Kinetics and Mechanism of Oxidation of L-Cystine by Cerium (IV) in Sulphuric Acid Medium

The kinetics of oxidation of L-cystine by cerium (IV) was studied at 0.5–2.0 mol dm–3 sulphuric acid in the range 25–40 °C. The reaction exhibits first order dependence with respect to concentration of cerium (IV) and cystine. Ionic strength has negligible effect on the rate. The rate of the reaction decreases with increase in hydrogen ion concentration up to 0.5–1.0 mol dm–3 and remains constant thereafter. Cysteic acid was identified as the main product. A suitable mechanism was proposed and activation parameters of the slow step are computed using linear least squares method and the values of activation energy and entropy were found to be 57.4 ± 1.9 kJ mol–1 and –122.7 ± 5.1 JK–1 mol–1 respectively.