1. Karl Fischer Titration
Regina Schlink
Metrohm Ltd, Herisau

2. Topics KF reaction Volumetric KF Titration Coulometric KF Titration
Endpoint indication
Drift as second endpoint indication
Parameters
KF Instruments

3. Topics KF reaction Volumetric KF titration Coulometric KF titration
Endpoint indication
Drift as second endpoint indication
Parameters
KF instruments

4. Method for water determination
in technical products
(oil, plastics and gases)
in cosmetic products
in pharmaceutical products
in food industry
In addition to determination of pH, weighing, acid-base titration, determination of water-content is one of the most frequently used methods in laboratories world wide.
In 1935 KF published his “New Method for the Determination of water…”. Many chemists have since been occupied with this reagent.
It is used in various industries, from quality control of raw materials to water determination in pharmaceutical products.

5. The KF reaction I. CH3OH + SO2 + RN  [RNH]SO3CH3
II. H2O + I2 + [RNH]SO3CH3 + 2RN  [RNH]SO4CH3 + 2[RNH]I
(RN = Base)
SO2 + ROH -> ester, which is neutralized by the base.
Anion of alkyl sulphurous acid = reacting compound, already present in the KF reagent.
Titration: Oxidation SO32- -> SO42- by I2, consumes water.
Stoichiometric reaction
suitable ROH to esterify SO2 completely
Suitable base for complete neutralization
pH 5-7 preferred (optimized by imidazole)
pH adjustment of prime importance => reaction rate is constant
Reaction only takes place as long as sample releases water

6. Basic ingredients of KF reagents
Iodine
Sulphur dioxide
Buffer
Solvent I2
SO2
Imidazole
Methanol
Reagents needed to carry out KFT
Water comes from the sample, other components of reaction from reagent

7. Optimum: pH range between 5 and 7
pH dependency
log K
< pH 5: log K increases linearly with pH
reaction rate to low (low results)
5.5-7: optimum, reaction rate constant
>8: reaction rate increases slightly (side reactions probably occur) pH
Optimum: pH range between 5 and 7

8. Topics KF reaction Volumetric KF titration Coulometric KF titration
Endpoint indication
Drift as second endpoint indication
Parameters
KF instruments

9. KF titration methods Volumetric KF titration Coulometric KF titration
Volumetric -> classical titration, I2-containing titrant is added via buret
Coulometric -> titrant is produced from I- in solution by anodic oxidation
H20 calculated out of applied current
Both methods:
-titration cell as impervious as possible (atmospheric water!)
-water adhered to wall of cell -> removed by swirling conditioned solvent
Working medium & titrant
Iodine is generated in titration cell (anodic oxidation)

10. Volumetric KF titration step by step
Fill titration vessel with solvent
Pretitration with KF reagent
Add the sample
Titrate with KF reagent
Volumetric
Higher (absolute) water contents
Solid or pasty samples can be directly introduced
Variety of suitable solvents, specifically adapted
Lower detection limit ( ppm)
No stable titre -> determination of titre regularly
Why pretitration?
O-ring and molecular sieve -> change regularly (in general 6 weeks, depends on air-humidity)

11. Volumetric KF reagents
One component reagents
Titrant contains iodine, sulphur dioxide, buffer and methanol/ethanol
Working medium contains only the methanol/ethanol
Disadvantage: the titre decreases 5% per year in the closed bottle!
Two component reagents
Titrant contains iodine
Solvent contains buffer and sulphur dioxide
Advantages: pH optimum in the solvent / fast reaction / titre is very stable
Two possibilities for working medium and titrant
1-Component: flexibility regarding solvent additives for improving solubility of sample
Titrant 5 mg/mL: 5 mg H2O can react with 1 mL
Titre determination: Water Std 10 recommended, Na-tartrat not as reproducible (bad solubility in methanol)
dest water possible, but very tiny amount, good practice needed

12. Topics KF reaction Volumetric KF titration Coulometric KF titration
Endpoint indication
Drift as second endpoint indication
Parameters
KF instruments

13. Coulometric KF titration step by stepH+ H I I- - +
current is our burette
iodine is produced from a iodide containing solvent by anodic oxidation
generating current is switched off as soon as a slight excess of free iodine is present
free iodine is indicated by a double platinum electrode
Not a classical titration -> “electronic buret”
Same chem. process as volumetric KFT: 1 H2O consumes 1 I2
I2 generated electronically
-> absolute method -> no titre
Instrument measures time + current flow to EP
-> prop. I2 -> prop. H2O
Primarily for small amounts of water
Metrohm: 10 g – 200 mg H2O, resolution 0.1 g

14. Coulometric KF titration
I2 produced by generator electrode, 2 different types:
Cell without diaphragm
- easy to handle and clean
- only 1 reagent (!!!Use only reagents intended for these electrodes!!!)
- electrode rapidly ready for use (no water on diaphragm)
- no cathode compartment -> reduction on Pt plate
b) Cell with diaphragm
- ketones/aldehydes in sample
(special reagents available only for this type)
- reagents with low conductivity
- lower trace range
Cell with diaphragm
Cell without diaphragm

15. How to fill coulometric cell with diaphragm?
Catholyte 5 mL
Reduction: 2 H+ +2 e- = H2
change catholyte weekly!
Anolyte: about 100 mL
2 I- = I2 + 2 e-
Reagents
anolyte -> titration vessel
catholyte -> cathode compartment
same level of both (prevents pressure compensation)
EP: double Pt electrode
Absolute -> no titre determination
Check from time to time with certified standard

16. Comparison with and without diaphragm
With diaphragm
Recommendable for most applications, sample should have a good solubility in alcohol
Reagents with low conductivity (the addition of chloroform or xylene > 10%),
with ketone reagents
absolute water content < 50 ppm
Generator I: 400 mA
Generator I: auto
With diaphragm
low water content, conductivity
Anode + cathode compartment separated
to avoid interferences
Allows application of lower current densities of A

17. Coulometric KF reagents
Capacity of more than 1000 mg of water (100 mL KF reagent)
Anodic and cathodic reagents
Combined reagents
Special reagents for ketones
2 reagent solutions necessary for cell with diaphragm:
anolyte: modified KF reagent, containing I- instead of I2
KF reaction occurs in anolyte
catholyte: enables counterpart cathode reaction which must proceed so that the by-products of the KF-reaction produced at the cathode are not disturbed
Combined reagents: Merck reagents and Coulomat E, in both compartments same solution
Keton reagents <- side reactions

18. Which is the right method for my application?
Volumetric titration
range of application 0.1 % % depends on sample size
Coulometric
range of application % - 1 %
(10 µg mg absolute water content), mainly liquids and gases

19. Topics KF reaction Volumetric KF titration Coulometric KF titration
Endpoint indication
Drift as second endpoint indication
Parameters
KF instruments

20. Endpoint indication Bivoltametry Ipol = 50 uA
Constant current applied to double Pt electrode
During titration: Excess H2O
 High voltage between Pt wires
Ipol: Small current is applied to between the electrodes (AC much more sensitive than DC) and resulting voltage is measured.
As long as there is H2O-> high voltage is applied (I2 -> I-)
End of titration: reaction turnover is decreasing (small excess of free I2) -> strong decrease of voltage
EP is determined by entering switch-off voltage. Reagent addition stops when reached
At end of titration: Small excess of free iodine
 Voltage decreases sharply

21. Endpoint indication Voltage: High as long as H2O in sample + I2 added
Once reaction turnover decreases, potential decreases
End point is not sufficient as criteria (titration would stop at signal at 88 sec)
-> Additionally drift as EP criteria
Drift <- water entering system

22. Topics KF reaction Volumetric KF titration Coulometric KF titration
Endpoint indication
Drift as second endpoint indication
Parameters
KF instruments

23. permanent consumption of KF reagent
 Drift
Aim: constant and low drift
Optimal: Volumetry <10 L/min
Coulometry g/min
Influence on the results  drift correction
Drift correction:
20 ug/min drift and 1 min measurement,
Sample 10 ug => 30 ug => correction necessary
Drift higher than sample -> lower drift recommended for trace analysis
Optimal drift is for europe, depends on humidity, air condition might be good in some countries for trace analysis

24. Drift
Start drift – acceptable drift value for start of determination (cond. OK)
Stop criteria drift – absolute or relative drift value
Absolute drift– the entered value is the stop drift
Relative drift – the stop drift is calculated from the measured drift value (start) and the entered value

25. Topics KF reaction Volumetric KF titration Coulometric KF titration
Endpoint indication
Drift as second endpoint indication
Parameters
KF instruments

26. Titration parameter Volumetry Coulometry EP at U 250 mV
Dynamics mV
Stop criteria drift/time
Stop drift 20 uL/min
Delay time 10 s
I(pol): uA
Coulometry
EP at U mV
Dynamics 70 mV
Stop criteria drift/rel drift
Stop drift 5 ug/min
Rel. Drift 5 ug/min
Start drift 20 ug/min
I(pol) : uA
Standard parameters
Dynamics: near to EP -> add smaller amount
Dynamics 100: from 350 smaller steps (volumetry), 120 (coulometry)
Special parameters:
Coulometer: Ketone reagents
Volumeter: Ethanol reagents

27. Topics KF reaction Volumetric KF titration Coulometric KF titration
Endpoint indication
Drift as second endpoint indication
Parameters
KF instruments

28. Volumetric KF Titration
Titrandos:
841 Titrando
PC Control / tiamo
Touch Control
Titrinos:
787 KF Titrino
795 KFP Titrino

29. Volumetric KF Titration
Modi
841 Titrando
KFT X
SET
Meas
High-end titrator for up to four 800 Dosino dosing systems.
Karl Fischer titration (KFT) and endpoint titration (SET), pH measurement (MEAS), liquid handling.
With four MSB connections, one measuring interface; USB connection.

30. Only a version with dosing units ?
Advantages:
The dosing unit can be totally emptied that means:
No crystallisation in cock and tubings
Rinsing the buret several times after changing the titrant is no longer necessary
Direction downwards -> no air-bubbles dosed
Possibility to empty totally -> saves reagents

31. Coulometric KF titration
756 Coulometer
with internal printer
831 Coulometer
Work relatively precisely
Both available with generator electrodes with or without diaphragm
Differences: Printer included 756
Optional: Automatic reagent changer (after a defined time)

32. Principle of the oven technique
Many substances release water slowly or at elevated temps
-> not suitable for direct KFT
Further problem: low solubility of sample in alcohol,
side reactions of other substances
=> Substance heated in oven, locate upstream from cell
Released water is transferred by flow of dry carrier gas to titration cell, where KFT takes place
=> Only water enters cell
=> No unwanted side reactions + matrix effects and contaminations
Coulometric cell
Vial placed in the oven

33. 832 Thermoprep 774 Oven Sample Processor
Oven for volumetric and coulometric KFT
832 -> stand alone
774 -> automated