Mechanism of the oxidation ofL-ascorbic acid by the pentaammineaquacobalt(III) ion in aqueous solution

Abstract

The kinetics of oxidation ofl-ascorbic acid (H2A) by the mononuclear complex [Co(NH3)5(OH2)] (ClO3)3 have been studied spectrophotometrically at 490 nm. The mechanism involves single one-electron transfers involving the pentaamminehydroxo complex and the ascorbate anions (HA− and A2−), subsequent formation of ascorbate radicals and > 90% CoII. The appropriate rate law in the 7.00 ⩽ pH ⩽ 8.40 and 0.005 ⩽ [AT] ⩽ 0.05 mol dm−3 ranges, has been established as: $$\begin{gathered} - d [ complex] /dt = \{ (k_{\text{3}} {\text{[H}}^{\text{ + }} {\text{]}} \hfill \\ {\text{ + }}k_{\text{4}} K_{{\text{a1}}} {\text{)}}K_{\text{2}} {\text{[}}A{\text{]}}_{\text{T}} {\text{[complex]}}_T \} / \hfill \\ {\text{ \{ ([H}}^{\text{ + }} {\text{ + }}K_{{\text{a1}}} {\text{)([H}}^{\text{ + }} {\text{] + }}K_{\text{2}} {\text{)\} }} \hfill \\ \end{gathered} $$