Kinetics And Mechanisms Of The Oxidation Of Aminocarboxylatocobaltate(ii) Complexes By Hypochlorite And Silver-catalysed Persulphate Ions In Aqueous Acidic Medium

The kinetics of the oxidation of aminocarboxylatocobaltate(II) complexes (hereafter,[Co(II)EDTA]2-and [Co(II)HEDTA(OH2)]-) by hypochlorite (ClO-) and silver-catalysedpersulphate ions (S2O82-) at 300 K; 520 nm and 299 K; 525 nm (temperature and λmax)respectively was studied in aqueous acidic mediumunder pseudo-first order conditions. For the reaction of complexes with persulphate ion, the concentrations were: I = 0.5 mol dm-3 (NaNO3), [H+] = 1.0 × 10-2 mol dm-3 (HNO3) and [Ag+] = 1.0 × 10-2mol dm-3 (AgNO3). While I = 0.5 moldm-3 and 0.2 mol dm-3(NaNO3) and [H+] = 4.0 × 10-2 mol dm-3 and 1.0 × 10-2 mol dm-3 (HNO3) arefor the reaction of hypochlorite with [Co(II)EDTA]2- and [Co(II)HEDTA(OH2)]- complexesrespectively. The stoichiometry obtained was 2:1 for the oxidation of complexes by persulphateion and 1:1 for the reaction of complexes with hypochlorite ion. First order kinetics in theconcentration of S2O82-and ClO- were observed for all the reactions except the reaction of[Co(II)HEDTA(OH2)]-with persulphate ion, where zero order with respect to concentration ofS2O82- was obtained. The reactions of complexes with were independent of [H+] butdependent on [Ag+] while with ClOthereaction rates were inversely dependent on [H+]. Thereactions conform to the following rate equations: where a = 5.90 × 10-2dm3 mol-1s-1, b = 5.4× 10-2dm3 mol-1s-1, c = 1.23 × 10-3dm3 mol-1s-1, d =2.28 × 10-4dm3 mol-1s-1. Zero salt effects for the reaction of complexes with S2O82-and positivesalt effects with ClO-were observed. The thermodynamic parameters obtained from thetemperature dependence studies showed that the activated complexes are ordered for theoxidation of [Co(II)EDTA]2-(∆H* = +28.67 kJ mol-1, ∆S* = -170.72 JK-1mol-1) and[Co(II)HEDTA(OH2)]-(∆H* = +53.12 kJmol-1, ∆S* = -113.65 JK-1mol-1) by persulphate ion.The free radical species were detected only in the reaction of complexes with persulphate ion.Addition of ions affected the rates for all the reactions except for the reaction of[Co(II)HEDTA(OH2)]- with persulphate ion, where both added cations and anions had no effecton the reaction rate. Moreover, Michaelis – Menten plot of 1/kobs versus 1/[oxidant] had zerointercept for all the reaction systems except for the oxidation of [Co(II)HEDTA(OH2)]-bypersulphate ion. Based on the above observations,an outer-sphere mechanism is proposed as theplausible mechanism for all the reactions except for the reaction of [Co(II)HEDTA(OH2)]-withpersulphate ion, where an inner-sphere mechanism is proposed.

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