Kinetics of the peroxodisulphate and hydrogen peroxide oxidations oftrans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetatocobaltate(II) in acidic and basic buffer media

Summary

Kinetics of the oxidation of Co(cydta)²⁻ (H₄cydta=trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid) by peroxodisulphate (S₂O ²⁻₈ ) and by hydrogen peroxide have been studied in the ranges of pH 3.6–5.8 (acetate buffer) and 6.0–8.0 (phosphate buffer), respectively. For the first oxidant, the reaction was shown to exhibit second-order kinetics, first-order in each of the reactants. No pH dependence on rate was observed. The hydrogen peroxide reaction was also second-order, being first-order in each of the reactants. The rate of reaction showed inverse [H⁺] dependence. The rate law is given by${\text{d[Co(cydta)}}^-- {\text{]/dt = 2\{ k}}_{{\text{H}}_{\text{2}} O_2 } {\text{ + k}}_{{\text{H}}O_2 - } {\text{K}}_{\text{a}} {\text{[H}}^{\text{ + }} {\text{]}}^{--1} {\text{\} [Co(cydta)}}^{{\text{2}}--} {\text{][H}}_{\text{2}} {\text{O}}_{\text{2}} {\text{]}}$. At 30 °C (I, 0.5 M), the ratio of$k_{HO_2 - } /k_{H_2 O_2 } \sim 10^5$. The corresponding activation parameters obtained from the temperature-dependence of rate are ΔH^≠= (136±9) kJ mol⁻¹, ΔS^≠=(119±10) JK⁻¹ mol⁻¹ (for the peroxodisulphate system) and ΔH^≠=(75±2) kJ mol⁻¹, ΔS^≠ =(−78±4) JK⁻¹ mol⁻¹ (for the hydrogen peroxide system).