# Karl Fischer method

A Karl Fischer titrator

The Karl Fischer method is understood to be the quantitative determination of water by titration, hence also Karl Fischer titration or simply KFT . The method was developed in 1935 by the German chemist Karl Fischer at Lazăr Edeleanu GmbH. The misnomer Karl Fisher Titration or Fisher Titration is also used occasionally . It found its way into practically all pharmacopoeias .

## theory

The method is specific for the quantitative determination of water . In its original form, it consists in titrating water with an anhydrous methanolic solution containing iodine , sulfur dioxide and excess pyridine as a buffer solution . Instead of pyridine, modern reagent solutions contain other less unpleasant and dangerous components for buffering. Eugen Scholz introduced imidazole in 1982 and thus created a fast and reliable reagent. The maximum reaction rate is between pH 5.5 and 8. Accordingly, basic components such as imidazole are used for acidic samples and acidic components such as salicylic acid for basic samples. One- and two-component reagents are commercially available. For its part, methanol can also be replaced by other alcohols which improve the titer stability. The stoichiometry (molar ratio H 2 O: I 2 ) depends on the type of solvent . Alcohol-containing solvents lead to a stoichiometry of H 2 O: I 2 1: 1, while non-alcoholic solvents result in a stoichiometry of 2: 1. The amount of water in the sample also affects the molar ratio. However, this only occurs from approx. 1 mol / l of the solvent.

## Chemical reaction

The decisive factor for the process is the fact that sulfur dioxide and iodine only react with one another in the presence of water. In the absence of alcohols, the following reaction occurs:

${\ displaystyle \ mathrm {2 \, H_ {2} O + SO_ {2} + I_ {2} \ longrightarrow SO_ {4} ^ {2 -} + 2 \, I ^ {-} + 4 \, H ^ {+}}}$

If methanol is present in the solution, it forms an acidic ester with sulfur dioxide , which is neutralized by the base (e.g. imidazole , hereinafter referred to as "RN"):

${\ displaystyle \ mathrm {CH_ {3} OH + SO_ {2} + RN \ longrightarrow (RNH) \ cdot (CH_ {3} OSO_ {2})}}$

In the case of titration, in which iodine in methanol is used as the standard solution , the methyl sulfite anion is oxidized by the iodine to methyl sulfate anion in the presence of water . The yellow-brown iodine is reduced to colorless iodide:

${\ displaystyle \ mathrm {(RNH) \ cdot (CH_ {3} OSO_ {2}) + I_ {2} + H_ {2} O + 2 \, RN \ longrightarrow (RNH) \ cdot (CH_ {3} OSO_ {3}) + 2 \, (RNH) \ cdot I}}$

During this process, water is consumed, so the reaction can only take place until all of the water in the sample is consumed.

If there is no more water available, the added iodine is no longer reduced. The resulting brown coloration is used for visual endpoint indication. In practice, electrometric indications (especially biamperometry ) are preferred because they are more sensitive and more accurate.

In the coulometric Karl Fischer titration, the iodine required for the reaction is generated by anodic oxidation of iodide. The devices used for this have two pairs of electrodes:

• a working electrode on which iodine is generated (the "used" charge is also measured here)
• a measuring electrode, which measures whether the iodine produced can be broken down by the reactions described above or whether it remains in the solution (end point).

Special coulometric reagents are required for these devices.

## Volumetric method

One advantage of the volumetric method compared to the coulometric method is the lower outlay on equipment. The volumetric method can also be used if the sample is colored. Here the end point of the titration is detected by UV / VIS spectrophotometry.

## References

1. K. Fischer: New method for the analytical determination of the water content of liquids and solids , in: Angewandte Chemie 1935 , 48 , 394–396; doi : 10.1002 / anie.19350482605 .
2. Production manager for inorganic chemicals at Riedel-de-Haen in Seelze near Hanover.
3. Eugen Scholz: Karl Fischer titration . Springer-Verlag 1984, ISBN 3-540-12846-8 .
4. Note: The usual proof of iodine excess as an iodine-starch complex in iodometry cannot be carried out in an anhydrous environment.
5. Tavčar, E., Turk, E., Kreft, S .: Simple Modification of Karl-Fischer Titration Method for Determination of Water Content in Colored Samples. Journal of Analytical Methods in Chemistry, Vol. 2012, Article ID 379724, doi : 10.1155 / 2012/379724 .