Non-Triiodide Based Autoinhibition by Iodide Ion in the Trithionate−Iodine Reaction

The trithionate−iodine reaction has been studied spectrophotometrically in a slightly acidic medium at 25.0 ± 0.1 °C in acetate/acetic acid buffer monitoring the absorbance at 468 nm at the isosbestic point of the iodine−triiodide ion system in the absence and presence of initially added iodide ion at I = 0.5 M ionic strength adjusted by sodium perchlorate. The stoichiometry of the reaction was found to be S3O6 2− + 4I2 + 6H2O → 3SO4 2− + 8I− + 12H+. It is also shown that the consumption of iodine is inhibited by iodide ion, but it cannot be simply explained via the fast equilibrium formation of triiodide ion. A five-step kinetic model with four fitted and fixed kinetic parameters is proposed and discussed in detail, on the basis of which all the most important characteristics of the measured kinetic curves are adequately explained.