Karl Fischer titration
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Karl-Fischer titration, often shortened to KF titration, is an iodometric titration technique for the determination of water content in a sample. The reagents contain, as a minimum, elemental [[iodine], sulfur dioxide, and a suitable base.
The overall reaction which occurs is:
- SO2 + I2 + 2H2O ⇌ H2SO4 + 2HI
A base must be present in order to drive the equilibrium to the right by reacting with the sulfuric acid and hydriodic acid produced. Fischer's original reagent used pyridine, however modern KF reagents tend to use other bases like imidazole or pyridine derivatives, due to the volatility, toxicity, and unpleasant odour of pyridine.
Normally, the reagent components are dissolved in a primary alcohol such as methanol, ethanol, or methyl cellosolve (ethylene glycol monomethyl ether). In these cases, sulfur dioxide reacts with the alcohol to produce the corresponding alkyl sulfite, which is an intermediate in the reaction.
Non-alcoholic KF reagents are also possible, however the reaction mechanism and stoichiometry changes, and the overall system becomes much more sensitive to sample composition. For this reason non-alcoholic KF reagents are seldom used.
