Kinetics and mechanism of the alkaline decomposition of hexathionate ion.

The alkaline decomposition of hexathionate has been investigated by high-performance liquid chromatography monitoring the concentration of polythionates (such as hexathionate, pentathionate, and tetrathionate) and that of thiosulfate simultaneously in buffered medium at a constant ionic strength. As a major final product, unlike in the case of the alkaline decomposition of tetrathionate and pentathionate, a significant amount of sulfur deposition was also observed. Nevertheless, the chromatograms clearly suggested that heptathionate also appears as a key intermediate in a detectable amount during the course of the reaction. On the basis of these observations an 11-step kinetic model with 9 fitted and 4 fixed rate coefficients is suggested to take all of the most important characteristics of the decomposition into account. The proposed model includes the well-known thiosulfate-assisted sulfur chain, increasing pathways for tetrathionate and pentathionate along with their rate coefficients, but among them that of the thiosulfate-assisted rearrangement of pentathionate has not been reported yet. Having these kinetic parameters in hand reveals a reasonable trend of these rate coefficients against the length of the sulfur chain, suggesting that the attack of the nucleophilic agents like hydroxide, thiosulfate, and sulfite ions is determined by the electron density in the vicinity of the given sulfur atom.