Reduction potentials of so3·- , so5·- , and S4O6·3- radicals in aqueous solution

Article abstract of DOI:10.1021/jp9900234

Reduction potentials of the SC₃^·- , 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^·-/SO₃^2- and SO₅^·-/ SO₅^2- 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^·-/SO₃^2- couple is found to be 0.73 ± 0.01 V vs^·NHE. The potential for SO₅^·-/SO₅^2- 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 λ_max 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- /2S₂O₃^2-.