KINETICS AND MECHANISM FOR THE REDOX REACTION BETWEEN HEXAAQUATHALLIUM(III) AND SULFUR-DIOXIDE IN ACIDIC AQUEOUS-SOLUTION

Oxidation of dissolved sulfur dioxide, SO2.nH2O, HSO3- and SO32-, by h exaaquathallium(III) has been studied in acidic aqueous solution with ionic strength 1.0 mol dm 3 at 25-degrees-C. The stoichiometry of the reaction has been determined: [Tl(H2O)6]3+ + SO2(aq) + 2H2O --> [Tl(H2 O)6]+ + HSO4- + 3H+. The kinetics has been studied by use of stopped-f low 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 resul ts are interpreted in terms of a reaction mechanism where sulfito comp lexes are formed by reaction between [Tl(H2O)6]3+ or [Tl(H2O)5(OH)]2and HSO3-. These complexes decompose to the products [Tl(HO)6]+ and HS O4- by intramolecular inner-sphere electron-transfer processes. The th allium(III) sulfito complexes are weak, as indicated by the absence of any transient absorbance peaks in the UV region. Potentiometric measu rements of the oxygen concentration and spectrophotometric measurement s showed that Tl(III) does not initiate a free-radical chain oxidation of S(IV) by dissolved oxygen.