Stanley M. Levenson

Kinetics of the reactions of dehydroascorbic acid in the presence of hydrogen sulfide.

Abstract Kinetic measurements have been made on the reaction of dehydroascorbic acid in the presence of hydrogen sulfide. Two separate reactions were measured: a reversible reduction to ascorbic acid, and an irreversible change to an undetermined product. The order of both of these reactions was found to be 1. The rates of reaction of each were determined at various pHs and temperatures. It was found that the buffers used had a definite influence on the reactions. The rate of the irreversible reaction is at a minimum at a pH of about 3.5, and the reduction of dehydroascorbic acid is essentially complete at this pH. Below this pH, however, less than complete reduction is obtained. The rates depend on pH in such a way as to suggest that the reaction dehydroascorbic acid→ ascorbic acid takes place through the dehydroascorbate ion. The rate of reduction of dehydroascorbic acid by hydrogen sulfide was found to vary with temperature according to the Arrhenius equation, and also with the concentration of hydrogen sulfide. After correction for the change of solubility of hydrogen sulfide with temperature, the energy of activation (Δ E ) of this reaction was calculated to be 32,875 cal./mole.