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ABSTRACT. The behaviour towards simultaneous oxidation by Cr(VI) in acidic environment of mixtures consisting of two structurally closely related thioethers (S-methylcysteine and DL-methionine) was tested, at 0.085 mol·l -1 H + and 298  0.1 K. The kinetic and mechanistic patterns of the two substrates when oxidized separately were already known and very similar. Based on that knowledge, the individual kinetic parameters (rate and equilibrium constants) were first computed. Theoretical predictive models for the value of the observed rate constant of a mixture, based on the kinetic data from non-mixtures, are compared with experimental findings and subsequently an empirical calibration surface obtained from all experiments is discussed. Due to differences in the kinetic parameters of the two reactions, in all models the obtained surfaces are asymmetrical, and this opens the possibility to univocally determine the concentrations of both the thioethers without separation.

Thiolactic acid, containing a reactive sulphydryl group, has been shown to easily reduce Cr(VI) and yield disulfide as oxidation product. The reaction was studied by the use of spectrophotometry. The spectral data as well as the kinetic information showed distinct evidence of the formation of an Sbonded Cr(VI)-thiolactate intermediate that was subsequently followed by a somewhat slower, bimolecular redox process leading to the formation of the final products. The rate laws for these two stages have been determined, showing a complex dependence on substrate and hydrogen ion concentration. Small experimental Arrhenius activation energies for the two successive steps were also obtained. The involvement of paramagnetic Cr(V) species and that of some organic free radicals were evidenced by ESR and, the latter, also by initialization of polymerization. A reaction mechanism has been proposed, that leads to a rate law in convincing agreement with the experimental one.