© Elsevier Sequoia S.A., Lausanne-- Printed in The Netherlands MECHANISM AND REACTION RATE OF THE KARL-FISCHER TITRATION REACTION PART I. POTENTIOMETRIC MEASUREMENTS

The reaction rate of the coulometr ic variant of the Karl-Fischer t itration reaction (in which electrolytically generated tri iodide is used as oxidant instead of iodine) has been measured in methanol. The reaction is first order in water, sulfur dioxide and tri iodide, respectively. For pH < 5 the reaction rate constant decreases logarithmically with decreas-ing pH. Addit ion of pyridine solely influences the pH (by fixing it to a value of about 6) and has no direct influence on the reaction rate. A linear relation exists between the reac-tion rate constant and the reciprocal value of the iodide concentration, from which we can calculate the individual reaction rates for the oxidation by iodine and tri iodide, re-spectively. While the reaction rate constant for tri iodide is relatively small (k 3 ~ 350 12 tool-2 s--l), the reaction rate constant for iodine is much larger (k 3 ~ 1.5 x 107 12 tool-2 s-~)