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)-O-. or N-3(.) radicals with sulfite, peroxymonosulfate, and thiosulfate anions, respectively. The potentials for the couples SO3.-/SO32- and SO5.-/SO52- an 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 lambda(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 N-3(.) 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-/2S(2)O(3)(2-).