Karl Fischer method – Methods for water determination

This method is the quantitative determination of water by titration. The method was developed in 1935 by the German chemist Karl Fischer at Lazăr Edeleanu GmbH.

Theory

This process consists of titrating water with an anhydrous methanol solution containing iodine, sulfur dioxide and excess pyridine as a buffer solution. The maximum reaction rate is set between pH 5.5 and 8. Accordingly, basic components like imidazole are used for acidic samples and acidic components like salicylic acid for basic samples. The amount of water in the sample also influences the molar ratio. However, this only occurs from about 1 mol/l of the solvent.

Chemical Reactions

Sulphur dioxide and iodine only react with each other in the presence of water:

2 H2O + SO2 + I2 → SO42- + 2 I- + 4 H+ (in the absence of alcohols)

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

CH3OH + SO2 + RN → (RNH)-(CH3SO3)

During titration, in which iodine in methanol is used as the measuring solution, the methyl sulfite anion is oxidized to methyl sulfate by the iodine in the presence of water. The yellow-brown iodine is reduced to a colorless iodide: (RNH)-(CH3SO3) + I2 + H2O + 2 RN → (RNH)-(CH3SO4) + 2 (RNH)-I

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