Comprehensive simultaneous kinetic study of sulfitolysis and thiosulfatolysis of tetrathionate ion: unravelling the unique pH dependence of thiosulfatolysis.

Abstract

The kinetics of the reactions of tetrathionate with S(IV) species and with thiosulfate in slightly acidic and neutral media were studied concurrently at 25.0 ± 0.1 °C by simultaneous high-performance liquid chromatography monitoring of the concentrations of polythionates (including trithionate, tetrathionate, and pentathionate), thiosulfate, and sulfite. The tetrathionate-sulfite and tetrathionate-thiosulfate reactions were found to be first-order with respect to both reactants. The tetrathionate-sulfite reaction was found to be pH-dependent under the conditions studied. In contrast, the tetrathionate-thiosulfate reaction was experimentally demonstrated to be pH-independent at neutral medium, where the pKa2 value of sulfurous acid plays a key role, whereas under slightly acidic conditions, between pH 4 and 5 the consumption of tetrathionate during the course of reaction was found to become pH-dependent. We show that the pH dependencies in both systems can be readily explained by the reactivity difference between sulfite and bisulfite toward the β-sulfur of the tetrathionate. A simple two-step kinetic model incorporating the protonation equilibrium of sulfite is proposed on the basis of the simultaneous evaluation of the kinetic curves of the two systems, which allowed us to determine reliable rate coefficients for both the forward and backward reactions. Furthermore, the powerful ability of simultaneously evaluating the two chemical systems to yield reliable rate coefficients of the kinetic model is demonstrated.