Inhibition of Aquated Sulfur Dioxide Autoxidation by Aliphatic, Acyclic, Aromatic, and Heterocyclic Volatile Organic Compounds

The oxidation of dissolved sulfur dioxide, sulfur(IV), by oxygen proceeds through the involvement of sulfoxy radicals among which sulfate radical anion is the main chain carrier. When organics are present, they inhibit the oxidation of sulfur(IV) via scavenging of SO 4 − radicals. In contrast to previous studies, which were limited mostly to aliphatic compounds, this paper presents the results of the effect of 13 new volatile organic compounds (VOCs) including aromatic and heterocyclic on uncatalyzed sulfur(IV) autoxidation at pH 8.2 and 25°C. In all cases, the kinetics was first order in the presence and absence of VOCs and experimental rate law was Eq. (1). 1− d [ S ( IV ) ] / d t = k obsS ( IV ) = k oS ( IV ) / 1 + B [ Inh ]where −d[S(IV)]/d t is the rate of sulfur(IV) disappearance, k obs is the first‐order rate constant in the presence of inhibitor, k o is the first‐order rate constant in the absence of inhibitor, [S(IV)] is concentration of sulfur(IV) at time, t , and B is an inhibition parameter. VOCs cause inhibition by scavenging sulfate radical anions, which propagate the autoxidation chain. An analysis of B (Eq. (1)) and k inh (Eq. (2)) values for 21 aliphatic, aromatic, acyclic, and heterocyclic organic compounds showed that these to be related by Eq. (3) for a subgroup and Eq. (4) for b subgroup. 2SO 4 − + VOC⟶ k inhSO 4 2 − + VOCa subgroup (benzamide, 2,2‐dimethyl‐1‐propanol, 1‐hexanol, methanol, ethanol, 1‐propanol, 2‐ propanol, 1‐butanol, 2‐butanol, ethylene glycol, rebaudioside A) 3B = 0.87 ± 0.21 × 10 − 3k inhb  subgroup ( o ‐toluic acid, m ‐toluic acid, p ‐toluic acid, 4‐hydroxybenzoic acid, 1‐heptanol, glycerol, sucralose, acesuifame K, glycine, 3‐pentanol) 4B = 1.2 ± 0.3 × 10 − 3k inh