Kinetics of the oxidation of ethylenediaminetetraacetato and N-(2-hydroxyethyl)ethylenediamine-N,N',N'-triacetato complexes of cobalt(II) by hydrogen peroxide in aqueous acidic medium.

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Kinetics of the oxidation of Co(II) complexes, Co(edta)2− and Co(hedta)− [H4edta=ethylenediaminetetra-acetic acid and H3hedta=N-(2-hydroxyethyl)ethylenediamine-N,N′,N′-triacetic acid] have been studied in aqueous acidic medium (pH 3.6–4.8) at 30–60 °C. The reactions were shown to exhibit second-order kinetics, first-order in each of the reactants. In the pH range studied, the rates of reaction showed inverse [H+] dependence which could be accounted for in terms of the acid-base equilibrium H2O2\oversetKa\rightleftharpoonsHO2−+H+. Applying the limiting conditions [H+]>>Ka, [H2O2]>>[HO2−], the rate-law is given by d[CoIII−L]⁄dt=2{kH2O2+kHO2−Ka[H+]−1} [CoII−L] [H2O2] where L denotes the general form of the ligand in either case. At 30 °C (I, 0.5 mol dm−3), values obtained are kH2O2=(5.6±0.3)×10−5 mol−1 dm3 s−1, kHO2−=(5.4±0.3)×102 mol−1 dm3 s−1 for the Co(edta)2− complex and kH2O2=(4.5±0.4)×10−5 mol−1 dm3 s−1, kHO2−=(5.1±0.5)×102 mol−1 dm3 s−1 for the Co(hedta)− complex. The corresponding activation parameters are ΔH\neweq=(71±8) kJ mol−1, ΔS\neweq=−(92±23) J K−1 mol−1 (Co(edta)2− system) and ΔH\neweq=(89±2) kJ mol−1 and ΔS\neweq=−(36±4) J K−1 mol−1 (Co(hedta)− system).
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