General Pathway of Sulfur-Chain Breakage of Polythionates by Iodine Confirmed by the Kinetics and Mechanism of the Pentathionate–Iodine Reaction

The pentathionate–iodine reaction has been studied spectrophotometrically at <i>T</i> = 25.0 ± 0.1 °C and at an ionic strength of 0.5 M in both the absence and presence of an initially added iodide ion at the pH range of 3.95–5.15. It was found that the pH does not affect the rate of the reaction; however, the iodide ion produced by the reaction strongly inhibits the oxidation. Therefore, it acts as an autoinhibitor. The kinetic curves also support the fact that iodide inhibition cannot be explained by the formation of the unreactive triiodide ion, and S₅O₆I^– along with the iodide ion has to be involved in the initiating rapid equilibrium being shifted far to the left. Further reactions of S₅O₆I^–, including its hydrolysis and reaction with the iodide ion, lead to the overall stoichiometry represented by the following equation: S₅O₆^2– + 10I₂ + 14H₂O → 5SO₄^2– + 20I^– + 28H⁺. A nine-step kinetic model with two fitted parameters is proposed and discussed, from which a rate equation has also been derived. A brief discussion about the general pathway of sulfur-chain breakage of polythionates supported by theoretical calculations has also been included