
the letter K in the commercial name of such solvents
72 Coulometric titration
Apparatus
Usually the apparatus is comprised of an electrolytic cell for iodine production a stirrer a
titration flask and a potentiometric titration system at constant current The iodine
production device is composed of an anode and a cathode separated by a diaphragm The
anode is immersed in the anolyte solution for water determination and the cathode is
immersed in the catholyte solution for water determination Both electrodes are usually
made of platinum-mesh
Because water determination TS is extremely hygroscopic the titration apparatus
should be protected from atmospheric moisture For this purpose silica gel or calcium
chloride for water determination is usually used Procedure Take a suitable volume of an
anolyte for water determination in a titration vessel immerse in this solution a pair of
platinum electrodes for potentiometric titration at constant current
Then immerse the iodide production system filled with a catholyte for water
determination in the anolyte solution Switch on the electrolytic system and make the
content of the titration vessel anhydrous Next take an accurately weighed amount of the
sample containing 15 mg of water add it quickly to the vessel and dissolve by stirring
Perform the titration to the end point under vigorous stirring When the sample is insoluble
in the anolyte powder it quickly and add an accurately weighed amount of the sample to
the vessel After stirring the mixture for 5 to 30 minutes while protecting from atmospheric
moisture perform the titration with vigorous stirring Determine the quantity of electricity
(C) [electric current (A) and time (s)] required for the production of iodine during the
titration and calculate the content () of the water in the sample by the formula
When the sample interferes with the Karl Fisher reaction water in the sample can be
removed by heating under a stream of nitrogen gas and introduced into the titration vessel
by using a water-evaporation device
Sheth M N Science College Patan Page no - 14
KARL - FISCHER Analysis Water Content Determination
8 Interference and Limitations of Karl Fischer Method
Interferences in the water determinations by Karl Fischer method are associated with
condensation or oxidation ndash reduction reactions with a number of substances and
compounds or by side reactions Other possible interferences to the KF reagent are certain
active metals metal oxides metal hydroxides chromates melamines etc
The Karl Fischer titration has a number of serious limitations due to possible
interferences tantamount to erroneous results namely
(i) Oxidizing agents for instance chromates Cu(II) Fe(III) Cr2O7 2ndash peroxides salts
higher oxides
Example MnO2 + 4C5H5NH+ + 2Indash rarr Mn2+ + 4C5H5N + I2 + H2O
(ii) Reducing agents such as Sn (II) salts sulphides and S2O32ndash and
(iii) Compounds that have a tendency to form water with the ingredients of the Karl Fischer
reagent for instance
(a) Basic oxides eg ZnO
Example ZnO + 2C5H5NH+ rarr Zn2+ + C5H5N + H2O
(b) Salts of weak oxy-acids eg NaHCO3
Example NaHCO3 + C5H5NH+ rarr Na+ + H2O + CO2 + C5H5N
[Note As H2CO3 carbonic acid is very unstable hence it splits up to yield a mole each of
water and CO2]
Use of certain reagents will minimize or eliminate the interferences of many classes
of compounds For example the use of non-methanol containing KF reagent and solvent will
reduce the interference from aldehyde and ketones Ammonia and amines can be eliminated
by the addition of salicylic acid to the solvent
Sheth M N Science College Patan Page no - 15
KARL - FISCHER Analysis Water Content Determination
9 Side reactions
Side Reactions are classified as
(1) Reactions Influenced by Methanol
(2) Reactions with Iodine
(1) Reactions Influenced by Methanol
It includes
bull Aldehydes react to form acetals also undergo Bisulfite addition
bull Ketones react to form ketals
bull Amines undergo methylation
bull Siloxanes undergo esterification
(2) Reactions with Iodine
It includes
bull Halogenated Hydrocarbons containing free halogens
bull Mercaptans (Thiols)
bull Phenols
bull Peroxides
Testing for Iodine Reactivity
bull Dissolve a few Iodine Crystals in Methanol and Introduce Sample
bull Watch for Solution to Change Color if Clear Iodine Reaction is Present
Sheth M N Science College Patan Page no - 16
KARL - FISCHER Analysis Water Content Determination
1 Mercaptans (Thiols) are oxidized by iodine Suppress this side reaction by treating
sample with N-ethylmaleimide in a buffered solvent system
2 Inhibition of Thiol Oxidation by Iodine
Most Phenols do not react with KF reagents higher molecular weight phenols and
Amino-phenols are most problematic Volumetric method is most suitable since the
Oxidation potential is high in coulometry or Acidify Solvent system to reduce interference
Sheth M N Science College Patan Page no - 17
KARL - FISCHER Analysis Water Content Determination
10 Applications of Karl Fischer method for determination of Water in
pharmaceutical analysis
101 PREDNISOLONE SODIUM PHOSPHATE
Materials Required Karl Fischer Reagent 100 ml prednisolone sodium phosphate
02gm anhydrous methanol 200 ml
Procedure Add about 20 ml of anhydrous methanol to the titration vessel and titrate to the
Amperometric End-point with the Karl Fischer reagent Quickly add 02 g of prednisolone
sodium phosphate sample stir for 1 minute and again titrate to the amperometric end-point
with the Karl Fischer reagent The difference between the two titrations gives the volume
(v) of Karl Fischer reagent consumed by the sample
The minimum water equivalent is 35 mg of water per ml of Karl Fischer reagent
Hence the percentage of water ww in the given sample may be calculated by the
following
Precautions
(1) The reagents and solutions used must be kept anhydrous and necessary care should be
taken throughout to prevent exposure to atmospheric moisture
(2) The Karl Fischer reagent should be protected from light and preferably stored in a bottle
fitted with an automatic burette and
(3) The water equivalent of Karl Fischer reagent should always be determined before use
102 COGNATE ASSAYS
A number of other official pharmaceutical substances may be assayed for their water
content by the Karl Fischer method as summarized in the following Table
Sheth M N Science College Patan Page no - 18
KARL - FISCHER Analysis Water Content Determination
11 Automated Electrochemical Karl Fischer Analysis
Commercially available Modern KF-Titrators are usually equipped with specifically
designed titration vessels that are exclusively meant to check and prevent the contact with
atmospheric moisture Quite a few such devices are armed with microprocessors that will
perform the requisite operations sequentially in a programmed manner automatically and
may also dish out a print-out of the desired results including the percentage moisture
content In fact these Modern KF-Titrators not only afford greater accuracy and precision
in results but also offer much ease and convenience in routine analysis as compared to the
classical techniques based on either caulometry or controlled current potentiometry using
two indicator electrodes
Thus the basis of the analysis rests upon the quantitative relationship existing
between charge passed and iodine produced by the reagent according to the above reaction
Therefore the generation of iodine is automatically stopped when an excess of it is detected
by the indicator electrode It essentially consists of two platinum electrodes across which an
AC is applied and subsequently a marked drop in voltage between the electrodes takes place
as soon as an excess of iodine is present Normally such automated instruments make use of
proprietory reagents exclusively
The major advantage of this approach to KF-analysis being that no calibration is
required as the method is absolute and is entirely based on the stoichiometry of the
aforesaid equation It is noteworthy that one may determine the amounts of water ranging
between 10 mcg and 10 mg in solid as well as liquid samples
Sheth M N Science College Patan Page no - 19
KARL - FISCHER Analysis Water Content Determination
12 Some of the innovative brands in Karl Fischer Auto ndash titrator
instruments
Sheth M N Science College Patan Page no - 20
KARL - FISCHER Analysis Water Content Determination
Sheth M N Science College Patan Page no - 21
KARL - FISCHER Analysis Water Content Determination
13 References
1 Chapter 14 Karl fishcer method for determination of water Page no 223-227
Pharmaceutical Drug Analysis Ashutosh Kar New age international limited publishers
2 Aqua Stars Karl fischer instruments wwwemdchemicalscom
3 Karl Fischer Volumetric Titration Theory and Practice by Radiometer analytical
SAS wwwradiometer-analyticalcom
4 MettlerToledo AG Analytical Switzerland wwwmtcom
5 Summary of Karl fischer titrations by Orion products Baverly MA Thermofischer
Scientific wwwthermocomwater
6 Moisture determination by Karl Fischer titration background of chemistry and
recent advances SupelcoSigma-Aldrich Bellefonte Pa wwwsigma-aldrichcom
7 Karl Fischer Titration Methods Methodology and water determination in 442
different substances European Pharmacopoeia (PharmEurope) 4th Edition plus
Supplements 2002 International Electrotechnical Commission Geneva
8 Karl ndash fischer titration wwwwikipediacom
Sheth M N Science College Patan Page no - 22
KARL - FISCHER Analysis Water Content Determination
71 Volumetric titration method
Apparatus
Generally the apparatus consists of an automatic burette a backtitration flask a stirrer and
equipment for amperometric titration at constant voltage or potentiometric titration at
constant current
Because water determination TS is extremely hygroscopic the titration apparatus
should be protected from atmospheric moisture Silica gel or calcium chloride for water
determination is usually used for moisture protection
Procedure
As a rule the titration of the sample with water determination TS should be performed at
the same temperature as that at the standardization of the TS while protecting from
moisture
The apparatus is equipped with a variable resistor in the circuit and the resistor is
adjusted to apply a definite voltage (mV) between a pair of platinum electrodes immersed in
the solution to be titrated The change in current (1048707A) is measured during the dropping of
water determination TS (Amperometric titration at constant voltage) As titration continues
the abrupt change in current in the circuit occurs but returns to the original state within
several seconds At the end of a titration the change in current persists for a certain time
(usually longer than 30 seconds) The end point of titration is determined at this electric
state
Otherwise by adjusting the resistor a definite current is passed between the two
platinum electrodes and the change in potential (mV) is measured during dropping water
determination TS (Potentiometric titration at constant current) With the progress of
titration the value indicated by the potentiometer in the circuit decreases suddenly from a
polarization state of several hundreds (mV) to the nonpolarization state but it returns to the
original state within several seconds At the end of titration the non-polarization state
persists for a certain time (usually longer than 30 seconds) The end point of titration is
determined when this electric state attains
In the case of back titration when the amperometric titration method is used at
constant voltage the needle of microammeter is out of scale while an excessive quantity of
water determination TS remains It returns rapidly to the original position when the titration
reaches the end point
Sheth M N Science College Patan Page no - 11
KARL - FISCHER Analysis Water Content Determination
Similarly when the Potentiometric titration method at constant current is used the
needle of the millivoltmeter is at the original position while an excessive quantity of water
determination TS remains A definite voltage is applied when the titration reaches the end
point Unless otherwise specified the titration of water with water determination TS is
performed by either of the methods below Usually the end point of the titration can be
observed more clearly in the back titration method than in the direct titration method
711 Direct titration
Unless otherwise specified proceed as directed below Take 25ml of methanol for water
determination in a dried titration flask and titrate with water determination TS to the end
point Unless otherwise specified weigh accurately a quantity of the sample containing 10
to 50 mg of water transfer it quickly into the titration flask and dissolve by stirring Titrate
the solution with water determination TS to the end point under vigorous stirring When the
sample is insoluble in the solvent powder the sample quickly weigh a suitable amount of
the sample accurately and transfer it quickly into the titration vessel stir the mixture for 30
minutes while protecting it from moisture
Perform a titration under vigorous stirring When the sample interferes with the Karl
Fisher reaction water in the sample can be removed by heating and under a stream of
nitrogen gas and introduced into the titration vessel by using a water-evaporation device
712 Back titration
Unless otherwise specified proceed as directed below Take 20ml of methanol for water
determination in the dried titration vessel and titrate with water determination TS Weigh
accurately a suitable quantity of the sample containing 10 to 50 mg of water transfer the
sample quickly into the titration vessel add an excessive and definite volume of water
determination TS stir for 30 min protecting from atmospheric moisture and then titrate the
solution with Water Methanol Standard Solution under vigorous stirring
Sheth M N Science College Patan Page no - 12
KARL - FISCHER Analysis Water Content Determination
Wheref = the number of mg of water (H2O) corresponding to 1 ml of water determination
TS
Where
f = the number of mg of water (H2O) corresponding to 1 ml of water determination TS
f rsquo = the number of mg of water (H2O) in 1 ml of Water Methanol Standard Solution
713 General Remarks
Volumetric Karl Fischer titration requires the determination of the titre (t) of the Karl
Fischer reagent It is usually quoted in mg of water per ml of Karl Fischer reagent Modern
reagents allow direct titration of water in the sample The sample may be introduced
directly into the KF cell or after an extraction or dissolution with a suitable solvent The
water concentration of the solvent must be determined previously in order to be subtracted
from the sample analysis In Radiometer Analytical titrators all these operations are
simplified and the different results are accounted for automatically The volumetric titration
of water allows the analysis of water concentrations between 01 and 100 If an aliquot
contains less than 1 mg of water coulometric determination will result in a more accurate
result For reasons of precision the titre of the titrant should be chosen so that the titration is
completed with a titrant demand between 1 and 10 ml
The working medium (ie the solvent required) can be freely chosen by the user
depending on the dissolution properties of the sample to be investigated For methanol-
based reagents the stoichiometry 11 of the Karl Fischer reaction is only fulfilled if there is
more than 25 methanol in the reaction mixture A methanol-free working medium can be
used however it is important to determine the titre of the KF reagent in the same working
medium The modern solvents available today present a high buffer and dissolution
capacity These solvents consist of sulphur dioxide a base and methanol or ethanol The
main advantages of these solvents are
bull A more rapid titration due to better reaction kinetics an advantage especially for the
titration of large amounts of water
bull A better reproducibility because the reaction environment is stable The pH and the
sulphur dioxide concentration remain constant
Sheth M N Science College Patan Page no - 13
KARL - FISCHER Analysis Water Content Determination
For the titration of samples producing side reactions (aldehydes ketones and
silanols) it is necessary to use an appropriate solvent Most reagent manufacturers include
the letter K in the commercial name of such solvents
72 Coulometric titration
Apparatus
Usually the apparatus is comprised of an electrolytic cell for iodine production a stirrer a
titration flask and a potentiometric titration system at constant current The iodine
production device is composed of an anode and a cathode separated by a diaphragm The
anode is immersed in the anolyte solution for water determination and the cathode is
immersed in the catholyte solution for water determination Both electrodes are usually
made of platinum-mesh
Because water determination TS is extremely hygroscopic the titration apparatus
should be protected from atmospheric moisture For this purpose silica gel or calcium
chloride for water determination is usually used Procedure Take a suitable volume of an
anolyte for water determination in a titration vessel immerse in this solution a pair of
platinum electrodes for potentiometric titration at constant current
Then immerse the iodide production system filled with a catholyte for water
determination in the anolyte solution Switch on the electrolytic system and make the
content of the titration vessel anhydrous Next take an accurately weighed amount of the
sample containing 15 mg of water add it quickly to the vessel and dissolve by stirring
Perform the titration to the end point under vigorous stirring When the sample is insoluble
in the anolyte powder it quickly and add an accurately weighed amount of the sample to
the vessel After stirring the mixture for 5 to 30 minutes while protecting from atmospheric
moisture perform the titration with vigorous stirring Determine the quantity of electricity
(C) [electric current (A) and time (s)] required for the production of iodine during the
titration and calculate the content () of the water in the sample by the formula
When the sample interferes with the Karl Fisher reaction water in the sample can be
removed by heating under a stream of nitrogen gas and introduced into the titration vessel
by using a water-evaporation device
Sheth M N Science College Patan Page no - 14
KARL - FISCHER Analysis Water Content Determination
8 Interference and Limitations of Karl Fischer Method
Interferences in the water determinations by Karl Fischer method are associated with
condensation or oxidation ndash reduction reactions with a number of substances and
compounds or by side reactions Other possible interferences to the KF reagent are certain
active metals metal oxides metal hydroxides chromates melamines etc
The Karl Fischer titration has a number of serious limitations due to possible
interferences tantamount to erroneous results namely
(i) Oxidizing agents for instance chromates Cu(II) Fe(III) Cr2O7 2ndash peroxides salts
higher oxides
Example MnO2 + 4C5H5NH+ + 2Indash rarr Mn2+ + 4C5H5N + I2 + H2O
(ii) Reducing agents such as Sn (II) salts sulphides and S2O32ndash and
(iii) Compounds that have a tendency to form water with the ingredients of the Karl Fischer
reagent for instance
(a) Basic oxides eg ZnO
Example ZnO + 2C5H5NH+ rarr Zn2+ + C5H5N + H2O
(b) Salts of weak oxy-acids eg NaHCO3
Example NaHCO3 + C5H5NH+ rarr Na+ + H2O + CO2 + C5H5N
[Note As H2CO3 carbonic acid is very unstable hence it splits up to yield a mole each of
water and CO2]
Use of certain reagents will minimize or eliminate the interferences of many classes
of compounds For example the use of non-methanol containing KF reagent and solvent will
reduce the interference from aldehyde and ketones Ammonia and amines can be eliminated
by the addition of salicylic acid to the solvent
Sheth M N Science College Patan Page no - 15
KARL - FISCHER Analysis Water Content Determination
9 Side reactions
Side Reactions are classified as
(1) Reactions Influenced by Methanol
(2) Reactions with Iodine
(1) Reactions Influenced by Methanol
It includes
bull Aldehydes react to form acetals also undergo Bisulfite addition
bull Ketones react to form ketals
bull Amines undergo methylation
bull Siloxanes undergo esterification
(2) Reactions with Iodine
It includes
bull Halogenated Hydrocarbons containing free halogens
bull Mercaptans (Thiols)
bull Phenols
bull Peroxides
Testing for Iodine Reactivity
bull Dissolve a few Iodine Crystals in Methanol and Introduce Sample
bull Watch for Solution to Change Color if Clear Iodine Reaction is Present
Sheth M N Science College Patan Page no - 16
KARL - FISCHER Analysis Water Content Determination
1 Mercaptans (Thiols) are oxidized by iodine Suppress this side reaction by treating
sample with N-ethylmaleimide in a buffered solvent system
2 Inhibition of Thiol Oxidation by Iodine
Most Phenols do not react with KF reagents higher molecular weight phenols and
Amino-phenols are most problematic Volumetric method is most suitable since the
Oxidation potential is high in coulometry or Acidify Solvent system to reduce interference
Sheth M N Science College Patan Page no - 17
KARL - FISCHER Analysis Water Content Determination
10 Applications of Karl Fischer method for determination of Water in
pharmaceutical analysis
101 PREDNISOLONE SODIUM PHOSPHATE
Materials Required Karl Fischer Reagent 100 ml prednisolone sodium phosphate
02gm anhydrous methanol 200 ml
Procedure Add about 20 ml of anhydrous methanol to the titration vessel and titrate to the
Amperometric End-point with the Karl Fischer reagent Quickly add 02 g of prednisolone
sodium phosphate sample stir for 1 minute and again titrate to the amperometric end-point
with the Karl Fischer reagent The difference between the two titrations gives the volume
(v) of Karl Fischer reagent consumed by the sample
The minimum water equivalent is 35 mg of water per ml of Karl Fischer reagent
Hence the percentage of water ww in the given sample may be calculated by the
following
Precautions
(1) The reagents and solutions used must be kept anhydrous and necessary care should be
taken throughout to prevent exposure to atmospheric moisture
(2) The Karl Fischer reagent should be protected from light and preferably stored in a bottle
fitted with an automatic burette and
(3) The water equivalent of Karl Fischer reagent should always be determined before use
102 COGNATE ASSAYS
A number of other official pharmaceutical substances may be assayed for their water
content by the Karl Fischer method as summarized in the following Table
Sheth M N Science College Patan Page no - 18
KARL - FISCHER Analysis Water Content Determination
11 Automated Electrochemical Karl Fischer Analysis
Commercially available Modern KF-Titrators are usually equipped with specifically
designed titration vessels that are exclusively meant to check and prevent the contact with
atmospheric moisture Quite a few such devices are armed with microprocessors that will
perform the requisite operations sequentially in a programmed manner automatically and
may also dish out a print-out of the desired results including the percentage moisture
content In fact these Modern KF-Titrators not only afford greater accuracy and precision
in results but also offer much ease and convenience in routine analysis as compared to the
classical techniques based on either caulometry or controlled current potentiometry using
two indicator electrodes
Thus the basis of the analysis rests upon the quantitative relationship existing
between charge passed and iodine produced by the reagent according to the above reaction
Therefore the generation of iodine is automatically stopped when an excess of it is detected
by the indicator electrode It essentially consists of two platinum electrodes across which an
AC is applied and subsequently a marked drop in voltage between the electrodes takes place
as soon as an excess of iodine is present Normally such automated instruments make use of
proprietory reagents exclusively
The major advantage of this approach to KF-analysis being that no calibration is
required as the method is absolute and is entirely based on the stoichiometry of the
aforesaid equation It is noteworthy that one may determine the amounts of water ranging
between 10 mcg and 10 mg in solid as well as liquid samples
Sheth M N Science College Patan Page no - 19
KARL - FISCHER Analysis Water Content Determination
12 Some of the innovative brands in Karl Fischer Auto ndash titrator
instruments
Sheth M N Science College Patan Page no - 20
KARL - FISCHER Analysis Water Content Determination
Sheth M N Science College Patan Page no - 21
KARL - FISCHER Analysis Water Content Determination
13 References
1 Chapter 14 Karl fishcer method for determination of water Page no 223-227
Pharmaceutical Drug Analysis Ashutosh Kar New age international limited publishers
2 Aqua Stars Karl fischer instruments wwwemdchemicalscom
3 Karl Fischer Volumetric Titration Theory and Practice by Radiometer analytical
SAS wwwradiometer-analyticalcom
4 MettlerToledo AG Analytical Switzerland wwwmtcom
5 Summary of Karl fischer titrations by Orion products Baverly MA Thermofischer
Scientific wwwthermocomwater
6 Moisture determination by Karl Fischer titration background of chemistry and
recent advances SupelcoSigma-Aldrich Bellefonte Pa wwwsigma-aldrichcom
7 Karl Fischer Titration Methods Methodology and water determination in 442
different substances European Pharmacopoeia (PharmEurope) 4th Edition plus
Supplements 2002 International Electrotechnical Commission Geneva
8 Karl ndash fischer titration wwwwikipediacom
Sheth M N Science College Patan Page no - 22
KARL - FISCHER Analysis Water Content Determination
Similarly when the Potentiometric titration method at constant current is used the
needle of the millivoltmeter is at the original position while an excessive quantity of water
determination TS remains A definite voltage is applied when the titration reaches the end
point Unless otherwise specified the titration of water with water determination TS is
performed by either of the methods below Usually the end point of the titration can be
observed more clearly in the back titration method than in the direct titration method
711 Direct titration
Unless otherwise specified proceed as directed below Take 25ml of methanol for water
determination in a dried titration flask and titrate with water determination TS to the end
point Unless otherwise specified weigh accurately a quantity of the sample containing 10
to 50 mg of water transfer it quickly into the titration flask and dissolve by stirring Titrate
the solution with water determination TS to the end point under vigorous stirring When the
sample is insoluble in the solvent powder the sample quickly weigh a suitable amount of
the sample accurately and transfer it quickly into the titration vessel stir the mixture for 30
minutes while protecting it from moisture
Perform a titration under vigorous stirring When the sample interferes with the Karl
Fisher reaction water in the sample can be removed by heating and under a stream of
nitrogen gas and introduced into the titration vessel by using a water-evaporation device
712 Back titration
Unless otherwise specified proceed as directed below Take 20ml of methanol for water
determination in the dried titration vessel and titrate with water determination TS Weigh
accurately a suitable quantity of the sample containing 10 to 50 mg of water transfer the
sample quickly into the titration vessel add an excessive and definite volume of water
determination TS stir for 30 min protecting from atmospheric moisture and then titrate the
solution with Water Methanol Standard Solution under vigorous stirring
Sheth M N Science College Patan Page no - 12
KARL - FISCHER Analysis Water Content Determination
Wheref = the number of mg of water (H2O) corresponding to 1 ml of water determination
TS
Where
f = the number of mg of water (H2O) corresponding to 1 ml of water determination TS
f rsquo = the number of mg of water (H2O) in 1 ml of Water Methanol Standard Solution
713 General Remarks
Volumetric Karl Fischer titration requires the determination of the titre (t) of the Karl
Fischer reagent It is usually quoted in mg of water per ml of Karl Fischer reagent Modern
reagents allow direct titration of water in the sample The sample may be introduced
directly into the KF cell or after an extraction or dissolution with a suitable solvent The
water concentration of the solvent must be determined previously in order to be subtracted
from the sample analysis In Radiometer Analytical titrators all these operations are
simplified and the different results are accounted for automatically The volumetric titration
of water allows the analysis of water concentrations between 01 and 100 If an aliquot
contains less than 1 mg of water coulometric determination will result in a more accurate
result For reasons of precision the titre of the titrant should be chosen so that the titration is
completed with a titrant demand between 1 and 10 ml
The working medium (ie the solvent required) can be freely chosen by the user
depending on the dissolution properties of the sample to be investigated For methanol-
based reagents the stoichiometry 11 of the Karl Fischer reaction is only fulfilled if there is
more than 25 methanol in the reaction mixture A methanol-free working medium can be
used however it is important to determine the titre of the KF reagent in the same working
medium The modern solvents available today present a high buffer and dissolution
capacity These solvents consist of sulphur dioxide a base and methanol or ethanol The
main advantages of these solvents are
bull A more rapid titration due to better reaction kinetics an advantage especially for the
titration of large amounts of water
bull A better reproducibility because the reaction environment is stable The pH and the
sulphur dioxide concentration remain constant
Sheth M N Science College Patan Page no - 13
KARL - FISCHER Analysis Water Content Determination
For the titration of samples producing side reactions (aldehydes ketones and
silanols) it is necessary to use an appropriate solvent Most reagent manufacturers include
the letter K in the commercial name of such solvents
72 Coulometric titration
Apparatus
Usually the apparatus is comprised of an electrolytic cell for iodine production a stirrer a
titration flask and a potentiometric titration system at constant current The iodine
production device is composed of an anode and a cathode separated by a diaphragm The
anode is immersed in the anolyte solution for water determination and the cathode is
immersed in the catholyte solution for water determination Both electrodes are usually
made of platinum-mesh
Because water determination TS is extremely hygroscopic the titration apparatus
should be protected from atmospheric moisture For this purpose silica gel or calcium
chloride for water determination is usually used Procedure Take a suitable volume of an
anolyte for water determination in a titration vessel immerse in this solution a pair of
platinum electrodes for potentiometric titration at constant current
Then immerse the iodide production system filled with a catholyte for water
determination in the anolyte solution Switch on the electrolytic system and make the
content of the titration vessel anhydrous Next take an accurately weighed amount of the
sample containing 15 mg of water add it quickly to the vessel and dissolve by stirring
Perform the titration to the end point under vigorous stirring When the sample is insoluble
in the anolyte powder it quickly and add an accurately weighed amount of the sample to
the vessel After stirring the mixture for 5 to 30 minutes while protecting from atmospheric
moisture perform the titration with vigorous stirring Determine the quantity of electricity
(C) [electric current (A) and time (s)] required for the production of iodine during the
titration and calculate the content () of the water in the sample by the formula
When the sample interferes with the Karl Fisher reaction water in the sample can be
removed by heating under a stream of nitrogen gas and introduced into the titration vessel
by using a water-evaporation device
Sheth M N Science College Patan Page no - 14
KARL - FISCHER Analysis Water Content Determination
8 Interference and Limitations of Karl Fischer Method
Interferences in the water determinations by Karl Fischer method are associated with
condensation or oxidation ndash reduction reactions with a number of substances and
compounds or by side reactions Other possible interferences to the KF reagent are certain
active metals metal oxides metal hydroxides chromates melamines etc
The Karl Fischer titration has a number of serious limitations due to possible
interferences tantamount to erroneous results namely
(i) Oxidizing agents for instance chromates Cu(II) Fe(III) Cr2O7 2ndash peroxides salts
higher oxides
Example MnO2 + 4C5H5NH+ + 2Indash rarr Mn2+ + 4C5H5N + I2 + H2O
(ii) Reducing agents such as Sn (II) salts sulphides and S2O32ndash and
(iii) Compounds that have a tendency to form water with the ingredients of the Karl Fischer
reagent for instance
(a) Basic oxides eg ZnO
Example ZnO + 2C5H5NH+ rarr Zn2+ + C5H5N + H2O
(b) Salts of weak oxy-acids eg NaHCO3
Example NaHCO3 + C5H5NH+ rarr Na+ + H2O + CO2 + C5H5N
[Note As H2CO3 carbonic acid is very unstable hence it splits up to yield a mole each of
water and CO2]
Use of certain reagents will minimize or eliminate the interferences of many classes
of compounds For example the use of non-methanol containing KF reagent and solvent will
reduce the interference from aldehyde and ketones Ammonia and amines can be eliminated
by the addition of salicylic acid to the solvent
Sheth M N Science College Patan Page no - 15
KARL - FISCHER Analysis Water Content Determination
9 Side reactions
Side Reactions are classified as
(1) Reactions Influenced by Methanol
(2) Reactions with Iodine
(1) Reactions Influenced by Methanol
It includes
bull Aldehydes react to form acetals also undergo Bisulfite addition
bull Ketones react to form ketals
bull Amines undergo methylation
bull Siloxanes undergo esterification
(2) Reactions with Iodine
It includes
bull Halogenated Hydrocarbons containing free halogens
bull Mercaptans (Thiols)
bull Phenols
bull Peroxides
Testing for Iodine Reactivity
bull Dissolve a few Iodine Crystals in Methanol and Introduce Sample
bull Watch for Solution to Change Color if Clear Iodine Reaction is Present
Sheth M N Science College Patan Page no - 16
KARL - FISCHER Analysis Water Content Determination
1 Mercaptans (Thiols) are oxidized by iodine Suppress this side reaction by treating
sample with N-ethylmaleimide in a buffered solvent system
2 Inhibition of Thiol Oxidation by Iodine
Most Phenols do not react with KF reagents higher molecular weight phenols and
Amino-phenols are most problematic Volumetric method is most suitable since the
Oxidation potential is high in coulometry or Acidify Solvent system to reduce interference
Sheth M N Science College Patan Page no - 17
KARL - FISCHER Analysis Water Content Determination
10 Applications of Karl Fischer method for determination of Water in
pharmaceutical analysis
101 PREDNISOLONE SODIUM PHOSPHATE
Materials Required Karl Fischer Reagent 100 ml prednisolone sodium phosphate
02gm anhydrous methanol 200 ml
Procedure Add about 20 ml of anhydrous methanol to the titration vessel and titrate to the
Amperometric End-point with the Karl Fischer reagent Quickly add 02 g of prednisolone
sodium phosphate sample stir for 1 minute and again titrate to the amperometric end-point
with the Karl Fischer reagent The difference between the two titrations gives the volume
(v) of Karl Fischer reagent consumed by the sample
The minimum water equivalent is 35 mg of water per ml of Karl Fischer reagent
Hence the percentage of water ww in the given sample may be calculated by the
following
Precautions
(1) The reagents and solutions used must be kept anhydrous and necessary care should be
taken throughout to prevent exposure to atmospheric moisture
(2) The Karl Fischer reagent should be protected from light and preferably stored in a bottle
fitted with an automatic burette and
(3) The water equivalent of Karl Fischer reagent should always be determined before use
102 COGNATE ASSAYS
A number of other official pharmaceutical substances may be assayed for their water
content by the Karl Fischer method as summarized in the following Table
Sheth M N Science College Patan Page no - 18
KARL - FISCHER Analysis Water Content Determination
11 Automated Electrochemical Karl Fischer Analysis
Commercially available Modern KF-Titrators are usually equipped with specifically
designed titration vessels that are exclusively meant to check and prevent the contact with
atmospheric moisture Quite a few such devices are armed with microprocessors that will
perform the requisite operations sequentially in a programmed manner automatically and
may also dish out a print-out of the desired results including the percentage moisture
content In fact these Modern KF-Titrators not only afford greater accuracy and precision
in results but also offer much ease and convenience in routine analysis as compared to the
classical techniques based on either caulometry or controlled current potentiometry using
two indicator electrodes
Thus the basis of the analysis rests upon the quantitative relationship existing
between charge passed and iodine produced by the reagent according to the above reaction
Therefore the generation of iodine is automatically stopped when an excess of it is detected
by the indicator electrode It essentially consists of two platinum electrodes across which an
AC is applied and subsequently a marked drop in voltage between the electrodes takes place
as soon as an excess of iodine is present Normally such automated instruments make use of
proprietory reagents exclusively
The major advantage of this approach to KF-analysis being that no calibration is
required as the method is absolute and is entirely based on the stoichiometry of the
aforesaid equation It is noteworthy that one may determine the amounts of water ranging
between 10 mcg and 10 mg in solid as well as liquid samples
Sheth M N Science College Patan Page no - 19
KARL - FISCHER Analysis Water Content Determination
12 Some of the innovative brands in Karl Fischer Auto ndash titrator
instruments
Sheth M N Science College Patan Page no - 20
KARL - FISCHER Analysis Water Content Determination
Sheth M N Science College Patan Page no - 21
KARL - FISCHER Analysis Water Content Determination
13 References
1 Chapter 14 Karl fishcer method for determination of water Page no 223-227
Pharmaceutical Drug Analysis Ashutosh Kar New age international limited publishers
2 Aqua Stars Karl fischer instruments wwwemdchemicalscom
3 Karl Fischer Volumetric Titration Theory and Practice by Radiometer analytical
SAS wwwradiometer-analyticalcom
4 MettlerToledo AG Analytical Switzerland wwwmtcom
5 Summary of Karl fischer titrations by Orion products Baverly MA Thermofischer
Scientific wwwthermocomwater
6 Moisture determination by Karl Fischer titration background of chemistry and
recent advances SupelcoSigma-Aldrich Bellefonte Pa wwwsigma-aldrichcom
7 Karl Fischer Titration Methods Methodology and water determination in 442
different substances European Pharmacopoeia (PharmEurope) 4th Edition plus
Supplements 2002 International Electrotechnical Commission Geneva
8 Karl ndash fischer titration wwwwikipediacom
Sheth M N Science College Patan Page no - 22
KARL - FISCHER Analysis Water Content Determination
Wheref = the number of mg of water (H2O) corresponding to 1 ml of water determination
TS
Where
f = the number of mg of water (H2O) corresponding to 1 ml of water determination TS
f rsquo = the number of mg of water (H2O) in 1 ml of Water Methanol Standard Solution
713 General Remarks
Volumetric Karl Fischer titration requires the determination of the titre (t) of the Karl
Fischer reagent It is usually quoted in mg of water per ml of Karl Fischer reagent Modern
reagents allow direct titration of water in the sample The sample may be introduced
directly into the KF cell or after an extraction or dissolution with a suitable solvent The
water concentration of the solvent must be determined previously in order to be subtracted
from the sample analysis In Radiometer Analytical titrators all these operations are
simplified and the different results are accounted for automatically The volumetric titration
of water allows the analysis of water concentrations between 01 and 100 If an aliquot
contains less than 1 mg of water coulometric determination will result in a more accurate
result For reasons of precision the titre of the titrant should be chosen so that the titration is
completed with a titrant demand between 1 and 10 ml
The working medium (ie the solvent required) can be freely chosen by the user
depending on the dissolution properties of the sample to be investigated For methanol-
based reagents the stoichiometry 11 of the Karl Fischer reaction is only fulfilled if there is
more than 25 methanol in the reaction mixture A methanol-free working medium can be
used however it is important to determine the titre of the KF reagent in the same working
medium The modern solvents available today present a high buffer and dissolution
capacity These solvents consist of sulphur dioxide a base and methanol or ethanol The
main advantages of these solvents are
bull A more rapid titration due to better reaction kinetics an advantage especially for the
titration of large amounts of water
bull A better reproducibility because the reaction environment is stable The pH and the
sulphur dioxide concentration remain constant
Sheth M N Science College Patan Page no - 13
KARL - FISCHER Analysis Water Content Determination
For the titration of samples producing side reactions (aldehydes ketones and
silanols) it is necessary to use an appropriate solvent Most reagent manufacturers include
the letter K in the commercial name of such solvents
72 Coulometric titration
Apparatus
Usually the apparatus is comprised of an electrolytic cell for iodine production a stirrer a
titration flask and a potentiometric titration system at constant current The iodine
production device is composed of an anode and a cathode separated by a diaphragm The
anode is immersed in the anolyte solution for water determination and the cathode is
immersed in the catholyte solution for water determination Both electrodes are usually
made of platinum-mesh
Because water determination TS is extremely hygroscopic the titration apparatus
should be protected from atmospheric moisture For this purpose silica gel or calcium
chloride for water determination is usually used Procedure Take a suitable volume of an
anolyte for water determination in a titration vessel immerse in this solution a pair of
platinum electrodes for potentiometric titration at constant current
Then immerse the iodide production system filled with a catholyte for water
determination in the anolyte solution Switch on the electrolytic system and make the
content of the titration vessel anhydrous Next take an accurately weighed amount of the
sample containing 15 mg of water add it quickly to the vessel and dissolve by stirring
Perform the titration to the end point under vigorous stirring When the sample is insoluble
in the anolyte powder it quickly and add an accurately weighed amount of the sample to
the vessel After stirring the mixture for 5 to 30 minutes while protecting from atmospheric
moisture perform the titration with vigorous stirring Determine the quantity of electricity
(C) [electric current (A) and time (s)] required for the production of iodine during the
titration and calculate the content () of the water in the sample by the formula
When the sample interferes with the Karl Fisher reaction water in the sample can be
removed by heating under a stream of nitrogen gas and introduced into the titration vessel
by using a water-evaporation device
Sheth M N Science College Patan Page no - 14
KARL - FISCHER Analysis Water Content Determination
8 Interference and Limitations of Karl Fischer Method
Interferences in the water determinations by Karl Fischer method are associated with
condensation or oxidation ndash reduction reactions with a number of substances and
compounds or by side reactions Other possible interferences to the KF reagent are certain
active metals metal oxides metal hydroxides chromates melamines etc
The Karl Fischer titration has a number of serious limitations due to possible
interferences tantamount to erroneous results namely
(i) Oxidizing agents for instance chromates Cu(II) Fe(III) Cr2O7 2ndash peroxides salts
higher oxides
Example MnO2 + 4C5H5NH+ + 2Indash rarr Mn2+ + 4C5H5N + I2 + H2O
(ii) Reducing agents such as Sn (II) salts sulphides and S2O32ndash and
(iii) Compounds that have a tendency to form water with the ingredients of the Karl Fischer
reagent for instance
(a) Basic oxides eg ZnO
Example ZnO + 2C5H5NH+ rarr Zn2+ + C5H5N + H2O
(b) Salts of weak oxy-acids eg NaHCO3
Example NaHCO3 + C5H5NH+ rarr Na+ + H2O + CO2 + C5H5N
[Note As H2CO3 carbonic acid is very unstable hence it splits up to yield a mole each of
water and CO2]
Use of certain reagents will minimize or eliminate the interferences of many classes
of compounds For example the use of non-methanol containing KF reagent and solvent will
reduce the interference from aldehyde and ketones Ammonia and amines can be eliminated
by the addition of salicylic acid to the solvent
Sheth M N Science College Patan Page no - 15
KARL - FISCHER Analysis Water Content Determination
9 Side reactions
Side Reactions are classified as
(1) Reactions Influenced by Methanol
(2) Reactions with Iodine
(1) Reactions Influenced by Methanol
It includes
bull Aldehydes react to form acetals also undergo Bisulfite addition
bull Ketones react to form ketals
bull Amines undergo methylation
bull Siloxanes undergo esterification
(2) Reactions with Iodine
It includes
bull Halogenated Hydrocarbons containing free halogens
bull Mercaptans (Thiols)
bull Phenols
bull Peroxides
Testing for Iodine Reactivity
bull Dissolve a few Iodine Crystals in Methanol and Introduce Sample
bull Watch for Solution to Change Color if Clear Iodine Reaction is Present
Sheth M N Science College Patan Page no - 16
KARL - FISCHER Analysis Water Content Determination
1 Mercaptans (Thiols) are oxidized by iodine Suppress this side reaction by treating
sample with N-ethylmaleimide in a buffered solvent system
2 Inhibition of Thiol Oxidation by Iodine
Most Phenols do not react with KF reagents higher molecular weight phenols and
Amino-phenols are most problematic Volumetric method is most suitable since the
Oxidation potential is high in coulometry or Acidify Solvent system to reduce interference
Sheth M N Science College Patan Page no - 17
KARL - FISCHER Analysis Water Content Determination
10 Applications of Karl Fischer method for determination of Water in
pharmaceutical analysis
101 PREDNISOLONE SODIUM PHOSPHATE
Materials Required Karl Fischer Reagent 100 ml prednisolone sodium phosphate
02gm anhydrous methanol 200 ml
Procedure Add about 20 ml of anhydrous methanol to the titration vessel and titrate to the
Amperometric End-point with the Karl Fischer reagent Quickly add 02 g of prednisolone
sodium phosphate sample stir for 1 minute and again titrate to the amperometric end-point
with the Karl Fischer reagent The difference between the two titrations gives the volume
(v) of Karl Fischer reagent consumed by the sample
The minimum water equivalent is 35 mg of water per ml of Karl Fischer reagent
Hence the percentage of water ww in the given sample may be calculated by the
following
Precautions
(1) The reagents and solutions used must be kept anhydrous and necessary care should be
taken throughout to prevent exposure to atmospheric moisture
(2) The Karl Fischer reagent should be protected from light and preferably stored in a bottle
fitted with an automatic burette and
(3) The water equivalent of Karl Fischer reagent should always be determined before use
102 COGNATE ASSAYS
A number of other official pharmaceutical substances may be assayed for their water
content by the Karl Fischer method as summarized in the following Table
Sheth M N Science College Patan Page no - 18
KARL - FISCHER Analysis Water Content Determination
11 Automated Electrochemical Karl Fischer Analysis
Commercially available Modern KF-Titrators are usually equipped with specifically
designed titration vessels that are exclusively meant to check and prevent the contact with
atmospheric moisture Quite a few such devices are armed with microprocessors that will
perform the requisite operations sequentially in a programmed manner automatically and
may also dish out a print-out of the desired results including the percentage moisture
content In fact these Modern KF-Titrators not only afford greater accuracy and precision
in results but also offer much ease and convenience in routine analysis as compared to the
classical techniques based on either caulometry or controlled current potentiometry using
two indicator electrodes
Thus the basis of the analysis rests upon the quantitative relationship existing
between charge passed and iodine produced by the reagent according to the above reaction
Therefore the generation of iodine is automatically stopped when an excess of it is detected
by the indicator electrode It essentially consists of two platinum electrodes across which an
AC is applied and subsequently a marked drop in voltage between the electrodes takes place
as soon as an excess of iodine is present Normally such automated instruments make use of
proprietory reagents exclusively
The major advantage of this approach to KF-analysis being that no calibration is
required as the method is absolute and is entirely based on the stoichiometry of the
aforesaid equation It is noteworthy that one may determine the amounts of water ranging
between 10 mcg and 10 mg in solid as well as liquid samples
Sheth M N Science College Patan Page no - 19
KARL - FISCHER Analysis Water Content Determination
12 Some of the innovative brands in Karl Fischer Auto ndash titrator
instruments
Sheth M N Science College Patan Page no - 20
KARL - FISCHER Analysis Water Content Determination
Sheth M N Science College Patan Page no - 21
KARL - FISCHER Analysis Water Content Determination
13 References
1 Chapter 14 Karl fishcer method for determination of water Page no 223-227
Pharmaceutical Drug Analysis Ashutosh Kar New age international limited publishers
2 Aqua Stars Karl fischer instruments wwwemdchemicalscom
3 Karl Fischer Volumetric Titration Theory and Practice by Radiometer analytical
SAS wwwradiometer-analyticalcom
4 MettlerToledo AG Analytical Switzerland wwwmtcom
5 Summary of Karl fischer titrations by Orion products Baverly MA Thermofischer
Scientific wwwthermocomwater
6 Moisture determination by Karl Fischer titration background of chemistry and
recent advances SupelcoSigma-Aldrich Bellefonte Pa wwwsigma-aldrichcom
7 Karl Fischer Titration Methods Methodology and water determination in 442
different substances European Pharmacopoeia (PharmEurope) 4th Edition plus
Supplements 2002 International Electrotechnical Commission Geneva
8 Karl ndash fischer titration wwwwikipediacom
Sheth M N Science College Patan Page no - 22
KARL - FISCHER Analysis Water Content Determination
For the titration of samples producing side reactions (aldehydes ketones and
silanols) it is necessary to use an appropriate solvent Most reagent manufacturers include
the letter K in the commercial name of such solvents
72 Coulometric titration
Apparatus
Usually the apparatus is comprised of an electrolytic cell for iodine production a stirrer a
titration flask and a potentiometric titration system at constant current The iodine
production device is composed of an anode and a cathode separated by a diaphragm The
anode is immersed in the anolyte solution for water determination and the cathode is
immersed in the catholyte solution for water determination Both electrodes are usually
made of platinum-mesh
Because water determination TS is extremely hygroscopic the titration apparatus
should be protected from atmospheric moisture For this purpose silica gel or calcium
chloride for water determination is usually used Procedure Take a suitable volume of an
anolyte for water determination in a titration vessel immerse in this solution a pair of
platinum electrodes for potentiometric titration at constant current
Then immerse the iodide production system filled with a catholyte for water
determination in the anolyte solution Switch on the electrolytic system and make the
content of the titration vessel anhydrous Next take an accurately weighed amount of the
sample containing 15 mg of water add it quickly to the vessel and dissolve by stirring
Perform the titration to the end point under vigorous stirring When the sample is insoluble
in the anolyte powder it quickly and add an accurately weighed amount of the sample to
the vessel After stirring the mixture for 5 to 30 minutes while protecting from atmospheric
moisture perform the titration with vigorous stirring Determine the quantity of electricity
(C) [electric current (A) and time (s)] required for the production of iodine during the
titration and calculate the content () of the water in the sample by the formula
When the sample interferes with the Karl Fisher reaction water in the sample can be
removed by heating under a stream of nitrogen gas and introduced into the titration vessel
by using a water-evaporation device
Sheth M N Science College Patan Page no - 14
KARL - FISCHER Analysis Water Content Determination
8 Interference and Limitations of Karl Fischer Method
Interferences in the water determinations by Karl Fischer method are associated with
condensation or oxidation ndash reduction reactions with a number of substances and
compounds or by side reactions Other possible interferences to the KF reagent are certain
active metals metal oxides metal hydroxides chromates melamines etc
The Karl Fischer titration has a number of serious limitations due to possible
interferences tantamount to erroneous results namely
(i) Oxidizing agents for instance chromates Cu(II) Fe(III) Cr2O7 2ndash peroxides salts
higher oxides
Example MnO2 + 4C5H5NH+ + 2Indash rarr Mn2+ + 4C5H5N + I2 + H2O
(ii) Reducing agents such as Sn (II) salts sulphides and S2O32ndash and
(iii) Compounds that have a tendency to form water with the ingredients of the Karl Fischer
reagent for instance
(a) Basic oxides eg ZnO
Example ZnO + 2C5H5NH+ rarr Zn2+ + C5H5N + H2O
(b) Salts of weak oxy-acids eg NaHCO3
Example NaHCO3 + C5H5NH+ rarr Na+ + H2O + CO2 + C5H5N
[Note As H2CO3 carbonic acid is very unstable hence it splits up to yield a mole each of
water and CO2]
Use of certain reagents will minimize or eliminate the interferences of many classes
of compounds For example the use of non-methanol containing KF reagent and solvent will
reduce the interference from aldehyde and ketones Ammonia and amines can be eliminated
by the addition of salicylic acid to the solvent
Sheth M N Science College Patan Page no - 15
KARL - FISCHER Analysis Water Content Determination
9 Side reactions
Side Reactions are classified as
(1) Reactions Influenced by Methanol
(2) Reactions with Iodine
(1) Reactions Influenced by Methanol
It includes
bull Aldehydes react to form acetals also undergo Bisulfite addition
bull Ketones react to form ketals
bull Amines undergo methylation
bull Siloxanes undergo esterification
(2) Reactions with Iodine
It includes
bull Halogenated Hydrocarbons containing free halogens
bull Mercaptans (Thiols)
bull Phenols
bull Peroxides
Testing for Iodine Reactivity
bull Dissolve a few Iodine Crystals in Methanol and Introduce Sample
bull Watch for Solution to Change Color if Clear Iodine Reaction is Present
Sheth M N Science College Patan Page no - 16
KARL - FISCHER Analysis Water Content Determination
1 Mercaptans (Thiols) are oxidized by iodine Suppress this side reaction by treating
sample with N-ethylmaleimide in a buffered solvent system
2 Inhibition of Thiol Oxidation by Iodine
Most Phenols do not react with KF reagents higher molecular weight phenols and
Amino-phenols are most problematic Volumetric method is most suitable since the
Oxidation potential is high in coulometry or Acidify Solvent system to reduce interference
Sheth M N Science College Patan Page no - 17
KARL - FISCHER Analysis Water Content Determination
10 Applications of Karl Fischer method for determination of Water in
pharmaceutical analysis
101 PREDNISOLONE SODIUM PHOSPHATE
Materials Required Karl Fischer Reagent 100 ml prednisolone sodium phosphate
02gm anhydrous methanol 200 ml
Procedure Add about 20 ml of anhydrous methanol to the titration vessel and titrate to the
Amperometric End-point with the Karl Fischer reagent Quickly add 02 g of prednisolone
sodium phosphate sample stir for 1 minute and again titrate to the amperometric end-point
with the Karl Fischer reagent The difference between the two titrations gives the volume
(v) of Karl Fischer reagent consumed by the sample
The minimum water equivalent is 35 mg of water per ml of Karl Fischer reagent
Hence the percentage of water ww in the given sample may be calculated by the
following
Precautions
(1) The reagents and solutions used must be kept anhydrous and necessary care should be
taken throughout to prevent exposure to atmospheric moisture
(2) The Karl Fischer reagent should be protected from light and preferably stored in a bottle
fitted with an automatic burette and
(3) The water equivalent of Karl Fischer reagent should always be determined before use
102 COGNATE ASSAYS
A number of other official pharmaceutical substances may be assayed for their water
content by the Karl Fischer method as summarized in the following Table
Sheth M N Science College Patan Page no - 18
KARL - FISCHER Analysis Water Content Determination
11 Automated Electrochemical Karl Fischer Analysis
Commercially available Modern KF-Titrators are usually equipped with specifically
designed titration vessels that are exclusively meant to check and prevent the contact with
atmospheric moisture Quite a few such devices are armed with microprocessors that will
perform the requisite operations sequentially in a programmed manner automatically and
may also dish out a print-out of the desired results including the percentage moisture
content In fact these Modern KF-Titrators not only afford greater accuracy and precision
in results but also offer much ease and convenience in routine analysis as compared to the
classical techniques based on either caulometry or controlled current potentiometry using
two indicator electrodes
Thus the basis of the analysis rests upon the quantitative relationship existing
between charge passed and iodine produced by the reagent according to the above reaction
Therefore the generation of iodine is automatically stopped when an excess of it is detected
by the indicator electrode It essentially consists of two platinum electrodes across which an
AC is applied and subsequently a marked drop in voltage between the electrodes takes place
as soon as an excess of iodine is present Normally such automated instruments make use of
proprietory reagents exclusively
The major advantage of this approach to KF-analysis being that no calibration is
required as the method is absolute and is entirely based on the stoichiometry of the
aforesaid equation It is noteworthy that one may determine the amounts of water ranging
between 10 mcg and 10 mg in solid as well as liquid samples
Sheth M N Science College Patan Page no - 19
KARL - FISCHER Analysis Water Content Determination
12 Some of the innovative brands in Karl Fischer Auto ndash titrator
instruments
Sheth M N Science College Patan Page no - 20
KARL - FISCHER Analysis Water Content Determination
Sheth M N Science College Patan Page no - 21
KARL - FISCHER Analysis Water Content Determination
13 References
1 Chapter 14 Karl fishcer method for determination of water Page no 223-227
Pharmaceutical Drug Analysis Ashutosh Kar New age international limited publishers
2 Aqua Stars Karl fischer instruments wwwemdchemicalscom
3 Karl Fischer Volumetric Titration Theory and Practice by Radiometer analytical
SAS wwwradiometer-analyticalcom
4 MettlerToledo AG Analytical Switzerland wwwmtcom
5 Summary of Karl fischer titrations by Orion products Baverly MA Thermofischer
Scientific wwwthermocomwater
6 Moisture determination by Karl Fischer titration background of chemistry and
recent advances SupelcoSigma-Aldrich Bellefonte Pa wwwsigma-aldrichcom
7 Karl Fischer Titration Methods Methodology and water determination in 442
different substances European Pharmacopoeia (PharmEurope) 4th Edition plus
Supplements 2002 International Electrotechnical Commission Geneva
8 Karl ndash fischer titration wwwwikipediacom
Sheth M N Science College Patan Page no - 22
KARL - FISCHER Analysis Water Content Determination
8 Interference and Limitations of Karl Fischer Method
Interferences in the water determinations by Karl Fischer method are associated with
condensation or oxidation ndash reduction reactions with a number of substances and
compounds or by side reactions Other possible interferences to the KF reagent are certain
active metals metal oxides metal hydroxides chromates melamines etc
The Karl Fischer titration has a number of serious limitations due to possible
interferences tantamount to erroneous results namely
(i) Oxidizing agents for instance chromates Cu(II) Fe(III) Cr2O7 2ndash peroxides salts
higher oxides
Example MnO2 + 4C5H5NH+ + 2Indash rarr Mn2+ + 4C5H5N + I2 + H2O
(ii) Reducing agents such as Sn (II) salts sulphides and S2O32ndash and
(iii) Compounds that have a tendency to form water with the ingredients of the Karl Fischer
reagent for instance
(a) Basic oxides eg ZnO
Example ZnO + 2C5H5NH+ rarr Zn2+ + C5H5N + H2O
(b) Salts of weak oxy-acids eg NaHCO3
Example NaHCO3 + C5H5NH+ rarr Na+ + H2O + CO2 + C5H5N
[Note As H2CO3 carbonic acid is very unstable hence it splits up to yield a mole each of
water and CO2]
Use of certain reagents will minimize or eliminate the interferences of many classes
of compounds For example the use of non-methanol containing KF reagent and solvent will
reduce the interference from aldehyde and ketones Ammonia and amines can be eliminated
by the addition of salicylic acid to the solvent
Sheth M N Science College Patan Page no - 15
KARL - FISCHER Analysis Water Content Determination
9 Side reactions
Side Reactions are classified as
(1) Reactions Influenced by Methanol
(2) Reactions with Iodine
(1) Reactions Influenced by Methanol
It includes
bull Aldehydes react to form acetals also undergo Bisulfite addition
bull Ketones react to form ketals
bull Amines undergo methylation
bull Siloxanes undergo esterification
(2) Reactions with Iodine
It includes
bull Halogenated Hydrocarbons containing free halogens
bull Mercaptans (Thiols)
bull Phenols
bull Peroxides
Testing for Iodine Reactivity
bull Dissolve a few Iodine Crystals in Methanol and Introduce Sample
bull Watch for Solution to Change Color if Clear Iodine Reaction is Present
Sheth M N Science College Patan Page no - 16
KARL - FISCHER Analysis Water Content Determination
1 Mercaptans (Thiols) are oxidized by iodine Suppress this side reaction by treating
sample with N-ethylmaleimide in a buffered solvent system
2 Inhibition of Thiol Oxidation by Iodine
Most Phenols do not react with KF reagents higher molecular weight phenols and
Amino-phenols are most problematic Volumetric method is most suitable since the
Oxidation potential is high in coulometry or Acidify Solvent system to reduce interference
Sheth M N Science College Patan Page no - 17
KARL - FISCHER Analysis Water Content Determination
10 Applications of Karl Fischer method for determination of Water in
pharmaceutical analysis
101 PREDNISOLONE SODIUM PHOSPHATE
Materials Required Karl Fischer Reagent 100 ml prednisolone sodium phosphate
02gm anhydrous methanol 200 ml
Procedure Add about 20 ml of anhydrous methanol to the titration vessel and titrate to the
Amperometric End-point with the Karl Fischer reagent Quickly add 02 g of prednisolone
sodium phosphate sample stir for 1 minute and again titrate to the amperometric end-point
with the Karl Fischer reagent The difference between the two titrations gives the volume
(v) of Karl Fischer reagent consumed by the sample
The minimum water equivalent is 35 mg of water per ml of Karl Fischer reagent
Hence the percentage of water ww in the given sample may be calculated by the
following
Precautions
(1) The reagents and solutions used must be kept anhydrous and necessary care should be
taken throughout to prevent exposure to atmospheric moisture
(2) The Karl Fischer reagent should be protected from light and preferably stored in a bottle
fitted with an automatic burette and
(3) The water equivalent of Karl Fischer reagent should always be determined before use
102 COGNATE ASSAYS
A number of other official pharmaceutical substances may be assayed for their water
content by the Karl Fischer method as summarized in the following Table
Sheth M N Science College Patan Page no - 18
KARL - FISCHER Analysis Water Content Determination
11 Automated Electrochemical Karl Fischer Analysis
Commercially available Modern KF-Titrators are usually equipped with specifically
designed titration vessels that are exclusively meant to check and prevent the contact with
atmospheric moisture Quite a few such devices are armed with microprocessors that will
perform the requisite operations sequentially in a programmed manner automatically and
may also dish out a print-out of the desired results including the percentage moisture
content In fact these Modern KF-Titrators not only afford greater accuracy and precision
in results but also offer much ease and convenience in routine analysis as compared to the
classical techniques based on either caulometry or controlled current potentiometry using
two indicator electrodes
Thus the basis of the analysis rests upon the quantitative relationship existing
between charge passed and iodine produced by the reagent according to the above reaction
Therefore the generation of iodine is automatically stopped when an excess of it is detected
by the indicator electrode It essentially consists of two platinum electrodes across which an
AC is applied and subsequently a marked drop in voltage between the electrodes takes place
as soon as an excess of iodine is present Normally such automated instruments make use of
proprietory reagents exclusively
The major advantage of this approach to KF-analysis being that no calibration is
required as the method is absolute and is entirely based on the stoichiometry of the
aforesaid equation It is noteworthy that one may determine the amounts of water ranging
between 10 mcg and 10 mg in solid as well as liquid samples
Sheth M N Science College Patan Page no - 19
KARL - FISCHER Analysis Water Content Determination
12 Some of the innovative brands in Karl Fischer Auto ndash titrator
instruments
Sheth M N Science College Patan Page no - 20
KARL - FISCHER Analysis Water Content Determination
Sheth M N Science College Patan Page no - 21
KARL - FISCHER Analysis Water Content Determination
13 References
1 Chapter 14 Karl fishcer method for determination of water Page no 223-227
Pharmaceutical Drug Analysis Ashutosh Kar New age international limited publishers
2 Aqua Stars Karl fischer instruments wwwemdchemicalscom
3 Karl Fischer Volumetric Titration Theory and Practice by Radiometer analytical
SAS wwwradiometer-analyticalcom
4 MettlerToledo AG Analytical Switzerland wwwmtcom
5 Summary of Karl fischer titrations by Orion products Baverly MA Thermofischer
Scientific wwwthermocomwater
6 Moisture determination by Karl Fischer titration background of chemistry and
recent advances SupelcoSigma-Aldrich Bellefonte Pa wwwsigma-aldrichcom
7 Karl Fischer Titration Methods Methodology and water determination in 442
different substances European Pharmacopoeia (PharmEurope) 4th Edition plus
Supplements 2002 International Electrotechnical Commission Geneva
8 Karl ndash fischer titration wwwwikipediacom
Sheth M N Science College Patan Page no - 22
KARL - FISCHER Analysis Water Content Determination
9 Side reactions
Side Reactions are classified as
(1) Reactions Influenced by Methanol
(2) Reactions with Iodine
(1) Reactions Influenced by Methanol
It includes
bull Aldehydes react to form acetals also undergo Bisulfite addition
bull Ketones react to form ketals
bull Amines undergo methylation
bull Siloxanes undergo esterification
(2) Reactions with Iodine
It includes
bull Halogenated Hydrocarbons containing free halogens
bull Mercaptans (Thiols)
bull Phenols
bull Peroxides
Testing for Iodine Reactivity
bull Dissolve a few Iodine Crystals in Methanol and Introduce Sample
bull Watch for Solution to Change Color if Clear Iodine Reaction is Present
Sheth M N Science College Patan Page no - 16
KARL - FISCHER Analysis Water Content Determination
1 Mercaptans (Thiols) are oxidized by iodine Suppress this side reaction by treating
sample with N-ethylmaleimide in a buffered solvent system
2 Inhibition of Thiol Oxidation by Iodine
Most Phenols do not react with KF reagents higher molecular weight phenols and
Amino-phenols are most problematic Volumetric method is most suitable since the
Oxidation potential is high in coulometry or Acidify Solvent system to reduce interference
Sheth M N Science College Patan Page no - 17
KARL - FISCHER Analysis Water Content Determination
10 Applications of Karl Fischer method for determination of Water in
pharmaceutical analysis
101 PREDNISOLONE SODIUM PHOSPHATE
Materials Required Karl Fischer Reagent 100 ml prednisolone sodium phosphate
02gm anhydrous methanol 200 ml
Procedure Add about 20 ml of anhydrous methanol to the titration vessel and titrate to the
Amperometric End-point with the Karl Fischer reagent Quickly add 02 g of prednisolone
sodium phosphate sample stir for 1 minute and again titrate to the amperometric end-point
with the Karl Fischer reagent The difference between the two titrations gives the volume
(v) of Karl Fischer reagent consumed by the sample
The minimum water equivalent is 35 mg of water per ml of Karl Fischer reagent
Hence the percentage of water ww in the given sample may be calculated by the
following
Precautions
(1) The reagents and solutions used must be kept anhydrous and necessary care should be
taken throughout to prevent exposure to atmospheric moisture
(2) The Karl Fischer reagent should be protected from light and preferably stored in a bottle
fitted with an automatic burette and
(3) The water equivalent of Karl Fischer reagent should always be determined before use
102 COGNATE ASSAYS
A number of other official pharmaceutical substances may be assayed for their water
content by the Karl Fischer method as summarized in the following Table
Sheth M N Science College Patan Page no - 18
KARL - FISCHER Analysis Water Content Determination
11 Automated Electrochemical Karl Fischer Analysis
Commercially available Modern KF-Titrators are usually equipped with specifically
designed titration vessels that are exclusively meant to check and prevent the contact with
atmospheric moisture Quite a few such devices are armed with microprocessors that will
perform the requisite operations sequentially in a programmed manner automatically and
may also dish out a print-out of the desired results including the percentage moisture
content In fact these Modern KF-Titrators not only afford greater accuracy and precision
in results but also offer much ease and convenience in routine analysis as compared to the
classical techniques based on either caulometry or controlled current potentiometry using
two indicator electrodes
Thus the basis of the analysis rests upon the quantitative relationship existing
between charge passed and iodine produced by the reagent according to the above reaction
Therefore the generation of iodine is automatically stopped when an excess of it is detected
by the indicator electrode It essentially consists of two platinum electrodes across which an
AC is applied and subsequently a marked drop in voltage between the electrodes takes place
as soon as an excess of iodine is present Normally such automated instruments make use of
proprietory reagents exclusively
The major advantage of this approach to KF-analysis being that no calibration is
required as the method is absolute and is entirely based on the stoichiometry of the
aforesaid equation It is noteworthy that one may determine the amounts of water ranging
between 10 mcg and 10 mg in solid as well as liquid samples
Sheth M N Science College Patan Page no - 19
KARL - FISCHER Analysis Water Content Determination
12 Some of the innovative brands in Karl Fischer Auto ndash titrator
instruments
Sheth M N Science College Patan Page no - 20
KARL - FISCHER Analysis Water Content Determination
Sheth M N Science College Patan Page no - 21
KARL - FISCHER Analysis Water Content Determination
13 References
1 Chapter 14 Karl fishcer method for determination of water Page no 223-227
Pharmaceutical Drug Analysis Ashutosh Kar New age international limited publishers
2 Aqua Stars Karl fischer instruments wwwemdchemicalscom
3 Karl Fischer Volumetric Titration Theory and Practice by Radiometer analytical
SAS wwwradiometer-analyticalcom
4 MettlerToledo AG Analytical Switzerland wwwmtcom
5 Summary of Karl fischer titrations by Orion products Baverly MA Thermofischer
Scientific wwwthermocomwater
6 Moisture determination by Karl Fischer titration background of chemistry and
recent advances SupelcoSigma-Aldrich Bellefonte Pa wwwsigma-aldrichcom
7 Karl Fischer Titration Methods Methodology and water determination in 442
different substances European Pharmacopoeia (PharmEurope) 4th Edition plus
Supplements 2002 International Electrotechnical Commission Geneva
8 Karl ndash fischer titration wwwwikipediacom
Sheth M N Science College Patan Page no - 22
KARL - FISCHER Analysis Water Content Determination
1 Mercaptans (Thiols) are oxidized by iodine Suppress this side reaction by treating
sample with N-ethylmaleimide in a buffered solvent system
2 Inhibition of Thiol Oxidation by Iodine
Most Phenols do not react with KF reagents higher molecular weight phenols and
Amino-phenols are most problematic Volumetric method is most suitable since the
Oxidation potential is high in coulometry or Acidify Solvent system to reduce interference
Sheth M N Science College Patan Page no - 17
KARL - FISCHER Analysis Water Content Determination
10 Applications of Karl Fischer method for determination of Water in
pharmaceutical analysis
101 PREDNISOLONE SODIUM PHOSPHATE
Materials Required Karl Fischer Reagent 100 ml prednisolone sodium phosphate
02gm anhydrous methanol 200 ml
Procedure Add about 20 ml of anhydrous methanol to the titration vessel and titrate to the
Amperometric End-point with the Karl Fischer reagent Quickly add 02 g of prednisolone
sodium phosphate sample stir for 1 minute and again titrate to the amperometric end-point
with the Karl Fischer reagent The difference between the two titrations gives the volume
(v) of Karl Fischer reagent consumed by the sample
The minimum water equivalent is 35 mg of water per ml of Karl Fischer reagent
Hence the percentage of water ww in the given sample may be calculated by the
following
Precautions
(1) The reagents and solutions used must be kept anhydrous and necessary care should be
taken throughout to prevent exposure to atmospheric moisture
(2) The Karl Fischer reagent should be protected from light and preferably stored in a bottle
fitted with an automatic burette and
(3) The water equivalent of Karl Fischer reagent should always be determined before use
102 COGNATE ASSAYS
A number of other official pharmaceutical substances may be assayed for their water
content by the Karl Fischer method as summarized in the following Table
Sheth M N Science College Patan Page no - 18
KARL - FISCHER Analysis Water Content Determination
11 Automated Electrochemical Karl Fischer Analysis
Commercially available Modern KF-Titrators are usually equipped with specifically
designed titration vessels that are exclusively meant to check and prevent the contact with
atmospheric moisture Quite a few such devices are armed with microprocessors that will
perform the requisite operations sequentially in a programmed manner automatically and
may also dish out a print-out of the desired results including the percentage moisture
content In fact these Modern KF-Titrators not only afford greater accuracy and precision
in results but also offer much ease and convenience in routine analysis as compared to the
classical techniques based on either caulometry or controlled current potentiometry using
two indicator electrodes
Thus the basis of the analysis rests upon the quantitative relationship existing
between charge passed and iodine produced by the reagent according to the above reaction
Therefore the generation of iodine is automatically stopped when an excess of it is detected
by the indicator electrode It essentially consists of two platinum electrodes across which an
AC is applied and subsequently a marked drop in voltage between the electrodes takes place
as soon as an excess of iodine is present Normally such automated instruments make use of
proprietory reagents exclusively
The major advantage of this approach to KF-analysis being that no calibration is
required as the method is absolute and is entirely based on the stoichiometry of the
aforesaid equation It is noteworthy that one may determine the amounts of water ranging
between 10 mcg and 10 mg in solid as well as liquid samples
Sheth M N Science College Patan Page no - 19
KARL - FISCHER Analysis Water Content Determination
12 Some of the innovative brands in Karl Fischer Auto ndash titrator
instruments
Sheth M N Science College Patan Page no - 20
KARL - FISCHER Analysis Water Content Determination
Sheth M N Science College Patan Page no - 21
KARL - FISCHER Analysis Water Content Determination
13 References
1 Chapter 14 Karl fishcer method for determination of water Page no 223-227
Pharmaceutical Drug Analysis Ashutosh Kar New age international limited publishers
2 Aqua Stars Karl fischer instruments wwwemdchemicalscom
3 Karl Fischer Volumetric Titration Theory and Practice by Radiometer analytical
SAS wwwradiometer-analyticalcom
4 MettlerToledo AG Analytical Switzerland wwwmtcom
5 Summary of Karl fischer titrations by Orion products Baverly MA Thermofischer
Scientific wwwthermocomwater
6 Moisture determination by Karl Fischer titration background of chemistry and
recent advances SupelcoSigma-Aldrich Bellefonte Pa wwwsigma-aldrichcom
7 Karl Fischer Titration Methods Methodology and water determination in 442
different substances European Pharmacopoeia (PharmEurope) 4th Edition plus
Supplements 2002 International Electrotechnical Commission Geneva
8 Karl ndash fischer titration wwwwikipediacom
Sheth M N Science College Patan Page no - 22
KARL - FISCHER Analysis Water Content Determination
10 Applications of Karl Fischer method for determination of Water in
pharmaceutical analysis
101 PREDNISOLONE SODIUM PHOSPHATE
Materials Required Karl Fischer Reagent 100 ml prednisolone sodium phosphate
02gm anhydrous methanol 200 ml
Procedure Add about 20 ml of anhydrous methanol to the titration vessel and titrate to the
Amperometric End-point with the Karl Fischer reagent Quickly add 02 g of prednisolone
sodium phosphate sample stir for 1 minute and again titrate to the amperometric end-point
with the Karl Fischer reagent The difference between the two titrations gives the volume
(v) of Karl Fischer reagent consumed by the sample
The minimum water equivalent is 35 mg of water per ml of Karl Fischer reagent
Hence the percentage of water ww in the given sample may be calculated by the
following
Precautions
(1) The reagents and solutions used must be kept anhydrous and necessary care should be
taken throughout to prevent exposure to atmospheric moisture
(2) The Karl Fischer reagent should be protected from light and preferably stored in a bottle
fitted with an automatic burette and
(3) The water equivalent of Karl Fischer reagent should always be determined before use
102 COGNATE ASSAYS
A number of other official pharmaceutical substances may be assayed for their water
content by the Karl Fischer method as summarized in the following Table
Sheth M N Science College Patan Page no - 18
KARL - FISCHER Analysis Water Content Determination
11 Automated Electrochemical Karl Fischer Analysis
Commercially available Modern KF-Titrators are usually equipped with specifically
designed titration vessels that are exclusively meant to check and prevent the contact with
atmospheric moisture Quite a few such devices are armed with microprocessors that will
perform the requisite operations sequentially in a programmed manner automatically and
may also dish out a print-out of the desired results including the percentage moisture
content In fact these Modern KF-Titrators not only afford greater accuracy and precision
in results but also offer much ease and convenience in routine analysis as compared to the
classical techniques based on either caulometry or controlled current potentiometry using
two indicator electrodes
Thus the basis of the analysis rests upon the quantitative relationship existing
between charge passed and iodine produced by the reagent according to the above reaction
Therefore the generation of iodine is automatically stopped when an excess of it is detected
by the indicator electrode It essentially consists of two platinum electrodes across which an
AC is applied and subsequently a marked drop in voltage between the electrodes takes place
as soon as an excess of iodine is present Normally such automated instruments make use of
proprietory reagents exclusively
The major advantage of this approach to KF-analysis being that no calibration is
required as the method is absolute and is entirely based on the stoichiometry of the
aforesaid equation It is noteworthy that one may determine the amounts of water ranging
between 10 mcg and 10 mg in solid as well as liquid samples
Sheth M N Science College Patan Page no - 19
KARL - FISCHER Analysis Water Content Determination
12 Some of the innovative brands in Karl Fischer Auto ndash titrator
instruments
Sheth M N Science College Patan Page no - 20
KARL - FISCHER Analysis Water Content Determination
Sheth M N Science College Patan Page no - 21
KARL - FISCHER Analysis Water Content Determination
13 References
1 Chapter 14 Karl fishcer method for determination of water Page no 223-227
Pharmaceutical Drug Analysis Ashutosh Kar New age international limited publishers
2 Aqua Stars Karl fischer instruments wwwemdchemicalscom
3 Karl Fischer Volumetric Titration Theory and Practice by Radiometer analytical
SAS wwwradiometer-analyticalcom
4 MettlerToledo AG Analytical Switzerland wwwmtcom
5 Summary of Karl fischer titrations by Orion products Baverly MA Thermofischer
Scientific wwwthermocomwater
6 Moisture determination by Karl Fischer titration background of chemistry and
recent advances SupelcoSigma-Aldrich Bellefonte Pa wwwsigma-aldrichcom
7 Karl Fischer Titration Methods Methodology and water determination in 442
different substances European Pharmacopoeia (PharmEurope) 4th Edition plus
Supplements 2002 International Electrotechnical Commission Geneva
8 Karl ndash fischer titration wwwwikipediacom
Sheth M N Science College Patan Page no - 22
KARL - FISCHER Analysis Water Content Determination
11 Automated Electrochemical Karl Fischer Analysis
Commercially available Modern KF-Titrators are usually equipped with specifically
designed titration vessels that are exclusively meant to check and prevent the contact with
atmospheric moisture Quite a few such devices are armed with microprocessors that will
perform the requisite operations sequentially in a programmed manner automatically and
may also dish out a print-out of the desired results including the percentage moisture
content In fact these Modern KF-Titrators not only afford greater accuracy and precision
in results but also offer much ease and convenience in routine analysis as compared to the
classical techniques based on either caulometry or controlled current potentiometry using
two indicator electrodes
Thus the basis of the analysis rests upon the quantitative relationship existing
between charge passed and iodine produced by the reagent according to the above reaction
Therefore the generation of iodine is automatically stopped when an excess of it is detected
by the indicator electrode It essentially consists of two platinum electrodes across which an
AC is applied and subsequently a marked drop in voltage between the electrodes takes place
as soon as an excess of iodine is present Normally such automated instruments make use of
proprietory reagents exclusively
The major advantage of this approach to KF-analysis being that no calibration is
required as the method is absolute and is entirely based on the stoichiometry of the
aforesaid equation It is noteworthy that one may determine the amounts of water ranging
between 10 mcg and 10 mg in solid as well as liquid samples
Sheth M N Science College Patan Page no - 19
KARL - FISCHER Analysis Water Content Determination
12 Some of the innovative brands in Karl Fischer Auto ndash titrator
instruments
Sheth M N Science College Patan Page no - 20
KARL - FISCHER Analysis Water Content Determination
Sheth M N Science College Patan Page no - 21
KARL - FISCHER Analysis Water Content Determination
13 References
1 Chapter 14 Karl fishcer method for determination of water Page no 223-227
Pharmaceutical Drug Analysis Ashutosh Kar New age international limited publishers
2 Aqua Stars Karl fischer instruments wwwemdchemicalscom
3 Karl Fischer Volumetric Titration Theory and Practice by Radiometer analytical
SAS wwwradiometer-analyticalcom
4 MettlerToledo AG Analytical Switzerland wwwmtcom
5 Summary of Karl fischer titrations by Orion products Baverly MA Thermofischer
Scientific wwwthermocomwater
6 Moisture determination by Karl Fischer titration background of chemistry and
recent advances SupelcoSigma-Aldrich Bellefonte Pa wwwsigma-aldrichcom
7 Karl Fischer Titration Methods Methodology and water determination in 442
different substances European Pharmacopoeia (PharmEurope) 4th Edition plus
Supplements 2002 International Electrotechnical Commission Geneva
8 Karl ndash fischer titration wwwwikipediacom
Sheth M N Science College Patan Page no - 22
KARL - FISCHER Analysis Water Content Determination
12 Some of the innovative brands in Karl Fischer Auto ndash titrator
instruments
Sheth M N Science College Patan Page no - 20
KARL - FISCHER Analysis Water Content Determination
Sheth M N Science College Patan Page no - 21
KARL - FISCHER Analysis Water Content Determination
13 References
1 Chapter 14 Karl fishcer method for determination of water Page no 223-227
Pharmaceutical Drug Analysis Ashutosh Kar New age international limited publishers
2 Aqua Stars Karl fischer instruments wwwemdchemicalscom
3 Karl Fischer Volumetric Titration Theory and Practice by Radiometer analytical
SAS wwwradiometer-analyticalcom
4 MettlerToledo AG Analytical Switzerland wwwmtcom
5 Summary of Karl fischer titrations by Orion products Baverly MA Thermofischer
Scientific wwwthermocomwater
6 Moisture determination by Karl Fischer titration background of chemistry and
recent advances SupelcoSigma-Aldrich Bellefonte Pa wwwsigma-aldrichcom
7 Karl Fischer Titration Methods Methodology and water determination in 442
different substances European Pharmacopoeia (PharmEurope) 4th Edition plus
Supplements 2002 International Electrotechnical Commission Geneva
8 Karl ndash fischer titration wwwwikipediacom
Sheth M N Science College Patan Page no - 22
KARL - FISCHER Analysis Water Content Determination
Sheth M N Science College Patan Page no - 21
KARL - FISCHER Analysis Water Content Determination
13 References
1 Chapter 14 Karl fishcer method for determination of water Page no 223-227
Pharmaceutical Drug Analysis Ashutosh Kar New age international limited publishers
2 Aqua Stars Karl fischer instruments wwwemdchemicalscom
3 Karl Fischer Volumetric Titration Theory and Practice by Radiometer analytical
SAS wwwradiometer-analyticalcom
4 MettlerToledo AG Analytical Switzerland wwwmtcom
5 Summary of Karl fischer titrations by Orion products Baverly MA Thermofischer
Scientific wwwthermocomwater
6 Moisture determination by Karl Fischer titration background of chemistry and
recent advances SupelcoSigma-Aldrich Bellefonte Pa wwwsigma-aldrichcom
7 Karl Fischer Titration Methods Methodology and water determination in 442
different substances European Pharmacopoeia (PharmEurope) 4th Edition plus
Supplements 2002 International Electrotechnical Commission Geneva
8 Karl ndash fischer titration wwwwikipediacom
Sheth M N Science College Patan Page no - 22
KARL - FISCHER Analysis Water Content Determination
13 References
1 Chapter 14 Karl fishcer method for determination of water Page no 223-227
Pharmaceutical Drug Analysis Ashutosh Kar New age international limited publishers
2 Aqua Stars Karl fischer instruments wwwemdchemicalscom
3 Karl Fischer Volumetric Titration Theory and Practice by Radiometer analytical
SAS wwwradiometer-analyticalcom
4 MettlerToledo AG Analytical Switzerland wwwmtcom
5 Summary of Karl fischer titrations by Orion products Baverly MA Thermofischer
Scientific wwwthermocomwater
6 Moisture determination by Karl Fischer titration background of chemistry and
recent advances SupelcoSigma-Aldrich Bellefonte Pa wwwsigma-aldrichcom
7 Karl Fischer Titration Methods Methodology and water determination in 442
different substances European Pharmacopoeia (PharmEurope) 4th Edition plus
Supplements 2002 International Electrotechnical Commission Geneva
8 Karl ndash fischer titration wwwwikipediacom
Sheth M N Science College Patan Page no - 22
KARL - FISCHER Analysis Water Content Determination