Reduction Potentials of SO₃•-, SO₅•-, and S₄O₆•3-Radicals in Aqueous Solution

Reduction potentials of the SO3•-, SO5•-, and S4O6•3- radicals in aqueous solutions are measured by pulse radiolysis at 294 K. These radicals are produced by reaction of •OH or N3• radicals with sulfite, peroxymonosulfate, and thiosulfate anions, respectively. The potentials for the couples SO3•-/SO32- and SO5•-/SO52- are determined from equilibrium constants with three reference couples of the phenoxyl/phenoxide type, i.e., those derived from phenol, 3-cresol, and tyrosine. The potential for PhO•/PhO- is redetermined against ClO2•/ClO2- and confirms the published value. The potentials for the other two phenols are determined against PhO•/PhO-. The potential for the SO3•-/SO32- couple is found to be 0.73 ± 0.01 V vs NHE. The potential for SO5•-/SO52- is found to be 0.81 ± 0.02 V. The reduction potential of the radical formed from the one-electron oxidation of thiosulfate, which exhibits a λmax at 375 nm, is also determined. This radical was identified before as either the monomeric S2O3•- or the dimeric S4O6•3-. Equilibrium measurements for this species, using N3• and the 4-cyanophenoxyl radical as references, support the dimeric assignment and yield a value of 1.07 ± 0.03 V for the reduction potential for the couple S4O6•3- /2S2O32-.