THE INFLUENCE OF ORGANIC-ACIDS AND METAL-IONS ON THE KINETICS OF THE OXIDATION OF SULFUR(IV) BY HYDROGEN-PEROXIDE

The kinetics of the oxidation of sulfur(IV) oxides by hydrogen peroxide was studied in the presence of two buffer systems as a function of pH, general acid concentration, temperature and iron(II/III) ion concentration by stopped-flow spectrophotometric techniques. The empirical rate law for the uncatalysed process is: -d([S(IV)])/dt={k(H).(a(H+)0.88+k(HX).[HX]}.[HSO3-].[H2O2] where k(H) = (8.6 +/- 0.5) x 10(6) dm5.64 mol-1.88 s-1; and k(HX) = (1.1 +/- 0.1) x 10(5) and (2.9 +/- 0.3) x 10(4) dm6 mol-2 s-1 for HX = formic and acetic acid, respectively, at 25-degrees-C and 1.0 mol dm-3 ionic strength. The activation parameters for these constants in the same seaquence are DELTAH# = 15.8 +/- 4.0, 14.3 +/- 0.9 and 21.8 +/- 1.8 kJ mol-1, and DELTAS# = -58 +/- 13, -99 +/- 3, and -85 +/- 6 JK-1 mol-1, respectively. The presence of iron(II) and iron(III) ions in the reaction mixture leads to a complicated kinetic behaviour. The S(IV)/H2O2 oxidation reaction is accelerated by Fe(II) ions if the iron is added to the H2O2 solution. No kinetic effect is observed if the Fe(II) is added to the S(IV) solution. The complicated kinetic behaviour can be ascribed to a contribution of a parallel free radical process.