Mechanism of reduction of chromium(VI) ion by 2-mercaptosuccinic acid in aqueous solution

The kinetics of the reduction of the chromium(VI) ion by 2-mercaptosuccinic acid (thiomalic acid) were studied by rapid scanning stopped flow spectrophotometry. The conditions used were [Cr(VI)]T=0.20 mM, [MSA]T=5–90 mM, 3.0≤pH≤5.6 in citric acid–phosphate buffer, or 3.3≤pH≤5.4 in 0.40 M acetic acid–acetate buffer, 20.0≤T≤35.0 °C at I=0.50 M (NaClO4). Spectrophotometric titration at 350 nm indicates the stoichiometry of the reaction to be 1:3. The kinetics of both formation and decay of the intermediate chromium(VI) thioester were followed at λmax=425 nm and rate expressions, specific rate constants and corresponding activation parameters were derived from the proposed mechanism. The acetic acid–acetate buffer was found to catalyze the formation but not the decay rate of the intermediate. The citric acid–phosphate buffer and dissolved oxygen did not have any significant effect on the reaction rates. The justification of the mechanism was discussed in terms of standard biological conditions.