Kinetics and Mechanism of the Concurrent Reactions of Hexathionate with S(IV) and Thiosulfate in a Slightly Acidic Medium

Reactions of hexathionate with thiosulfate and sulfite have been investigated by high-performance liquid chromatography via monitoring the concentration-time series of tetrathionate, pentathionate, hexathionate, and thiosulfate simultaneously within the pH range of 4.0-5.0. In both reactions, elementary sulfur forms; more significant sulfur precipitation may be observed in the case of the hexathionate-thiosulfate reaction, but slight turbidity in the other system means that elementary sulfur also appears in a detectable amount in the hexathionate-sulfite reaction. Initial rate studies have revealed that the formal kinetic orders of both reactants in both systems are clearly unity but pH-dependence can only be observed in the case of the hexathionate-sulfite reaction. The proposed kinetic model appears to suggest that nucleophilic attack of sulfite and thiosulfate may also occur on the β- or γ-sulfur of the polythionate chain and breakages of the α-β, β-γ, and γ-γ' bonds are all conceivable possibilities to drive the reactions. Consequently, the generally accepted sulfur-chain elongating effect of thiosulfate on longer polythionates is also proven to be accompanied by sulfur-chain shortening pathways, eventually leading to the formation of elementary sulfur.