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1Taiwan, September 2002
Determination of Water
by the
Karl Fischer Titration:
Theory
2Taiwan, September 2002
Motivation Volumetric KF titration
one an two-component reagentsresolution and detection limits
Coulometric KF titrationcell with or without diaphragmresolution and detection limits
Indication, control algorithm, termination parameters
KF titration: important points
Support
Program
3Taiwan, September 2002
Why measure water or moisture?
Butter: max 16.5% water content by law
Sugar: too much moisture will not flow
Drugs: too much moisture decomposition
Compact Disc: too much moisture bad music quality
Brake Fluid: too much water brake do not work
Kerosene: too much water blocked tubing
Flour: too little moisture dust explosion
4Taiwan, September 2002
Methods for the Determination of Water
Chromatography
Spectroscopy (IR, MS)
Thermogravimetry / DSC
Balance with IR /Halogen / Microwave heater
Drying oven
Karl Fischer Titration
5Taiwan, September 2002
Coulometric KF Volumetric KF
Karl Fischer Titration: Why?
Fast (e.g. 1...2 minutes) Selective for water Accurate and precise (0.3% srel) Wide measuring range : ppm to %
6Taiwan, September 2002
Karl Fischer
Bunsen reaction: 2 H2O + SO2 + I2 = H2SO4 + 2 HI
Pyridine happened to be around in the Lab
German petrochemist,1901 – 1958
Publication:1935
7Taiwan, September 2002
KF Titration
KF Reaction
SO2 + RN + ROH ------> (RNH)SO3R
a sulfite compound
(RNH)SO3R + H2O + I2 + 2RN ------> (RNH)SO4R + 2(RNH)I a sulfate compound
Summary
H2O + I2 + SO2 + 3RN + ROH ----->(RNH)SO4R + 2(RNH)I
The solvent (generally methanol) is involved in the reaction A suitable base keeps the pH 5 - 7
8Taiwan, September 2002
Solvent
pH range
optimal
slow
side reactions
pH
log K
2 4 6 8 10
2
4
0
buffer neededoptimal pH 5 - 7
9Taiwan, September 2002
Volumetric / Coulometric Titration
Volumetric Karl Fischer TitrationIodine is added by burette during titration.Water as a major component:100 ppm - 100 %
Coulometric Karl Fischer TitrationIodine is generated electrochemically during titration.Water in trace amounts:1 ppm - 5 %
+-
10Taiwan, September 2002
Volumetric KF Titration
Iodine is added by burette during titration. Water as a major component: 100 ppm - 100 %
11Taiwan, September 2002
Volumetric KF Titration
One - component reagent Titrant:
I2 , SO2, imidazole, methanol and diethylene glycol monoethyleter
Solvent:Methanol
Two - component reagent Titrant:
I2 and Methanol Solvent:
SO2, Imidazole, Methanol
-> fast reaction, chemically stable, higher cost
12Taiwan, September 2002
Volumetric KF Reagents
Titrant Concentration
1-2-5 mg H2O/mL
Titer stability
-----> Check by Standardization
Standardization materials
Water 100%Sodium tartrate 15.66%
Standard solution 5 mg/mLWater Standard 1% (10 mg/g)
13Taiwan, September 2002
Air humidity:
0.5 - 3 mg water / 10 mL air
Air Humidity
Conditioning of the titration stand
Well sealed titration cell
Tropical countries: Air conditioning
Protect titration stand, titrant and solvent from ingress of water.
14Taiwan, September 2002
Drift determination
Automatic drift compensation in the result calculation.
The titration stand is not 100 % tight against air humidity.
Drift determination
The drift is the amount of water entering into the titration stand per minute.
1 - 20 µg H20 / minute
15Taiwan, September 2002
Volumetric Karl Fischer Titration
Titrant: 5 mg H20/mLResolution: 2.5 µg H20/stepDetection limit: 125 µg H20For 5 g sample: 25 ppm
Resolution of burette: 10,000 stepsDetection limit : 50 x ResolutionBurette size: 5 mL
Titrant: 2 mg H20/mLResolution: 1 µg H20/stepDetection limit: 50 µg H20For 5 g sample: 10 ppm
Resolution and Detection Limit
16Taiwan, September 2002
Coulometric KF Titration
Iodine is generated electrochemically during titration Water in trace amounts: 1 ppm - 5 %
- +
17Taiwan, September 2002
Anolyte(sulfur dioxide, imidazole, iodide, different solvent for different application - methanol, ethanol with chloroform, octanol, ethyleneglycol )
Double platinum pin electrode
Catholyte(similar or
modified solution)
Generator electrode
Diaphragm
AnodeCathode+–
Coulometric KF Titration
Titration cell and reagents
18Taiwan, September 2002
Coulometric KF Titration
Same reaction as volumetric KF Titration but Iodine is produced just in time from iodide
+–
H+
-H
I--
I 2 I- I2 + 2 e-
AnodeIodine production by oxidation
Side reaction:Reduction of sulfur components.After 1 - 2 weeks, smells like mercaptansChange catholyte every week!
H2 2 H+ + 2 e-
Cathode
19Taiwan, September 2002
Coulometry Theory
One Coulomb C is the quantity of charge transported by an electric current of one Ampere (A) during one second (s).
1 C = 1 A • 1 s
Absolute method, no standardization!
Charles Augustin de Coulomb14.6.1736 - 23.8.1806
To produce one mol of a chemicalcompound, using one electron,96484 C are required.
2 I- ions react to form I2 which in turnreacts with water
1 mol of water (18g) is equivalent to 2 x 96484 C or 10.72 C/mg water.
20Taiwan, September 2002
Filling the Titration Cell
Anolyte:Fill in ~ 100 mL anolyte
Catholyte:Fill in 5 mL catholyte.
Anode+–
Cathode
Catholyte
Anolyte
The level of the anolyte should be 3 - 5 mm higher than the level of catholyte sothat the flow is from the anolyte compartment to catholyte compartment.
Low drift value
With stirring the level difference of anolyte and catholyte will be stable.
21Taiwan, September 2002
Filling the Titration Cell
Anode+–
Cathode
Catholyte
Anolyte
Catholyte always contains water!
If the catholyte level ishigher or at the same levelas the anolyte,there is a flow of moisture into the anolyte compartment.
High drift value
22Taiwan, September 2002
With or Without Diaphragm
What are the differences?
23Taiwan, September 2002
With Diaphragm
+–
Without Diaphragm
I- -I
Iodine is only in the anodecompartment and reacts with water.
+–
I- -I
-I I-
It is possible that iodine can go to the cathodeand convert to iodide.
With or Without Diaphragm
24Taiwan, September 2002
Without Diaphragm
Prevention:
– bigger sample error has no effect
– high stirrer speed iodine reacts faster with water
Iodine I2 can go to the cathode and convert to iodide.
– Small cathode surface less chance to contact iodine
Only a little less accuratefor samples
with very low water content.
+–
I- -I
-
H+ H
– high iodine production speed hydrogen protects cathode
25Taiwan, September 2002
Without Diaphragm
The hydrogen produced at the cathode is a very good reducing agent.
Easily reducible samples (nitro compounds) get reduced, which produces water.
too high result
Not recommended for easily reducible samples: e.g. nitrobenzene, unsaturated fatty acids, etc.
–
I- -I
-H+ H
R-NO2 R-NH2 + H2O+
26Taiwan, September 2002
Without Diaphragm
– A little bit less accuracy for very small water content (< 50 µg/sample).
– Not recommended for easily reducible samples: nitro compounds, unsaturated fatty acids, etc.
+ Titration cell easier to clean.+ Long-term drift value more stable.+ Only one reagent.
Titration cell without diaphragm is the standard set-up for: Hydrocarbons, halogenated hydrocarbons, alcohols, esters, ethers, acetamides, mineral oils, edible oils, ethereal oils
27Taiwan, September 2002
Resolution: 0.1 µg water
Detection limit: 5 µg waterfor 5 g sample 1 ppm
Measuring range:10 µg - 100 mg water/sample1 ppm - 5 % water
+-
Coulometric Karl Fischer Titration
Resolution and Detection Limit
28Taiwan, September 2002
srel > 5 %
srel 5 - 0.5 %
srel < 0.5 %
Not suitable for coulometry
coulometry
Not suitable for volumetry
srel > 5 %
srel 5 - 0.5 %
srel < 0.5 %
volumetry
1 ppm
10 ppm
100 ppm
1000 ppm
1 %
10 %
100 %
Coulometry versus Volumetry
Repeatability
29Taiwan, September 2002
Ipol = 20µAU = 650mV
2
KF Indication Principle (1/2)
Bivoltametric indicationconstant current at the double platinum pin electrode ==> polarization current (Ipol)
During titration:
I2 reacts with water
no free I2 in the solution
high potential
30Taiwan, September 2002
Ipol = 20µAU = 84mV
2
+ -
ee
I2
I2 + 2e- -> 2 I-2 I- -> I2 + 2e-
2I- I2
KF Indication Principle (2/2)
At endpoint all water has reacted with I2
After the endpoint
free I2 in the solution
I2 is reduced to I- at the cathode ionic conductivity occurs and the
measured potential drops potential change = endpoint
31Taiwan, September 2002
V/mL
E/mV
EP
KF Fuzzy logic
V/mL
E/mV
Control range EP
KF Classical
KF Control: Titrator Algorithm
Karl Fischer Fuzzy Logic Control DL31/38 No control band required
(typical 300 mV)
The titrant addition rate depends on: the distance to the endpoint EP the potential change/increment
Advantages: Simpler control: Only two control parameters
Vmin , Vmax (smallest/largest increment) Faster, more accurate, and better precision
even at low water content(toluene: n = 5, 115 ppm, srel 0.17% )
32Taiwan, September 2002
E (mV)
t(s)
EP
15 s
KF Control: Termination Parameters (1/3)
Delay time
the actual potential is lower than the EP for a defined time after the last titrant increment
typical delay : 15 - 20 sec
Note:Adapt the smallest increment to the drift and to the concentration of the titrant
33Taiwan, September 2002
KF Control: Termination Parameters (2/3)
Absolute drift stop
the actual drift is less then the predefined value
typical value : 30 g/min
Note:Adapt the value to the initial drift
t(s)
Drift (µg/min)
abs. drift stop= 30 µg/min
EP
34Taiwan, September 2002
Drift (µg/min)
t(s)
Initial drift
Rel. Drift stop= 20 µg/min
KF Control: Termination Parameters (3/3)
Relative drift stop
the sum of the initial and the relative drift has been reached
typical value : 15 g/min
independent from the initial drift and of titrant concentration
ideal with side reactions that cannot be suppressed otherwise
35Taiwan, September 2002
Karl Fischer Titration : Checks
Relevant points to be checked System tightness : Check carefully Ambient moisture : Drift determination Stability of titrant : Standardisation Side reactions : Check literature Sample handling : Accuracy, precision Free water only : Sample preparation
36Taiwan, September 2002
www.titration.net
Complete Solution : Solutions and Support
Application brochures Internet databases