Chloride and iodide mediated oxidation of antimony(III) by cerium(IV) in aqueous sulphuric acid medium

The micro amounts of iodide (10(-7)) (mol dm(-3)) and chloride (10(-2)) (mol dm(-3)) mediated oxidation of antimony(III) by cerium(IV) in an aqueous sulphuric acid medium have been studied spectrophotometrically at 25 degrees C and mu = 3.10 mol dm(-3). The stoichiometry is 1:2 in chloride and iodide mediated reactions. i.e. one mole of antimony(III) requires two moles of cerium(IV). In the case of chloride mediated reaction, the reaction was first order in cerium(IV) and halide concentrations, whereas in the case of iodide mediated reaction the order with respect to [cerium(IV)] was unity and with respect to iodide concentrations was more than unity (ca. 1.4). In both chloride and iodide mediated reactions the order with respect to antimony(III) concentrations was less than unity. Increase in sulphuric acid concentration increased the rate. The order with respect to H+ ion concentration was less than unity. Added products, cerium(III) and antimony(V) did not have any significant effect on the reaction rate. The active species of oxidant was understood to be H3Ce(SO4)(4)(-), whereas that of reductant as SbCl3 in the case of chloride and SbI2+ in case of iodide mediated reactions. The possible reaction mechanisms were proposed and the activation parameters were determined and discussed.