Prerequisites for attaining 1:1 stoichiometry between water and iodine using dimethylformamide as solvent in coulometric Karl Fischer titrations

Summary Conditions for attaining 1:1 stoichiometry between water and iodine in a non-alcoholic, dimethylformamide-based Karl Fischer reagent have been investigated. It has been made clear that the stoichiometric ratio is determined by the rate of the main KF-reaction consuming iodine, in relation to the rate of the side reaction between water and sulphur trioxide, which is a reaction product of the KF-reaction. The rate constants for these reactions were determined to be 9.8±0.6 M^–2s^–1 and 0.072±0.007 M^–1s^–1, respectively. It is shown theoretically as well as experimentally by means of coulometric titrations that the attainment of a 1:1 stoichiometric ratio requires that the titration is carried out at very high iodine concentrations.Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday