Photochemistry of sodium thiosulfate in aqueous solutions revisited

A mechanistic study of sodium thiosulfate (S2O32−) photolysis in aqueous solutions at neutral pH was performed using stationary and nanosecond laser flash photolysis. S2O32− photochemistry was found to be oxygen-dependent. Quantum yield of S2O32− disappearance was measured; it was found to depend on irradiation wavelength and presence of dissolved oxygen in solutions. The photochemical properties were explained by means of model containing two primary reactions, namely photoionization and photodissociation of thiosulfate to S•− and SO3•− radical anions. In the presence of dissolved oxygen we were forced to propose the formation of a weak complex formed by thiosulfate and dioxygen, [S2O32−…O2]. The photoexcitation of this complex directly yields S2O3•− and O2•−. Kinetic behavior of intermediate absorption in laser flash photolysis experiments was found to be rather complicated because of the presence of an aquated electron and several sulfur-containing radical anions. In oxygen-free solutions these radicals are S2O3•−, •S4O63−, S•−, and SO3•−. In the presence of dissolved oxygen SO2•−, SO5•− and S2O5•− radicals as well as superoxide anion radical should be added to explain the kinetics of intermediate absorption. The existence of the S2O5•− radical anion was proposed for the first time. Several rate constants of these radicals’ reactions were measured.