Mechanism of the bromate ion oxidation of the cobalt(II) complex of trans-cyclohexane-1,2-diamine-NNN′N′-tetra-acetic acid

Abstract

Kinetics of the oxidation of [Co(cdta)]^2–(H₄cdta =trans-cyclohexane-l,2-diamine-NNN′N′-tetra-acetic acid) by bromate ion have been studied in the ranges of [H⁺]= 1 × 10^–2– 1 × 10^–3 mol dm^–3 and [BrO₃^–]= 5 × 10^–2– 37.5 × 10^–2 mol dm^–3 at 60 °C with I=1.0 mol dm^–3(KNO₃). The second-order rate constant k in the rate law d[Co(cdta)^–]/dt=k[Co(cdta)^2–][BrO₃^–] is dependent on [H⁺] : k=a[H⁺]²/(b+[H⁺]). A reasonably good fit of the data to this equation is obtained with a=(13.1 ± 1.2)× 10^–2 dm⁶ mol^–2 s^–1 and b=(5.31 ± 0.2)× 10^–3 mol dm^–3. A possible mechanism has been deduced. This involves an inner-sphere electron transfer path in which a binuclear species containing the cobalt(II) substrate and BrO₂⁺ is formed in the slow step.