KINETICS AND MECHANISM OF SULFUR(IV) OXIDATION BY HYDROGEN-PEROXIDE IN AQUEOUS PHASE - THE NONLINEAR PARTS OF THE PH-PROFILE

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) (K(R1) and K(s1): acid dissociation constants of H3O2+ and SO2, respectively; [H] = proton activity). [GRAPHICS] (1) K(exp) 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(VI) formation. The pK(a) of the species H3O2+ as derived from the kinetic data, is found to be pK(R1) = 1.5-2.0. - In the pH range 6-8, the rate is given by (2) (pK(s2) = pK(a)(HSO3-)). [GRAPHICS] (2) At pH > 8, the rate is governed by Eq. (3) (pK(R2) = pK(a)(H2O2)). [GRAPHICS] (3) The data for k, K, k(H), K(HOH), k(SO3) and k(SO3') are presented and the mechanistic implications of rate laws (1)-(3) are discussed.