Kinetics and Mechanism of the Oxidation of Tetrathionate by Iodine in a Slightly Acidic Medium

The iodine-tetrathionate reaction has been reinvestigated spectrophotometrically at T = 25.0 +/- 0.1 degrees C and at an ionic strength of 0.5 M adjusted by sodium acetate as a buffer component in both the absence and presence of the iodide ion in the pH range of 4.25-5.55. The reaction was found to be independent of pH within the range studied, and it was clearly demonstrated that the reaction proceeds via an intermediate S4O6I- formed in a pre-equilibrium. Iodide dependence of the kinetic curves strongly suggests that the iodide ion has to be involved in this equilibrium. Further reactions of the intermediate, including its hydrolysis and reaction with iodide, leads to the strict stoichiometry characterized by S4O62- + 7I2 + 10H2O --> 4SO42- + 14I- + 20H+. A seven-step kinetic model with three fitted kinetic parameters is suggested and discussed. A rate equation is also derived from which a sound explanation of the iodide dependence of the apparent rate coefficient is presented. Furthermore, it has also been pointed out that formation of the triiodide ion alone is not sufficient to take the retardation effect of the iodide ion into account quantitatively.