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 S-bonded 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.

Keywords: kinetics, redox, chromium VI, thiolactic acid