Karl Fischer titration
Under the Karl Fischer method refers to the quantitative determination of water by titration, therefore Karl Fischer titration or simply KFT. The method was developed in 1935 by the German chemist Karl Fischer at the Lazar Edeleanu GmbH. Occasionally, the wrong name Karl Fisher titration is used or Fisher titration. It entered practically all pharmacopoeias.
Theory
The method is specific for the quantitative determination of water. In its original form it is in the titration of water with an anhydrous methanol solution containing iodine, sulfur dioxide and excess pyridine as a buffer solution. Modern reagent solutions containing pyridine instead of other less unpleasant and dangerous components for buffering. Eugen Scholz 1982, the imidazole introduced, creating a fast and reliable reagent. The maximum reaction rate is between a pH of 5.5 and 8. Accordingly, it uses basic components such as imidazole for acidic samples and acidic components such as salicylic acid for basic samples. The market offers one- and two-component reagents available. Methanol, in turn, may also be replaced by other alcohols, which improve the titer stability. The stoichiometry (molar ratio of H2O: I2) depends on the nature of the solvent. Alcoholic solvents lead to a stoichiometry of H2O: I2 1:1, while non-alcoholic solvents yield a stoichiometry of 2:1. Also the amount of water in the sample affects the Mole ratio. However, this only occurs from about 1 mole / liter of the solvent.
Chemical Reaction
Critical to the process is the fact that sulfur dioxide and iodine react with each other only in the presence of water:
Methanol is present in the solution, so as to form sulfur dioxide with an acid ester represented by the base (e.g., imidazole, hereinafter referred to as "RN ") is neutralized:
In the titration, in which iodine is used as a standard solution in methanol, the methyl sulfite anion in the presence of water is oxidized to methyl sulfate by the iodine. The yellow-brown iodine is thereby reduced to the colorless iodide:
In this process, water is consumed, the reaction can therefore only run until all of the water contained in the analyte is consumed.
If no water is present, iodine is no longer metered reduced. The browning occurring thereby serves the visual endpoint indication. In practice electrometric indications are preferably (especially biamperometry ), since they are more sensitive and more accurate.
In the coulometric Karl Fischer titration is necessary for the reaction of iodine is produced by anodic oxidation of iodide. Thus, the devices used for this purpose have two pairs of electrodes:
- A working electrode is produced on the iodine (in this case also, the " spent" charge measured )
- A measuring electrode, in which is measured if the iodine formed can be reduced by the reactions described above or in the remaining solution (end point).
For these devices, special coulometric reagents are required.