Iodide mediated oxidation of arsenic(III) by cerium(IV) in aqueous sulphuric acid and sulphate media- A kinetic and mechanistic study

P. G. Department of Studies in Chemistry, Karnatak University, Dharwad-580 003, Karnataka, India E-mail : schimatadar@gmail.com Manuscript received online 23 February 2012, revised 10 May 2012, accepted 22 May 2012 A minute (10 -8 mol dm -3 ) quantity of iodide mediated oxidation of arsenic(III) by cerium(IV) in an aqueous sulphuric acid and sulphate media has been studied spectrophotometrically at 25 °C. The stoichiometry is 1 : 2, i.e. one mole of arsenic(lll) requires two moles of cerium(IV). The reaction is first order each in cerium(IV) and iodide concentrations. The order with respect to arsenic(lll) is less than unity. Increase in sulphuric acid concentration increased the rate. In situ H + ion concentration is calculated by the knowledge of equilibrium constants and the order with respect to H + ion concentration is found to be less than unity. The ionic strength did not have any significant effect on the rate of reaction, whereas rate constant increased with decreasing dielectric constant of the medium. Added products cerium(lll) and arsenic(V) did not have any significant effect on the reaction rate. The active species of oxidant and reductant are Ce(SO 4 ) 2 + and Asl 2 + respectively. A suitable mechanism is proposed. The catalytic constant and activation parameters are determined. The.results obtained in the present study are similar to that of bromide, chloride and iodide mediated oxidation of antimony(III) by cerium(IV) in aqueous sulphuric acid-sulfate media and are compared and discussed.