The Karl Fischer technique for water determination is a titration reaction based on the Bunsen equation as follows:
(1) ROH + SO2 + R'N - [R'NH]SO3R
(2) [R'NH]SO3R + H2O + I2 + 2R'N - 2[R'NH]I + [R'NH]SO4R
Remarks: R'N = base, ROH = alcohol, typically methanol
The oxidation of alkylsulfite to alkylsulfate in reaction (2) consumes water, which should come only from the sample. Since water and iodine (I2) are consumed in a 1:1 stoichiometric ratio, the amount of water in the original sample is calculatedby measuring the concentration of I2 remaining after the reaction is complete. The I2 is measured either volumetrically or coulometrically.
