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

Abstract

Abstract Stopped‐flow spectrophotometry was used to study the oxidation of S(IV) by H 2 O 2 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 NaClO 4 , respectively). The reaction of HSO 3 − with H 2 O 2 is subject to general‐acid catalysis whereas that of the SO 3 2‐ ion is not. The dependence of the experimental first‐order rate constant on the concentration of the excess partner [H 2 O 2 ] 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 H 3 O 2 + and SO 2 , respectively; [H] = proton activity). magnified image 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(V1) formation. The p K a of the species H 3 O 2 + as derived from the kinetic data, is found to be p K R1 = 1.5–2.0. —In the pH range 6–8, the rate is given by p K a (HSO 3 − )). magnified image At pH > 8, the rate is governed by Eq. (3) p K a (HSO 3 − magnified image The data for k , K , k H , k HOH , k and k are presented and the mechanistic implications of rate laws (1)–(3) are discussed.