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.