Kinetics and Mechanistic Study of Redox Reaction of Cobalt (III) Complex with L-Ascorbic Acid in Aqueous Acid Medium

The kinetics of the electron transfer reaction of L-ascorbic acid with a cobalt (III) complex has been studied spectrophotometrically over the range 0.01 ≤ [ascorbic acid], mol dm-3 ≤ 0.07, 3.3 ≤ pH ≤ 5, and at 298 ≤ T, K ≤ 318 in aqueous acid medium. Kinetics measurements were run under pseudo-first order conditions. The rate of electron transfer reaction was found to be first order with respect to ascorbic acid. The results showed that the rate constant depends on size and strength of the ligand. The activation parameters favoured the electron transfer reaction. In the absence of evidence of direct coordination of ascorbic acid to Co(III) centre, an outer-sphere electron transfer mechanism was suggested. The reduction of Co(III) complex by L-ascorbic acid depends on pH and the nature of the ligand. Considering the dependence of kobs with pH, it was suggested that HA─ reacts much faster than H2 A and A2─ is not reactive at pH< 5. HA─ is the predominant reacting species under the experimental pH of the solution.