KINETIC INVESTIGATION OF SULFUR(IV) OXIDATION BY PEROXO COMPOUNDS R-OOH IN AQUEOUS-SOLUTION

Normal and rapid-scan stopped-flow spectrophotometry in the range of 260-300 nm was used to study the kinetics of sulfur(IV) oxidation by peroxo compounds R-OOH (such as hydrogen peroxide, R = H; peroxonitrous acid, R = NO; peroxoacetic acid, R = Ac; peroxomonosulfuric acid, R = SO3-) in the pH range 2-6 in buffered aqueous solution at an ionic strength of 0.5 M (NaClO4) or 1.0 M (R = NO; Na2SO4). The kinetics follow a three-term rate law, rate = (k(H)[H] + k(HX)[HX] + k(p))[HSO3-][ROOH] ([H] = proton activity; HX = buffer acid = chloroacetic acid, formic acid, acetic acid, H2PO4-). Ionic strength effects (I = 0.05 - 0.5 M) and anion effects (Cl-, ClO4-, SO4(2-)) were not observed. In addition to proton-catalysis (k(H)[H]) and general acid catalysis (k(HX)[HX]), the rate constant k(p) characterizes, most probably, a water induced reaction channel with k(p) = k(HOH)[H2O]. It is found that k(H) not-equal-to f(R) with k(H)(mean) = 2.1.10(7) M-2 s-1 at 298 K. The rate constant k(HX) ranges from 0.85 . 10(6) M-2 s-1 (HX = ClCH2- COOH; R = NO; 293 K) to 0.47.10(4) M-2 s-1 (HX = H2PO4-; R = H; 298 K) and the rate constant k(p) covers the range 0.2 . M-1 s-1 (R = H) to 4.0.10(4) M-1 s-1 (R = NO). LFE relationships can be established for both k(HX), correlating with the pK(a) of HX, and k(p), correlating with the pK(a) of the peroxo compounds R-OOH. These relationships imply interesting aspects concerning the mechanism of sulfur(IV) oxidation and the possible role of peroxonitrous acid in atmospheric chemistry. A UV-spectrum of the unstable peroxo acid ON - OOH is presented.