KINETICS AND MECHANISM OF THE OXIDATION OF SULFUR(IV) BY OZONE IN AQUEOUS-SOLUTIONS

The oxidation kinetics of sulfur(IV) by ozone was studied in the absence and presence of a buffer system as a function of the pH and temperature by stopped-flow spectrophotometric techniques. The reaction showed a first-order dependence on the sulfur(IV) concentration. The reaction rate drastically increased with increasing pH of the reaction medium and is accelerated by the presence of dissolved oxygen. Under pseudo-first-order conditions ([S(IV)[]>>[O3]), the process can be described by the rate law: -d([O3])/dt = {k(a)alpha0 + k(b)alpha1 + k(c)alpha2} [S(IV)]Total [O3] [H+]-1/2 where alpha0, alpha1 and alpha2 denote the fraction of S(IV) in the SO2.H2O, HSO3- and SO3(2-) form, respectively, and k(a) = 9.42 x 10(3) dm1.5 mol-0.5 s-1, k(b) = 1.27 x 10(4) dm1.5 mol-0.5 s-1 and k(c) = 1.74 x 10(5) dm1.5 mol-0.5 s-1. The activation parameters for the overall process are: DELTAH# = 17.0 +/- 0.9 kJ mol-1 and DELTAS# = -91.9 +/- 3.2 J K-1 mol-1 at pH = 2 in the absence of a buffer and DELTAH# 23.1 +/- 0.2 kJ mol-1 and DELTAS# = -48.8 +/- 0.7 J K-1 mol-1 at pH = 4 in the presence of an acetate buffer. No catalytic effect of Fe2+, Fe3+, Mn2+ or Cu2+ ions on the oxidations of S(IV) by O3 was observed. The kinetic data fit a mechanism based on a sulfite-induced radical process.