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, HOOSO3- (= PMS), at 285 K in the p H range 0-12.5 in buffered aqueous solution under pseudo-first-order c onditions (I = 1.0 M, Na2SO4). The reaction of the HSO3- ion with PMS is subject to general-acid catalysis, that of the SO3(2-) ion is not. Rate constants for the general-acid catalysed oxidation of S(IV) are r eported for various buffer acids. Rate law (equation (a)) is valid for the pH range 0-5 at zero buffer concentration. The overall equilibriu m constant K(exp) correlates [GRAPHICS] the various protonation steps involved in the formation of the intermediates -O3SOOSO2- and/or -O3SO OSO2H according to equation (b). [GRAPHICS] From K(exp) = f([H]) the e quilibrium constant K(= 19.1+/-2.1 M-1) for the formation of the inter mediate -O3SOOSO2- from PMS and HSO3- as well as the acid dissociation constant K(S) of the conjugate acid -O3SOOSO2H (K(S) = (4.1+/-0.9).10 (-4) M) were obtained ([H]=proton activity; pK(S1) = pK(a)(SO2)).The f irst-order decay of the species -O3SOOSO2H with k'(H) = 134+/-33 s-1 c ontrols the rate of S(IV) oxidation. The pK(a) of the species H2SO5+/- , as derived from the kinetic data, is pK(R1) almost-equal-to 0.4. In the pH range 5-8, the rate is given by equation (c) (pK(S2) = pK(a)(HS O3-)). [GRAPHICS] At pH >8, the rate is governed by equation (d) (pK(R 2) = pK(a)(HSO5-)). [GRAPHICS] The following rate constants were obtai ned: k(H) = (6.2+/-0.43).10(6) M-2 s-1, k(HOH) = 3.4+/-0.96 M-2 s-1, k (SO3) = (2.3+/-0.28).10(3) M-1 s-1, and k'(SO3) = 16+/-1.4 M-1 s-1. A mechanistic interpretation of rate laws (a)-(d) is presented. The UV a bsorption spectra of the species -O3SOOH and SO5(2-) are given.