Kinetic investigation of sulfur(IV) oxidation by peroxo compounds R-OOH in aqueous solution

Summary

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=SO ⁻₃ ) 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)[HSO ⁻₃ ][ROOH] ([H] = proton activity; HX = buffer acid = chloroacetic acid, formic acid, acetic acid, H₂PO ⁻₄ ). Ionic strength effects (I=0.05–0.5 M) and anion effects (Cl⁻, ClO ⁻₄ , SO ²⁻₄ ) 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≠f(R) with k_H(mean)=2.1·10⁷ M⁻² s⁻¹ at 298 K. The rate constant k_HX ranges from 0.85·10⁶ M⁻² s⁻¹ (HX=ClCH₂−COOH; R=NO; 293 K) to 0.47·10⁴ M⁻² s⁻¹ (HX=H₂PO ⁻₄ ; R=H; 298 K) and the rate constant k_p covers the range 0.2·M⁻¹ s⁻¹ (R=H) to 4.0·10⁴ 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.