Kinetics of iron(III)-catalyzed autoxidation of sulfur(IV) in acetate buffered medium

Abstract

The kinetics of the environmentally important oxidation of sulfur(IV) by oxygen in acetate buffered medium in the presence of Fe(III) and the pH range 5.27–5.70 has been studied. The results were in agreement with the rate law: $$ -\hbox{d}[\hbox{S}(\hbox{IV})]/\hbox{d}t=k_{0}[\hbox{S}(\hbox{IV})] +k_{\rm Fe}[\hbox{Fe}(\hbox{III})][\hbox{S}(\hbox{IV})][\hbox{H}^{+}]^{-1} $$ The role of iron(III) appears to be that of production of SO 3 − radicals in Fe(III)SO 3 2− complex by an internal 1-equivalent redox reaction. Subsequently, a radical mechanism involving oxysulfur radicals, viz., SO 3 − , SO 4 − , and SO 5 − operates. Addition of ethanol leads to the introduction of an induction period and decrease in reaction rate, most likely due to scavenging of SO 4 − radicals. The value of apparent energy of activation is 45.4 kJ mol−1.