The kinetics of oxidation of Pd II by Ce IV have been studied spectrophotometrically in HClO 4 media at 40°C. The reaction is first order each in [Ce IV ] and [Pd II ] at constant [H + ]. Increasing [H + ] accelerates the reaction rate with fractional order in [H + ]. The initially added products, palladium(IV) and cerium(III) do not have any significant effect on the reaction rate. At constant acidity, increasing the added chloride concentration enhances the rate of reaction. H 3 Ce(SO 4 ) 4) and PdCl 4 2) are the active species of oxidant and reductant respectively. The possible mechanisms are proposed and the reaction constants involved have been determined.

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The kinetics of oxidation of tyrosine by Fe(CN)63- have been studied in 40% AcOH-H2O containing 4.0 mol dm-3 HCl. The oxidation is made possible by varying the difference in redox potential by adding HCl and AcOH to the reaction mixture. The reaction is first order with respect to [Fe(CN)63- and fractional (0.72) order with respect to [tyrosine]. Increasing the acid concentration at constant chloride concentration accelerates the reaction. The order with respect to acid concentration is nearly two and H2Fe(CN)6- is regarded as the active oxidant. The initially formed product, Fe(CN)64- retards the reaction. A suitable mechanism is proposed and the reaction constants of the different steps involved have been evaluated. Activation parameters have also been calculated with respect to the slow step of the mechanism and discussed.

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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°C and l = 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 H 3 CeðSO 4 Þ À 4 , whereas that of reductant as SbCl 3 in the case of chloride and SbI 2þ in case of iodide mediated reactions. The possible reaction mechanisms were proposed and the activation parameters were determined and discussed.