Kinetics of the Two-Stage Oxidation of Sulfide by Chlorine Dioxide.

The sulfide-chlorine dioxide reaction was found to have two distinct kinetic stages at alkaline conditions. The first stage proceeds so rapidly that it can only be measured by a stopped-flow technique at low temperature and leads to the parallel formation of polysulfide and sulfate as sulfur-containing products. At the same time, chlorite, chlorate, and chloride are produced from chlorine dioxide in detectable amounts, suggesting a complex stoichiometry. A nine-step kinetic model including short-lived intermediates like sulfide radical and •HSClO2- is proposed to describe the kinetic data in this rapid stage. In an excess of chlorine dioxide, the first stage is followed by a significantly slower one to be measured by conventional UV-vis spectroscopy at room temperature. Considering that tetrasulfide is formed during the first rapid course of the reaction, the subsequent slow kinetic stage can only be described by the direct oxidation of tetrasulfide by chlorine dioxide and, surprisingly, the tetrasulfide-catalyzed disproportionation of chlorine dioxide.