KARL FISCHER MOISTURE DETERMINATION 1

Karl Fischer MoistureDetermination

Katrin SchoffskiSigma–Aldrich Laborchemikalien, Seelze,Germany

Dieter StrohmMetrohm Ltd, Herisau, Switzerland

1 Introduction 1

2 History 2

3 Definitions 3

4 Instrumentation 44.1 The Titration Stand 44.2 Volumetric Karl Fischer Titration 44.3 Coulometric Karl Fischer

Titration 54.4 Thermostatic Titration Cells 54.5 Karl Fischer Oven 54.6 Automation of the Volumetric Karl

Fischer Titration 54.7 Oven Sample Processor 6

5 Reagents 65.1 One-component Reagent 65.2 Two-component Reagents 65.3 Auxiliary Products 7

6 Standardization, Calculation and Controlof the Results 76.1 Titer Determination 76.2 Sample Size and Calculation of the

Water Content 76.3 Control of the Result 76.4 Interferences 8

7 Sample Preparation 87.1 Solids 87.2 Liquids 97.3 Gases 97.4 Oven Method 9

8 Important Applications 98.1 Chemicals 98.2 Pharmaceutical Products 108.3 Petroleum Products 118.4 Plastics 118.5 Foodstuffs 12

9 International Standard Procedures 12

10 Comparison with Loss-on-drying 12

Abbreviations and Acronyms 13

Related Articles 13

References 13

The Karl Fischer (KF) titration is a method of determiningthe water content of solid, liquid and gaseous samples. It isthe technique preferred for use in industrial quality control.In principle it involves the oxidation of sulfur dioxide byiodine, in the presence of water, in a buffered solution.An alcohol is used as the preferred solvent. The water isconverted stoichiometrically and therefore its quantityis determined indirectly. The end-point (EP) is reachedwhen there is an excess of iodine. It can be indicatedvisually, photometrically or electrochemically. Accordingto state-of-the-art technology a double platinum electrodedetermines a voltammetric indication. The KF titration canbe carried out either volumetrically or by coulometry. Fora volumetric titration the iodine is added by volume tothe titration cell containing the sample. In a coulometrictitration iodide is oxidized at a platinum electrode andthe iodine formed reacts with the water. The amount ofcurrent necessary in the generation of the iodine is directlyrelated to the quantity of iodine generated, according toFaraday’s first law. In practice, special titrators for the KFtitration are available. Preformulated reagents are similarlyavailable ready for use. Water in an extensive number ofmaterials can be determined by KF titration. Chemicals,pharmaceuticals, oils, plastics and foodstuffs are examplesof typical sample types. The measurement range spans afew ppm to 100% water. The KF titration is describedin ISO 760 and as part of many other internationalstandard procedures. Other methods for the determinationof water content include loss-on-drying, IR spectroscopyor azeotropic distillation.

1 INTRODUCTION

The water content of various products is an importantquality consideration in industrial processes, and, for araw material, is often taken as a correction factor in themeasure of its worth. For example, for oils and cereals,the proportion of water in the gross weight of a deliveryis taken into consideration in the price. For intermediateproducts, such as plastic granulates or tablet powders, thewater content influences the mechanical characteristics,e.g. the flow capability in a press. In the final productof a foodstuff the water content must sometimes bedetermined by law and can also influence the storagestability. For these reasons an exact determination of thewater component of the material is necessary.

It is necessary to distinguish between moisture andwater determination. To be exact, moisture is referred to

Encyclopedia of Analytical ChemistryR.A. Meyers (Ed.) Copyright John Wiley & Sons Ltd

14289

2 GENERAL ARTICLES

as bound water. It can also be used to describe a boundsolvent or an evaporated substance. In general, moistureis determined by using an oven. Water, on the other hand,is determined chemically by the KF titration. Both surfacewater and water bound in crystals can be determined inthis way.

Water determination according to the KF method is ananalytical technique for determining the water contentin various matrixes. Chemically, it involves the oxidationof sulfur dioxide by iodine, with the consumption ofwater, in a buffered solution. Organic solvents are used asthe solvent for the sample as well as for the workingmedium in the titration cell. Methanol, ethanol andhigher propylene glycol mixtures are most often used.The KF titration can be carried out both volumetricallyand coulometrically.

The volumetric titration is carried out with a one- ortwo-component reagent. When a one-component reagentis used, the buret of the titrator contains all the necessaryreagents dissolved in an inert solvent. Methanol or amixture of alcohols serves as the working medium in thetitration cell. This is then pretitrated by the KF reagentin order to remove any traces of water from the cell andworking medium. Then the sample is added and its watercontent determined automatically.

In a two-component reagent the reactive compo-nents are separated. The so-called ‘‘titrant’’ containsa preset amount of iodine in a solvent, which can beeither methanol or ethanol. The buret of the titra-tor is then filled with this solution. A base and sulfurdioxide are dissolved in the ‘‘KF solvent’’, which isadded to the titration cell and pretitrated to dryness.The sample is then added and its water content deter-mined.

The one-component reagent has the advantage thatit can be used with a wide variety of different working

6

EP (b)

(a)4

2

02 4 6 8

Vol

ume

of r

eage

nt

adde

d (m

L)

Time (min)

Figure 1 Typical Karl Fischer titration curve. (a) Reagentadding mode; (b) conditioning mode.

media. The two-component reagent is distinguished byits faster titration rate and greater polarity. Typical watercontents for volumetric KF determinations are between0.5 and 50 mg water. Figure 1 shows a typical KF titrationcurve of time vs reagent added.

A different composition is used for KF coulometry. Thereagents contain a solution of iodide rather than iodine,which is then generated from the iodide at a platinumelectrode. The iodine then reacts with sulfur dioxideaccording to the KF chemistry, with the loss of water.The current necessary to generate the iodine is measured.The amount of iodine is determined using Faraday’slaw and the water content consequently calculated. Tocarry out a coulometric determination the reagents areadded to the titration cell. These solutions must firstbe pretitrated as in the volumetric method. Any waterin the titration cell or dissolved by the reagent fromthe air is thereby removed. Since coulometry is moresensitive, this pretitration process takes longer than avolumetric titration. The sample is injected, via a septum,into the reagent. Modern titrators calculate the amount ofwater automatically after injection of the sample. Typicalabsolute values of water analyzed by coulometry arebetween 50 and 2000 µg.

2 HISTORY

In 1935 the German chemist Karl Fischer was required todetermine the water content of liquid sulfur dioxide.Neither loss-on-drying nor distillation were suitablefor liquid samples, and he had to find an alternativemethod. During his investigations he stumbled acrossthe Bunsen reaction, Equation (1), which describesthe iodometric determination of sulfate in a buffered,aqueous solution.

SO2 C I2 C 2H2O(H) H2SO4 C 2HI .1/

According to Equation (1), a determination of thewater content should also be possible if the sulfurdioxide is present in excess and the protons that areconsequently produced are taken up by a base. Fischerput together his reagent using sulfur dioxide, iodineand pyridine, with methanol as solvent. He formulatedEquation (2),.1/

SO2 C I2 C 2H2OŁPy(H) H2SO4 C 2HIŁPy .2/

where Py represents pyridine. The EP was indicated bythe color change from yellow to brown. Using this one-component reagent, he carried out the first volumetricKF titrations and determined the water content of sulfurdioxide and various different solvents.

14290

KARL FISCHER MOISTURE DETERMINATION 3

In 1939 the reaction mechanism was re-examinedby an American research group. Smith et al. for-mulated the two-step reaction, Equation (3), witha different molar relationship of water to iodineof 1 : 1,.2/

H2OC SO2ŁPyC PyŁI2

C PyCMeOH(H) 2HIŁPyC PyŁSO3 .3a/

PyŁSO3 CMeOH(H) PyŁMeSO3H .3b/

where MeOH represents methanol. By 1943 Wernimontand Hopkinson had introduced the electrochemical dead-stop EP detection for the KF titration..3/

During the years that followed, there were furtherimprovements in KF reagents,.4/ but the next bigstep came in 1959 when Meyer and Boyd publishedthe coulometric KF titration..5/ Further investigationswere published in the 1970s concerning the kineticsof the reaction. In 1974 Cedergren discovered thatthe reaction rate is dependent on the concentrationof iodine, sulfur dioxide and water..6/ Verhoef andBarendrecht found that pyridine functioned only as abase and was not part of the reaction itself..7/ Fromthese observations Scholz formulated Equation (4), theversion of the reaction equation that has been acceptedsince 1984,.8/

SO2 CHO-RC B(H) R-SO3� C BHC .4a/

R-SO3� C I2 CH2OC 2B(H) R-SO4

� C 2I� C 2BHC

.4b/

where B is a base. An alkyl sulfite is formed within thereagent: from the alcohol used as solvent and the sulfurdioxide (Equation 4a). Wuensch and Seubert isolated thisreaction product in 1988..9/ The alkyl sulfite is oxidizedby iodine in the presence of water in the second step(Equation 4b). The equations illustrate two essentialpoints. Firstly, in order to guarantee exact stoichiometry,an alcohol must be present in the KF titration. In practice,it is found that approximately 50% methanol or ethanolis necessary in the working medium. Secondly, since thepyridine does not take part directly in the reaction, itshould be possible to replace it by other bases. Pyridinehas been unwelcome in laboratories since the end of the1970s due to its unpleasant odor and the fact that it isharmful. Since the 1980s pyridine-free reagents have beenthe standard.

Comprehensive application support and informationon the titration of many different samples is available inthe form of the monograph the HYDRANAL Manual..10/

Imidazole is used most often as the base in pyridine-freereagents. The reagents manufactured with imidazole asbase have a higher pH because imidazole is a stronger

4

3

2

1

0

2 4 6 8 10

Log

K

pH

Figure 2 The relationship between reaction rate, K, and pH.

base than pyridine. Since the reaction rate, K, of theKF reaction is dependent on the pH (Figure 2), afaster titration and more stable EP can be achieved..11/

By 1998 further advances in the field of the KF waterdetermination included the development of a coulometriccell without a diaphragm, and reagents free of harmfulmaterials..12,13/

3 DEFINITIONS

One-component reagent. A volumetric reagent containingsulfur dioxide, base and iodine in an organic solvent. It isused with working media such as alcohols, or mixtures oforganic solvents and alcohols.Two-component reagent. A volumetric reagent combina-tion consisting of a titrant and solvent. The solutions arenot mixed prior to use but are used separately, in contrastto the former pyridine-containing KF reagents.Titrant. An alcoholic iodine solution.Solvent. The solution containing the base and sulfurdioxide and which is added to the titration cell.Anodic reagent D anolyte. A reagent consisting of iodide,a base and sulfur dioxide in an alcohol. It is the reagentin which the coulometric KF reaction takes place.Cathodic reagent D catholyte. A reagent consisting oforganic salts in an alcohol. It is the reagent in whichthe coulometric cathode reaction takes place, i.e. thereduction reaction. It counteracts the iodide oxidation andnormally involves the reduction of protons to hydrogen.Titer. The water equivalent of the measurement reagentgiven in mg water per ml reagent.Drift. The blank value in the titration cell prior to theaddition of water. Drift is caused by moisture ingressinto the cell and by side-reactions which consume iodine.Modern titrators determine and display the drift auto-matically. The drift is given in different units dependingon the instrument supplier: µg min�1, mg min�1, µL min�1

or µg s�1 are in use.

14291

4 GENERAL ARTICLES

4 INSTRUMENTATION

Due to the special qualities of the water content determi-nation by KF titration it is perhaps understandable thatspecific analytical instruments for this titration have beenon offer for many years. As a result of the enormousdevelopments in the field of microprocessor technology,many of these analytical instruments are now drivenby microprocessors. In practice, there are two differentinstruments accepted for the KF water determination: thevolumetric KF titrator and the coulometric KF titrator.Both techniques use the EP titration method, and thetitration stand consisting of the titration cell and stirrerplays a particularly critical role.

4.1 The Titration Stand

The atmospheric moisture present in every laboratory is acommon source of error in KF titration. This moisture canpenetrate the sample, titration reagent and titration cell,and thereby cause incorrect results. Modern analyticalinstruments for the KF water determination (such as thatin Figure 3) reduce the influence of atmospheric moistureas far as possible by ensuring that the following measuresare incorporated into the titration stand.

The stand is particularly airtight and has special inputports for the addition of solid, liquid or gas samples. Inorder to minimize the ingress of atmospheric moisturewhen changing the reagents, a pump is often integratedinto the stand. This pump can suck out used reagentand pump in fresh without having to open the titrationcell. During the pumping, it is impossible to avoid airbeing sucked into the titration cell. Moisture ingress inthis way is normally avoided by passing the air over amolecular sieve. No titration stand is absolutely closedto the atmosphere and very small quantities of water canalways seep in. Modern KF titrators continually condition

Figure 3 A typical, automated volumetric Karl Fischer titrator.

the titration cell and determine the quantity of waterthat has seeped into the cell over a particular period oftime. This value is displayed as drift, e.g. as µg H2O orµL of titration reagent required per minute. The driftis continually on display and the user therefore has aconstant measure of the condition of the titration stand.The drift value is also measured at the beginning of thetitration and it can therefore be taken into considerationin the final result.

As with all EP titrations, good mixing of the reagentsis an important prerequisite for a fast and exact KFtitration. Therefore a magnetic stirrer is built into thetitration stand of a modern analytical instrument for KFwater determination.

4.2 Volumetric Karl Fischer Titration

KF titrators of today are, without exception, fullyautomatic. The water determination is carried outindependently by the press of a button, with the watercontent calculated and displayed or printed out, accordingto good laboratory practice (GLP), on an attachedprinter. To ensure that an automatic titration is possiblethe volumetric KF titrator has the following essentialcomponents.

4.2.1 Motorized Volumetric Buret with a Titration Stand

This buret enables a precise dosing of the titration rea-gent. To ensure that the hub can accurately add reagentin the microliter range, it is set up with 10 000 steps.

4.2.2 Exchange Unit

The exchange unit was developed to dose the titrationreagent. It consists of a cylinder, buret, tap, buret tipand the stock bottle containing the titration reagent. Inorder to protect the reagent from ingress of atmosphericmoisture, the stock bottle is fitted with a drying tube.

4.2.3 Indication System

Normally a bi-voltammetric indication is used as theindication system for the EP of a volumetric KF titration.For this a double platinum electrode with a constantcurrent (e.g. 50 µA) is used. Whilst iodine is reacting withwater in the titration cell, there is no free iodine in thetitration solution. A high voltage is necessary to maintainthe preset polarization current of the electrode. As soonas all the iodine has reacted with the water, free iodinewill remain in the titration solution. This free iodineconsiderably reduces the voltage, which is necessary tomaintain the preset polarization current.

14292

KARL FISCHER MOISTURE DETERMINATION 5

4.2.4 Controls for the End-point Titration

In a KF titration the titration reagent must be added asfast as possible from the buret and exchange unit, and theaddition should be stopped exactly at the EP. The controlof modern KF titrators adapts itself to the course of thetitration curve and thus leads to short titration times andvery accurate results.

4.2.5 Integrated Communications

A display and keypad are available to the user in order toinput control parameters, sample data for the calculationand the documentation and to be able to read the results.Top-of-the-range KF titrators display a live titration curvein the display, which can give the user useful informationabout the course of the titration.

4.2.6 Method of Storage

A storage capability is necessary to save preset KFtitration methods. In each laboratory at least two methodsare necessary: the titer determination and the watercontent determination. If various different samples areto be analyzed the optimum titration conditions can besaved for each different sample type. Then all that has tobe done is to call up the right sample, add the sample andstart the determination.

4.2.7 Interfaces

Interfaces for a balance and data system (laboratoryinformation management system (LIMS)) enable theKF titrator to be integrated into the modern analyticallaboratory.

4.3 Coulometric Karl Fischer Titration

Coulometric KF titrators are also exclusively automatic.The water determination is carried out independently atthe press of a button, with the water content calculatedand displayed and documented on a printer connectedto the titrator. They are differentiated from volumetrictitrators by the following aspects. Instead of the motorizedburet to dose the iodine solution, coulometers have agenerator current circuit such that iodine can be generatedelectrochemically. For the iodine production there isa generator in the cell rather than the buret tip. Inpractice there are two types of generator electrode: thosewith and those without a diaphragm. Wherever possiblea generator without a diaphragm is preferred becausehandling is then easier.

Owing to the smaller amounts of water being deter-mined, a more sensitive indication set-up is preferred.The indication principle is, in effect, identical to that in

the volumetric KF titration, only here an alternating cur-rent is used for the polarization of the double platinumelectrode. Coulometric KF titration is a micro methodand particularly suitable for the determination of smallquantities of water. Owing to the high sensitivity, par-ticular consideration has to be paid to contamination bymoisture from external sources.

The water content of a wide range of samples, fromdiverse industrial processes, can be determined by usingthe KF technique. For this reason, special instrumentsare available for the preparation of samples that areanalyzed regularly. Additionally, automated instrumentsare available.

4.4 Thermostatic Titration Cells

Substances which dissolve slowly in methanol or the KFworking medium, cause drifting EPs. The same is true forsolids, for example foodstuffs, which give up their wateronly very slowly. In such cases, titration at 50–60 °C canaccelerate the dissolution of the sample or the release ofits water. Slow side-reactions sometimes involve iodineand therefore a stable EP is impossible to achieve. It canthen be advantageous to carry out the titration at low tem-peratures, which can reduce the effect of the side-reaction.

Special titration cells are available for these techniques.They are constructed with an outer glass jacket throughwhich a liquid can be pumped. When attached to athermostat or cryostat the titration temperature can beadjusted.

4.5 Karl Fischer Oven

Many substances only release their water at hightemperatures and are therefore unsuitable for the KFtitration. These samples can be heated in an oven tobetween 100 and 300 °C and their water evaporated. Theevaporated water is then carried over into the titrationcell via a heated, inert stream of gas and titrated eithercoulometrically or volumetrically. The KF oven can beused for insoluble solids (e.g. plastics and salts), where thewater can be released at temperatures above 60 °C fairlyrapidly, and also for solids and liquids that react with theKF titration reagents (e.g. ascorbic acid, mineral oil).

4.6 Automation of the Volumetric Karl FischerTitration

The aim of many laboratories is to increase the throughputof samples and so improve business efficiency and relievethe personnel of routine work. For this reason, sampleexchangers were developed. Samples are weighed intosample beakers, which are then closed with aluminum foilin order to protect them from the ingress of atmosphericmoisture. When they are at the titration position they are

14293

6 GENERAL ARTICLES

Figure 4 A typical, automated sample processor.

raised and the foil is pierced. A defined volume of solventis added. Then the desired dissolving time or extractiontime elapses before the KF titration is started. Using suchequipment, large numbers of samples can be processedfully automatically. Sample exchangers will also workovernight or during the weekend.

4.7 Oven Sample Processor

Water determination in plastics, oils, emulsions or saltsoften cause problems. They can cause contamination ofthe oven and titration cell, side-reactions, reduced lifetimeof the reagent, long analysis times and high cost due tothe fact that the possibility of automation is limited. Anoven sample processor (Figure 4) has been developed toavoid direct contact between the sample and titrationcell and thereby avoid the above-mentioned problems.In addition, the full capacity of the reagent is utilized tothe optimum. Both the KF volumetric and coulometrictitrator can be set up in coordination with the oven sampleprocessor. The samples are weighed in the small (glass)sample boat, which is then sealed with a septum andplaced on the exchanger. When the small sample boatreaches the workstation, the oven sample processor islowered and the septum is pierced with a needle. At thesame time the sample boat is pushed into the aluminumheating block and the heater is switched on. The needlehas two openings: an inert gas enters the sample boatvia the first one, and leaves through the second. Theinert gas then passes into the titration cell along with theevaporated water of the sample. The water is then titrated.

5 REAGENTS

The reagent Karl Fischer described has a number ofdisadvantages. Apart from the strong smell of pyridine, it

displays a very fast fall in the titer value. One-componentKF reagents are generally unstable. A slow-side reaction,which involves iodine, takes place within the reagent, andso diminishes the titer..14/ Pyridine-containing reagentsoften decompose completely within a very few weeks buta modern pyridine-free reagent has a titer decrease ofless than 10% per year if stored correctly. Additionally,the ingress of atmospheric water into the reagent cancause the titer value to decrease. Air humidity has agreat influence on the stability. A liter of air containsup to 15 mg water and therefore it must be driedbefore being pumped into the titration cell or bottleof reagent.

5.1 One-component Reagent

A pyridine-containing one-component reagent has beendescribed..15/ The titer of these reagents is not given, butmust be determined daily since it drops rapidly. Com-mercially available, pyridine-free reagents are offeredwith various different concentrations. The water equiv-alence is often contained in the name of the reagent.Reagents of titer 5, 2 and 1 are the norm. They areused in conjunction with methanol or other special work-ing media. The working media can be varied in orderto improve the solubility of the sample or to minimizeside-reactions. A chloroform/methanol mixture would beused, for example, to aid the solubility of fats. An ethano-lic medium will reduce a side-reaction of the sample withmethanol.

Aldehydes and ketones react with the components ofa usual KF reagent, and therefore special reagents areavailable for such samples. They have a K in the name,referring to ketone. The water determination is carriedout with a one-component reagent of titer 5 in a special,methanol-free working medium. The main componentsof this working medium are chlorinated or methoxylatedalcohols.

5.2 Two-component Reagents

Two-component reagents consist of a titrant and aKF solvent. The titrant is available with a titer of 5or 2. For general use both methanol- and ethanol-basedKF solvents are used. Solvents such as chloroform orformamide can be added to the KF solvent to aid samplesolubility. There are also special products for nonpolarsamples available.

5.2.1 Coulometric Reagents

Reagents for coulometry are in the form of an anolyteand catholyte. The KF reaction takes place in the anodereagent. This contains sulfur dioxide, an organic base anda soluble iodide in an alcoholic solvent. Anode reagents

14294

KARL FISCHER MOISTURE DETERMINATION 7

have an A in the name. For general purposes an anolytethat does not contain a chlorinated hydrocarbon is used.For oils, a chloroform-containing anolyte is used; forketones, a methanol-free anolyte. The reduction takesplace in the associated catholyte. The sample does notcome into direct contact with the catholyte, and thereforean alcoholic catholyte usually based on methanol canbe used for almost all applications. The methanol-freereagent is necessary only for a ketonic sample. Catholytescan be recognized from the C in their name.

5.3 Auxiliary Products

In addition to the actual KF reagents, a whole array ofauxiliary products is available. Buffers and acids allowthe setting of an optimal pH. Karl Fischer standards witha predetermined water content are used to carry out atiter determination and to check the functioning of theinstrumentation.

6 STANDARDIZATION, CALCULATIONAND CONTROL OF THE RESULTS

6.1 Titer Determination

Although a titer is given for volumetric reagents, thismust be checked at regular intervals. The titer of a one-component reagent falls due to a side-reaction and thereis always the possibility of ingress of moisture from theatmosphere, which also reduces its value. Since the titeris actually a measurement of volume, there is also atemperature dependency.

The titer can be determined using pure water, salts witha constant water content and liquid water standards. Purewater is injected into the cell using a microliter syringe.The titer is calculated according to Equation (5):

titer D water equivalent D amount of watervolume of reagent

.5/

where the amount of water is in mg, and the volume ofreagent is in mL. Sodium tartrate-2-hydrate is used as thesolid standard. This salt is not hygroscopic, it dissolves inmethanol and has a constant water content of 15.66%.The titer is calculated according to Equation (6):

titer D water equivalent D sample sizeð 0.1566volume of reagent

.6/

where the sample size is in mg, and the volume ofreagent is in mL. Liquid water standards consist of anon-hygroscopic solvent mixture that contains 0.01–1%water. The standard (1–2 g) is weighed into the cell by

difference. The calculation for the 1% standard would beas in Equation (7):

titer D water equivalent D sample sizeð 0.01volume of reagent

.7/

where the sample size is in mg, and the volume of reagentis in mL.

The titer determination is not only an exact measure ofthe water equivalent but is also a control of the workingconditions. If the titer falls dramatically, this can indicatemoisture in the reagent. If the values vary considerablyfrom one another the titration cell may not be firmlyclosed.

A titer determination or any other calibration proce-dure is unnecessary in the case of coulometry. It is anabsolute method in which the amount of iodine is calcu-lated by using Faraday’s constant.

6.2 Sample Size and Calculation of the Water Content

Sample size depends on the method, water content of thesample and the desired accuracy. Table 1 gives some ideaof appropriate sample sizes.

6.3 Control of the Result

When validating the KF titration, various sources of errorneed to be considered. The result can be incorrect dueto an error in the titer value, or a deficient reagent orinstrument. Additional errors due to a reaction betweenthe sample and reagent are also possible. The wholetitration system, i.e. the instrumentation together withthe reagents, can be checked using a certified waterstandard. The water content of the standard is determinedduring its manufacture by obtaining numerous results atvarious sample weights. In order to guarantee the KFsystem, the result obtained with the standard must liewithin a predetermined standard deviation limit. Table 2shows a typical overview of results.

This procedure checks both the KF apparatus andreagents together. If the results do not fall within theaccepted limits of error then the source of the error must

Table 1 Recommended sample sizes

Water content Sample weight (g) for Sample weight (g) forof sample (%) volumetric titration coulometric titration

100 0.05 –10 0.5 0.05

1 5 0.20.1 10 20.01 10 50.001 20 100.0001 – 10

14295

8 GENERAL ARTICLES

Table 2 Typical results from a reliability test of a KFcoulometer

Sample Weight of Water Water Waterno. water (calculated) (found) content

standard (g) (mg) (mg) (mg g�1)

1 0.5355 0.5301 0.5366 1.00212 0.5001 0.4951 0.5089 1.01763 0.4785 0.4737 0.4794 1.00194 1.0268 1.0165 1.0405 1.01335 0.9911 0.9812 0.9866 0.99556 0.9994 0.9894 1.0046 1.00527 1.9515 1.9320 1.9575 1.00318 1.9800 1.9602 1.9964 1.00839 2.0169 1.9967 2.0060 0.9946

Water content:Mean of nine determinations, 1.0046Relative standard deviation, 0.75%Standard deviation control limit, 2%Control requirements fulfilled? Yes.

be found and eradicated. Then the procedure is repeatedwith fresh reagents. The drift and titration curve shouldbe noted. The balance being used must also be checked.If the error persists then the buret of the volumetrictitrator can be checked. In order to do this, definitevolumes of water are dosed at 20 °C from the buret andthese volumes are then compared with their exact weight.Further details can be obtained from the manufacturersof the instrumentation..15/ If the source of error remains,the manufacturer should check the instrumentation.

Manufacturers of the instruments are able to certifythem, and also support the user in many ways in termsof quality management in the analytical laboratory. Prac-tically all manufacturers are ISO 9001 accredited andconsider the requirements of the international standardsof quality management during the development of theirinstrumentation. This support involves all phases ofinstrument procurement and use. Advice involves the set-up of the equipment, the necessary solutions to problems,delivery of the instrument, instructions of use, operatortraining, instrument servicing, and solving customer prob-lems. On request, the manufacturer will help the operatorwith installation qualification (IQ), the operation quali-fication (OQ) and performance qualification (PQ). Withthe integration of GLP tools, for example the monitor-ing of validation intervals, reagent life and result limits,the operator is efficiently supported by the instrumentitself during daily determination of water content by KFtitration.

6.4 Interferences

If it is to be determined whether or not a sample isinterfering with the KF chemistry, the method of ‘‘subse-quent standard addition’’ is recommended. The working

medium of the volumetric titration or the anolyte incoulometry is titrated to dryness. Then the sample isadded and its water content found. A known amount ofwater is then added to the titration cell still containing thetitrated sample. If the amount of water titrated is 100%,within a predetermined standard deviation control limit,interference with the sample can be eliminated.

7 SAMPLE PREPARATION

Due to the wide area of application for the KF titration,there are countless different samples where the watercontent needs to be determined. It is only possiblehere to cover the sample preparation and methods forthe various different classes of substance rather thanindividual materials.

7.1 Solids

Solids usually have water bound in two different states.It can be adsorbed on the surface of the solid or be inthe form of crystallized water or trapped water in thesolid. In order to determine the whole water content it ispreferable that the sample is fully dissolved. Since manymaterials do not dissolve in methanol, additional solventsare often added to aid solubility. For lipophilic substances,e.g. fats, a working medium is used which consists of 50%methanol or KF solvent and the remainder is chloroformor a suitable alcohol. To dissolve polar substances theworking medium is made up of 50% formamide.

Mechanical sample preparation is also possible. Forbetter solubility it is always recommended to reduce thesize of large pieces. The sample can be crushed using apestle and mortar, ground in a laboratory mill, or cutinto small pieces. If there is no working medium thatdissolves the sample, an extraction titration is sometimesalso possible. The sample is ground as fine as possibleand suspended in the working medium. The water isthen titrated. The hygroscopic solvent extracts the waterfrom the solid. After the EP has been reached it isrecommended that a subsequent standard addition iscarried out, as described in section 6.4, to ensure that thewater is completely extracted. To improve the distributionof the sample, a high-speed stirrer or homogenizer(Ultra-Turrax, IKA Werke, Stauffen, Germany) can beinstalled inside the titration cell. The sample is dispersedduring the titration, which optimizes the conditions forsuch samples.

A further possibility for determining the water contentof insoluble samples is external extraction. Here, aknown amount of a hygroscopic solvent, typically driedmethanol, is added to the sample in a volumetric flask.After a period of stirring, an aliquot is taken and the

14296

KARL FISCHER MOISTURE DETERMINATION 9

water content titrated. The result must take into accountthe blank value of the methanol. When small quantities ofwater are analyzed, not only the blank value of the solventbut also a blank of the procedure should be determined.For this an identical volumetric flask is taken, filled withthe same volume of methanol, stirred under the sameconditions for the same length of time and an aliquottaken and titrated to the same volume. Already titratedKF solvent can also be used to extract water from samples.Pretitrated anolyte from coulometry is best suited heredue to the very small amounts of water involved. A syringeis rinsed numerous times with the pretitrated anolyte untilthe syringe is completely free of water. A distinct volumeof water-free anolyte is then used as extracting solvent.In this case the blank is always zero.

7.2 Liquids

Liquids are ideal samples for the KF titration. They areinjected into the titration cell, via a septum, by using asyringe. The cell remains in an ideal water-free conditionbecause ingress of atmospheric water from outside can beminimized. The coulometric cell should, in general, neverbe opened for sample addition. Therefore, a sample ina liquid or gaseous state is a prerequisite for the KFtitration by coulometry. The mass of sample titrated ismeasured by weighing by difference. Dosing by volumeis generally less exact than by weight.

Liquids can also present a solubility problem or, putanother way, a problem with mixing, if the sampleand working medium have very different polarities. Inprinciple, a titration in two phases is possible withsufficient stirring, but, where possible, only one phaseshould be present. To improve the solubility of fats, oilsand other nonpolar samples, chloroform, propanol orother higher alcohols, e.g. hexanol, can be used. Polarsamples can be dissolved if formamide is added.

7.3 Gases

The water content of gases can also be determined bythe KF titration. The gas is directed into the titration cell.The water content of the gas is then transferred to the KFreagent. Sometimes the gas dissolves in the reagent andsometimes it bubbles out. If the bubbles are not too largethis does not affect the KF titration. In practice, a finestream of bubbles is achieved by using a sieve-type tip tothe insert tube and therefore good dispersion of the gas.Gases normally contain very small amounts of water andtherefore coulometry is the preferred method of analysis.Volumetric titration can also be carried out, although areagent with a low titer must be used. The amount of gaspassed through the titration cell is measured in volumeand the water content result obtained is consequently

in mg L�1. It must be noted that the water content ofgases is not always distributed homogeneously. If thewater is adsorbed on the inside of the gas cylinder, highervalues are obtained at the start of the determination thanwhen equilibrium has been reached. In order to ensurethat there is a homogeneous distribution of water in thegas, a constant flow of the gas should be allowed tobubble through the titration cell for an hour and the driftobserved. A titration should only then be carried outwhen the drift has reached a stable value.

7.4 Oven Method

For the water content of samples that react with theKF reagents or that are completely insoluble, the ovenmethod is recommended. This method is described insection 4.5.

8 IMPORTANT APPLICATIONS

The KF reaction is a redox reaction that takes place in anorganic solvent. Oxidizing and reducing agents affect thereaction just like substances that react with the workingmedium. Other interferences take place when a sampleis not fully dissolved. Here the water is only releasedslowly and the titration displays a drifting EP. In order toensure that the sample fully dissolves, different solventsor physical techniques can be used. The following sectiongives an overview of the most frequently titrated sampletypes. A more complete coverage of sample applicationsis given in the HYDRANAL Guide..16/

8.1 Chemicals

8.1.1 Inorganic Compounds

Inorganic salts rarely dissolve in methanol. In order toguarantee the determination of the entire water content,the salt must first be fully dissolved. A mixture of 50%formamide and 50% methanol dissolves some alkaliand alkali earth salts. Prior grinding of the sample andwarming of the titration cell accelerates solubility. Mostinorganic salts do not dissolve in organic solvents. Theoven method is then used. The water is evaporated fromthe sample, carried into the titration cell via an inertgas, and titrated according to the KF method. Manyoxides and carbonates react with the KF reagent and adirect titration is not possible.

Inorganic salts and bases can also be titrated by KFtitration, in principle, although the pH in the titration cellmust be considered. The reaction proceeds too slowly ifthe environment is too acid, and the titrator consequentlyfinds no EP. In an alkaline environment there is a side-reaction that consumes iodine and therefore causes a high

14297

10 GENERAL ARTICLES

result or a vanishing EP. When titrating acids, a suitablebuffer, e.g. one based on imidazole and sulfur dioxide, isrecommended and, for basic samples, salicylic or benzoicacid is added to the working medium.

8.1.2 Organic Compounds

Organic compounds are very diverse and often charac-terized by many different functional groups. Whether thecompound causes a side-reaction or not can often bejudged prior to titration, by taking into consideration thefunctional groups present. If the substance is sensitive tooxidation or reduction it could react with iodine or iodide.Active carbonyl groups can react with methanol; iodineand sulfur dioxide can add across double bonds. Figure 5shows titration curves with and without a side-reaction.

Hydrocarbons, halogenated hydrocarbons, alcohols,ethers, esters and other similar substances normally poseno difficulties for KF titration. Propanol or chloroformmust sometimes be added to aid the solubility oflong-chain compounds. Hydrocarbons that are mainly

8.07.06.05.04.03.02.0

1.0

0.0 2.0 4.0 6.0 8.0 10.0

0.0 0.5 1.0 1.5 2.0 2.5

Vol

ume

of r

eage

nt (

mL)

3.22.82.42.01.61.20.80.4

Vol

ume

of r

eage

nt (

mL)

5.585 mLa

1.888 mLb

Time (min)

(a)

(b)

Figure 5 Titration curves with and without side-reaction.(a) Titration of benzylamine in an unbuffered solution.(b) Titration of benzylamine in a buffered solution. a This isthe volume of reagent consumed until the titration is stoppedmanually; b this is the volume of reagent consumed until thetitration is stopped automatically.

unsaturated behave inertly in the KF reagent. However,if a multiple bond is present, an addition reaction cantake place which consumes iodine. Most carboxylic acidsdo not have to be neutralized in the same way as mineralacids. However, very strong acids, such as dichloroaceticacid or bromoacetic acid, require the addition of abuffer based on imidazole. Some carboxylic acids formesters with alcohols and therefore need to be titratedvery rapidly or require a modified working medium.Working media based on ethanol esterify slower thanthose based on methanol. Salts of carboxylic acids canbe dissolved, often with addition of formamide. Basicorganic compounds such as amines require the additionof benzoic or salicylic acid to maintain the optimum pH.

Polar organic compounds such as sugars and proteinsare titrated volumetrically because they are solids. Thesesubstances are dissolved by using a working mediumof formamide–methanol 1 : 1. Warming the titration cellaccelerates the dissolving process.

Aldehydes and ketones present particular difficultiesfor the KF titration. These substances form acetalsand ketals with methanol and these reactions releasea stoichiometric quantity of water. There are specialreagents available for these sample types which containalcohols that react according to the KF reaction shownin Equation (4) but which do not form acetals andketals with the sample. Alcohols such as chloroethanol,or sterically hindered compounds such as 2-methoxy-propanol, can be used as suitable alcohols. Aldehydesalso react according to the bisulfite addition reactionwith the KF reagent. Sulfur dioxide reacts with aldehydegroups and this reaction requires water. Therefore withconventional KF reagents a result is obtained which istoo low. Special reagents for aldehydes have a reducedsulfur dioxide content to decelerate the bisulfite additionreaction and enable the KF titration to be carriedout accurately. Only for very reactive aldehydes, e.g.acetaldehyde, does the side-reaction predominate.

An example of substances that are sensitive to oxidationand which react with iodine is the mercaptans. Peroxidesare sensitive to reduction and react with iodide to generateiodine. There are many exceptions within each group ofcompounds and these can behave differently in the KFenvironment. It is therefore worthwhile carrying out a testtitration in each case. A two-step validation is necessarywith addition of standard after the titration has beencarried out, as described in section 6.4, and the titrationcurve should be investigated.

8.2 Pharmaceutical Products

Different substance groups can be thought of under theterm pharmaceuticals: raw materials, intermediates andfinished products. Raw materials consist of the active sub-stances, drugs derived from vegetable material, inorganic

14298

KARL FISCHER MOISTURE DETERMINATION 11

salts, e.g. sodium phosphate, solvents, e.g. ethanol orpolyethylene glycol, and non-active components such assaccharin and isomalt. For these materials there is oftena valid, quality control method according to the relevantpharmacopeia. The methods quoted by the pharmacopeiafor the determination of water content are KF titrationand loss-on-drying..17 – 19/

Interference with the KF reagents can occur and canbe avoided as described in section 8.1. Normally, rawmaterials are pure substances whose behavior can bedetermined by looking at their chemical structure.

If a reagent being used has a different composition tothat described by the relevant pharmacopeia, it has to bevalidated as described in section 6.4. This validation mustbe carried out for each individual product being tested.Sometimes loss-on-drying is quoted as the preferredmethod for water determination. A KF titration can beused instead if a validation is carried out. This validationis necessary since in some cases loss-on-drying and KFtitration give results that are not identical. The watercontent given in the specification of the raw materialmust also be taken into consideration. If the water contentvalue found by the KF titration is within the specificationcontrol limit then this method can be employed.

Intermediates are mixtures of active and non-activesubstances that are not yet in their final state. The watercontent is an important quality criterion since it affectsthe physical characteristics of the product, such as itsflow ability or its adhesiveness. In comparison to thequality requirements of raw materials, where, as a rule,only a maximum value for water content is quoted, forintermediates there is normally both an upper and lowerlimit specified. Tablet powders and other similar solidsoften display solubility problems. External extraction(section 7.1) or the KF oven (section 4.5) must then beused.

Similarly, in the finished product, the water contentis an important quality criterion because it influencesphysical properties and storage stability. The water intablets and capsules can be determined using either theKF oven or by external extraction. Direct titration israrely possible. Effervescent tablets that release waterand carbon dioxide are particularly problematic. A KFtitration is not possible in this situation. Liquid medicinecan be added direct to the titration cell. Creams andsuppositories can also be added to the titration cell anddissolved by using either propanol or chloroform, andwarming the cell if necessary.

8.3 Petroleum Products

Oils, lubricants and related products generally do notdissolve in the methanolic or ethanolic working mediumof KF titration. In addition, they contain very small

amounts of water (with the exception of crude oil), andso the determination must be carried out very accurately.Coulometry is recommended when used in conjunctionwith an anolyte modified specifically for oils. These specialreagents contain chloroform or a long-chained alcohol toaid solubility of the oils. Sometimes xylene is added.

Crude oils must be homogenized before a sample istaken, because the water is often unevenly dispersed.Homogenization can be carried out either by a high-speed mixer or in an ultrasonic bath. Crude oil containstar, which can stick to the electrodes. Xylene or tolueneshould therefore be added to the working medium ofa volumetric titration and likewise to the anolyte incoulometry. In this way the tar stays in solution andcannot stick to the electrodes. Refined products such asbenzene, diesel and kerosene are added to the speciallymodified anolytes designed for oils and fats. Disturbanceswith such products are not expected.

Lubricants, insulating oil and motor oils containadditives to improve their performance and increase theiractive life. Many of these additives cause problems forthe KF titration. Antioxidants and mercaptans react withiodine, ketone groups form ketals, and metal oxides createwater. Generally, the water content determined for theseproducts by direct titration is far too high. Thereforethe oven method (section 7.4) is carried out. The oilis heated at a temperature between 120 and 140 °C.Coulometry is absolutely necessary because the watercontents are usually in the ppm region. The water contentdetermination of petroleum products is controlled by alarge number of ASTM (American Society for Testingand Materials) and ISO standards.

8.4 Plastics

The water content of polymers, like pharmaceuticals,determines their physical characteristics and therefore itis a very important criterion. The samples to be analyzedtend to be in the form of granules, fibers or solutions. Thewater content is often very low and therefore coulometryis the preferred choice of instrumentation. Many of thepolymers can be brought into solution by the addition ofchloroform or methyl pyrrolidone and can then be titrateddirectly. Ketones, which are also often used as solventsfor polymers, are not recommended for KF titrationbecause they cause side-reactions (see section 8.1). Smallamounts of ketone can be titrated when special reagentsfor ketones are used.

A volumetric titration can be tried if the poly-mer is in the form of a fine powder. The sample isadded directly to the methanolic working medium. Themethanol extracts the water from the very finely groundpolymer without dissolving it.

14299

12 GENERAL ARTICLES

The oven method is used for less fine powders, fibersand granules. The sample is heated to between 120 and170 °C and its water carried over to the coulometric cellin a stream of dried gas. Since the water is released veryslowly a minimum determination time of 10 min must beset on the titrator to avoid it switching off prematurely.Finding an optimum oven temperature is extremelydifficult because the polymers can further polymerizeand form extra water. Sometimes they can decompose athigher temperatures. It is extremely difficult to distinguishbetween free water and water formed during the heatingprocess and so in practice the oven temperature isstandardized for each particular type of polymer.

8.5 Foodstuffs

The water content of foodstuffs is an important qualityparameter. It influences, amongst other things, taste, shelflife and appearance. An upper legal limit is set for manyfoods. At the same time the water content determinationof food is very difficult because of the very complexmaterial involved, and requires very careful samplepreparation. Food is often a mixture of polar material,e.g. protein, and nonpolar material, e.g. fat. Additionally,ethereal oils and other chemicals that enhance the tasteof a food can cause side-reactions. The water can also bedistributed nonhomogeneously. Sometimes it is dissolved,sometimes suspended and sometimes, in the case of plantmaterial, trapped in cellular structures. If the samples areheated they often easily degrade, releasing water. In suchcases the results have to be carefully validated. A titrationcurve yields a great deal of information about possibleside-reactions and the course of the titration.

Volumetric titrations are carried out almost exclusivelyfor the titration of foodstuffs. Coulometry is only used asan exception, such as for the titration of a vegetable oil.The sample must be dissolved or very finely dispersed.A working medium mixture of methanol/formamide isused for polar ingredients such as proteins and sugars.To enable the ingredients to dissolve faster the cellcan be warmed to 50 °C. Fats, and samples containingfats, are dissolved in a working medium of methanoland chloroform. A mixture of methanol, formamide andchloroform is used for samples containing both fat andsugar, e.g. milk powder.

Samples containing starch never dissolve completelyand their water is therefore extracted from the fineparticles in warm methanol. Generally, vegetable materialis very difficult to dissolve or does not dissolve at all. Inorder to extract the water satisfactorily, the materialmust be chopped as finely as possible. Homogenizers orhigh-speed mixers are state-of-the-art technology. Thesemixers can be set up inside the titration cell through aspecial opening in the lid. The sample is added and the

mixer switched on for a certain set time. Short titrationtimes and sharp EPs are possible due to the very finedispersion of the material. Heterogeneous samples suchas chocolate bars, biscuits and noodles can be investigatedusing this technique. Extensive coverage of the titrationmethods for foodstuffs can be found in Isengard..20,21/

9 INTERNATIONAL STANDARDPROCEDURES

The International Standard ISO 760 controls the volu-metric water determination according to the KF method.Reagents, instruments and procedures are quoted.Sodium tartrate-2-hydrate or water is used for the titerdetermination of the reagents. Since ISO 760 was writtenin 1978 it does not take into consideration new methods,reagents and instrumentation. There is no internationalstandard available for the coulometric KF titration.

The general KF titration is also quoted in the vari-ous national pharmacopeias. The US Pharmacopeia andthe Japanese Pharmacopeia describe the volumetric andcoulometric KF titration..18,19/ The European Pharma-copeia only covers the volumetric technique..17/

In addition to the standards that quote the generalmethods for the KF titration, there are also nationaland international standards that describe the titration ofindividual matrixes. Such standards, given by the Amer-ican Society for Testing and Materials (ASTM), are, forexample, ASTM D 1533-79 ‘‘Standard Test Method forWater in Insulation Liquids’’, ASTM D 1744-83 ‘‘Stan-dard Test Method for Water in Liquid Petroleum’’ andthe DIN ISO 51869 ‘‘Coulometric Water Determinationin Gases’’.

10 COMPARISON WITH LOSS-ON-DRYING

Loss-on-drying and the KF titration do not determinethe same parameters of a sample, and therefore theanalytical values found can not really be compared withone another. In the KF titration the water within a samplereacts chemically. During loss-on-drying the amount ofmaterial that escapes is measured. For some samples thewater content found according to KF titration and loss-on-drying is identical. Such samples would be those thatrelease water easily and do not alter on heating. Samplesbelonging to this category are the salts of carboxylicacids, some inorganic salts and mineral samples. Loss-on-drying is, however, still the preferred method quoted inthe pharmaceutical and food industries, where samplescan yield different results in loss-on-drying and the KFtitration. Results found by KF titration are, in comparison,

14300

KARL FISCHER MOISTURE DETERMINATION 13

mainly high because the water is fully extracted andreacted. Foodstuffs often yield higher results due to thedegradation that often takes place under the conditionsof loss-on-drying.

ABBREVIATIONS AND ACRONYMS

ASTM American Society for Testing and MaterialsEP End-pointGLP Good Laboratory PracticeIQ Installation QualificationKF Karl FischerLIMS Laboratory Information Management

SystemOQ Operation QualificationPQ Performance Qualification

RELATED ARTICLES

Food (Volume 5)Food Analysis Techniques: Introduction ž Sample Prepa-ration for Food Analysis, General žWater Determinationin Food

Pesticides (Volume 7)Pesticide Analysis: Introduction

Petroleum and Liquid Fossil Fuels Analysis (Volume 8)Hydrocarbons Analysis: Introduction ž Diesel FuelsAnalysis ž Fuels Analysis, Regulatory Specificationsfor ž Full Range Crudes, Analytical Methodology ofž Lubricant Base Oils: Analysis and Characterization of

Pharmaceuticals and Drugs (Volume 8)Pharmaceuticals and Drugs: Introduction

Polymers and Rubbers (Volume 8)Polymers and Rubbers: Introduction

Electroanalytical Methods (Volume 11)Electroanalytical Methods: Introduction

General Articles (Volume 15)Quality Assurance in Analytical Chemistry žTraceabilityin Analytical Chemistry

REFERENCES

1. K. Fischer, ‘Neues Verfahren zur massanalytischen Bes-timmung des Wassergehaltes von Fluessigkeiten undfesten Koerpern’, Angew. Chemie, 48, 394 (1935).

2. D.M. Smith, W.M.D. Bryant, J. Mitchell, ‘Analytical Pro-cedures Employing Karl Fischer Reagent I’, J. Am. Chem.Soc., 61, 2407–2412 (1939).

3. G. Wernimot, F.J. Hopkinson, ‘The Dead-stop End-point. As Applied to the Karl Fischer Method forDetermining Moisture’, Ind. Eng. Chem. Anal. Ed., 15,272–274 (1943).

4. E.D. Peters, J.L. Jungnickel, ‘Improvements in Karl Fis-cher Method for Determination of Water’, Anal. Chem.,27, 450–453 (1955).

5. A.S. Meyer, C.M. Boyd, ‘Determination of Water byTitration with Coulometric Generated Karl FischerReagent’, Anal. Chem., 31, 215–219 (1959).

6. A. Cedergren, ‘Reaction Rates Between Water andthe Karl Fischer Reagents’, Talanta, 21, 265–271(1974).

7. J.C. Verhoef, E. Barendrecht, ‘Mechanism and ReactionRate of the Karl Fischer Titration Reaction’, J. Elec-troanal. Chem. (Lausanne), 71, 305–315 (1976).

8. E. Scholz, Karl Fischer Titration, Springer Verlag, Berlin,Heidelberg, New York, Tokyo, 1984.

9. G. Wunsch, A. Seubert, ‘Stoechiometrie und Kinetikder Karl-Fischer-Reaktion in Methanol als Reaktion-smedium’, Fresenius’ Z. Anal. Chem., 334, 16–21(1988).

10. ‘HYDRANAL-Manual’, RdH Laborchemikalien, Seelze,Germany, 1999. (http://www.netvertise.de/hydranal)

11. E. Scholz, ‘Karl-Fischer-Reagenzien ohne Pyridin (3):Die Genauigkeit der Wasserbestimmung’, Fresenius’Z. Anal. Chem., 306, 304–396 (1981).

12. W. Richter, W. Huerlimann, ‘Diaphragmalose Messzellefuer die Karl-Fischer-Titration’, EP 0 390 727, CA114(26):258756b, Metrohm Ltd, Herisau, Switzerland.

13. K. Schoeffski, ‘Der lange Weg zur giftfreien Karl-Fischer-Titration’, GIT Lab. Fachz., 10/98, 996–997(1998).

14. G. Wuensch, K. Schoeffski, ‘Die iodierende Nebenreak-tion im Karl-Fischer-System’, Anal. Chim. Acta, 239,283–290 (1990).

15. ‘Application Bulletins’, Metrohm Ltd, Herisau, Switzer-land. (email: sales@metrohm.ch)

16. HYDRANAL Guide PC, RdH Laborchemikalien, Seelze,Germany, 1999.

17. ‘Europaeisches Arzneibuch’, 3. Ausgabe, DeutscherApotheker Verlag, Stuttgart, 1997.

18. United States Pharmacopeia, 24th edition, The UnitedStates Pharmacopeia Convention Inc., Rockville, Mary-land, USA, 1999.

19. Japanese Pharmacopeia, 13th edition, Hirokawa Shoten,Tokyo, 1996.

20. H.-D. Isengard, ‘Bestimmung von Wasser in Lebensmit-teln nach Karl Fischer’, Eur. Food Science, ZFL, 42, 1–6(1991).

21. H.D. Isengard, ‘Karl Fischer Titration in Boiling Metha-nol’, Fresenius’ Z. Anal. Chem., 342, 287–291 (1992).

14301