Kinetics and Mechanism of Sulfur(IV) Oxidation by Hydrogen Peroxide in Aqueous Phase: The Non-Linear Parts of the pH-Profile

Abstract

Stopped-flow spectrophotometry was used to study the oxidation of S(IV) by H2O2 at 285 K and 298 K, respectively, in the pH range −0.3 to 13 in buffered aqueous solution under pseudo-first-order conditions (I = 0.5, 1.0 and 2.0 M NaClO4, respectively). The reaction of HSO3− with H2O2 is subject to general-acid catalysis whereas that of the SO32- ion is not. The dependence of the experimental first-order rate constant on the concentration of the excess partner [H2O2] was studied in detail. — In the pH range −0.3 to 6 the rate is given by Eq. (1) (KR1 and KS1: acid dissociation constants of H3O2+ and SO2, respectively; [H] = proton activity). Kexp is the experimental overall equilibrium constant for the various protonation steps involved in the formation of the intermediate peroxosulfurous acid (= PSA), whereas K is the equilibrium constant for the formation of PSA. Its decay (k = 740 s−1 at 285 K) controls the rate of S(V1) formation. The pKa of the species H3O2+ as derived from the kinetic data, is found to be pKR1 = 1.5–2.0. —In the pH range 6–8, the rate is given by pKa(HSO3−)). At pH > 8, the rate is governed by Eq. (3) pKa(HSO3− The data for k, K, kH, kHOH, k and k are presented and the mechanistic implications of rate laws (1)–(3) are discussed.