Kinetics and mechanism of the oxidation of sulfur (IV) by peroxomonosulfuric acid anion

Stopped-flow spectrophotometry was used to study the oxidation of S(IV) by peroxomonosulfuric acid anion, HOOSO 3 − (=PMS), at 285 K in the pH range 0–12.5 in buffered aqueous solution under pseudo-first-order conditions ( I = 1.0 M, Na 2SO 4). The reaction of the HSO 3 − ion with PMS is subject to general-acid catalysis, that of the SO 3 2− ion is not. Rate constants for the general-acid catalysed oxidation of S(IV) are reported for various buffer acids. Rate law (equation (a)) is valid for the pH range 0–5 at zero buffer concentration. The overall equilibrium constant K exp correlates Rate= k·K exp [ PMS] 0[S(IV) ] 0 1+K exp [ S(IV)] 0 the various protonation steps involved in the formation of the intermediates −O 3SOOSO 2 − and/or −O 3SOOSO 2H according to equation (b). K exp = K·(1+ [ H] K S1 ) (1+ [1+ H] K R1 )·(1+ [ H] K S1 ) From K exp = ƒ([ H]) the equilibrium constant K(=19.1 ±2.1 M −1) for the formation of the intermediate −O 3SOOSO 3 − from PMS and HSO 3 − as well as the acid dissociation constant K s of the conjugate acid −O 3SOOSO 2H ( K s=(4.1 ± 0.9) · 10 −4 M) were obtained ([H] = proton activity; pK s1 = pK a (SO 2)). The first-order decay of the species −O 3SOOSO 2H with k H ′ = 134 ± 33 s −1 controls the rate of S(IV) oxidation. The pK a of the species H 2SO 5 ±, as derived from the kinetic data, is pK R1≈0.4. In the pH range 5–8, the rate is given by equation (c) ( pK s2= pK a (HSO 3 −)). Rate=[ k H [ H]+k HOH [H 2O 1+ K S2 [ H] + k′ SO3 1+ K S2 [ H] ] ·[ S(IV)] 0[PMS] 0 At pH>8, the rate is governed by equation (d) ( pK R2 = pK a (HSO 5 −)). Rate= [ k SO3 [ H] k R2 +k′[ SO3 1+[ H] K R2 ] ·[ S(IV)] 0[PMS] 0 The following rate constants were obtained: k H = (6.2 ± 0.43)·10 6 M −2 s −1, k HOH = 3.4 ± 0.96 M −2 s −1, k SO 3 = (2.3±0.28)·10 3 M −1 s −1, and k SO 3 ′ = 16 ± 1.4 M −1 s −1. A mechanistic interpretation of rate laws (a)–(d) is presented. The UV absorption spectra of the species −O 3SOOH and SO 5 2− are given.