Abstract

The kinetics of the copper(II) catalysed reduction of ethylenediaminetetraacetatocobaltate(III) ion (hereafter [Co(III)EDTA]–) by L-ascorbic acid (hereafter H2A) has been investigated spectrophotometrically in aqueous acidic medium at λmax = 535 nm. The reaction follows a stoichiometry of 1:2 for H2A to [Co(III)EDTA]–. The order of reaction was first order with respect to both H2A and [Co(III)EDTA]– with a second order rate constant, k2, value of (9.88 ± 0.065) × 10-3 dm3 mol-1 s-1. Pseudo – first order rate constant, kobs, values were stable with changes in [H+] {(0.5 - 8.0) ×10-3 mol dm-3}, while a negative kinetic salt effect was noticed with changes in ionic strength (Na2SO4). The experimental data fits the rate law Rate = K_eq k_1 [〖Cu〗^(2+) ][H_2 A][Co(EDTA)^-]. Activated parameters were obtained as ΔH‡ = 61.85 KJ mol–1 and ΔS‡ = ̶ 77.09 JK–1 mol. The effect of added anions inhibition and absence of intermediate as seen from the Michaelis-Menten plot led to the conclusion that the reaction follows an outer – sphere mechanistic path.