Kinetics and mechanism of the oxidation of thiosulfate ion by hexachloroiridate(IV)

Abstract

In the oxidation of thiosulfate ion by hexachloroiridate(IV) ion the value Δ[IrCl₆^2–]/Δ[S₂O₃^2–] has been determined to be 1.08 ± 0.06 (where Δ indicates concentrations used in the reaction). The reaction is first-order with respect to [S₂O₃^2–] whereas K_obs, the pseudo-first-order rate constant ([IrCl₆^2–][S₂O₃^2–]), increased with increasing pH. The dependence of K_obs on [H⁺] indicated that both HS₂O^–₃ and S₂O^2–₃ are involved in the reduction which can only be explained on the basis of an asymmetrical structure for thiosulfuric acid. The reactivity of the S₂O₃^2– ion is almost 10⁴ times greater than that of the HS₂O₃^– ion. The ΔH^‡ values for the two paths is almost identical indicating that the energy barrier for the electron transfer is the same in the two cases. The electron is probably transferred from the thiolic sulfur.