Kinetics and mechanism for the redox reaction between hexaaquathallium(III) and sulfur dioxide in acidic aqueous solution

Abstract

Oxidation of dissolved sulfur dioxide, SO₂·nH₂O, HSO₃^– and SO₃^2–, by hexaaquathallium(III) has been studied in acidic aqueous solution with ionic strength 1.0 mol dm^–3 at 25 °C. The stoichiometry of the reaction has been determined: [Tl(H₂O)₆]³⁺+ SO₂(aq)+ 2H₂O →[Tl(H₂O)₆]⁺+ HSO₄^–+ 3H⁺. The kinetics has been studied by use of stopped-flow spectrophotometry under pseudo-first-order conditions with either S^IV or Tl^III in excess. The reaction is first order in both [S^IV] and [Tl^III] and has a complex [H⁺] dependence. The rate decreases with increasing [H⁺] in the range 0.1 < [H⁺] < 1.0 mol dm^–3. The results are interpreted in terms of a reaction mechanism where sulfito complexes are formed by reaction between [Tl(H₂O)₆]³⁺ or [Tl(H₂O)₅(OH)]²⁺ and HSO₃^–. These complexes decompose to the products [Tl(H₂O)₆]⁺ and HSO₄^– by intramolecular inner-sphere electron-transfer processes. The thallium(III) sulfito complexes are weak, as indicated by the absence of any transient absorbance peaks in the UV region. Potentiometric measurements of the oxygen concentration and spectrophotometric measurements showed that Tl^III does not initiate a free-radical chain oxidation of S^IV by dissolved oxygen.