Oxidation of aqueous sulfide solutions by dioxygen .1. Autoxidation reaction

The autoxidation of aqueous sulfide solutions by dioxygen is studied at pH 9 and 14 and at initial sulfide concentrations between 1 and 100 mM. Disulfide as a primary intermediate is proposed to enhance the sulfide oxidation by autocatalytically forming polysulfides. The postulate is supported by the observed acceleration of the process at increasing pH favoring the presence of disulfide rather than that of sulfur. The final reaction products thiosulfate and sulfate are proposed to result from the hydrolysis of the highly instable intermediate polythiosulfite in parallel reaction pathways, assuming probabilities for breaking the sulfur chain at the alpha (sulfate) and beta (thiosulfate) positions to be responsible for the selectivity. A reaction scheme is proposed, considering all experimental observations.