DETERMINATION OF KINETICS OF THE KARL FISCHER REACTION BASED ON COULOMETRY AND TRUE POTENTIOMETRY

摘要

A new measurement technique based on a combination of coulometry and zero-current potentiometry is described for determination of the kinetics of rapidly reacting Karl Fischer (KF) reagents, This makes it possible to determine the order as well as the rate constant for large variations in the concentrations of iodine and water present during a titration, It was shown that for imidazole-based methanolic reagents exposed to a large variation in the concentration of water, the KF reaction is first order with respect to iodine, sulfur dioxide, and water only for reagents in which the concentration of nonprotonated imidazole is very low. The rate constant determined for such a reagent (1 M imidazole, 0.8 M sulfur dioxide, 0.1 M iodine) was equal to that reported earlier in the literature, Regions showing first-order kinetics were also found for low concentrations of water when imidazole concentrations up to 2 mol/L were used, provided that these reagents had a quotient [Im](free)/[ImH(+)] around 4, In the interval 2-8 mol/L of imidazole, the order of the reaction with respect to iodine was, in most cases, one-half, while it was changed to between one-half and one with respect to water, The rate of the KF reaction was found to increase by nearly 5 orders of magnitude for a reagent in which the concentration of nonprotonated imidazole was increased from 0 (rate constant equal to 2.6 x 10(3) L(2) mol(-2) s(-1)) to about 7 mol/L. For most of these reagents, a recovery rate close to 100% was attained, A high concentration of nonprotonated imidazole in combination with a high concentration of sulfur dioxide could, however, lead to a change in stoichiometry of the KF reaction when larger amounts of water were determined (250 mu g of water added to 3.4 mL of reagent solution). A reaction scheme is proposed which might explain this change in stoichiometry observed for some reagent compositions. By use of the described most rapidly reacting reagents, it was shown to be possible to carry out titrations even at such a low end-point concentration as 10(-10) M of iodine within 1-2 min.