Mechanism of oxidation of L-ascorbic acid by the pentaamminechromatocobalt(III) complex ion in aqueous solution

Abstract

Oxidation of L-ascorbic acid by pentaamminechromatocobalt(III) nitrate has been investigated over the ranges 6.1 ⩽ pH ⩽ 8.7, 21.0 ⩽ θ ⩽ 30.0 °C, in excess of L-ascorbic acid at a constant ionic strength of 0.50 mol dm ^-3 (NaClO ₄ ). Overall the reaction occurs in two distinct stages. The first, involving saturation kinetics, proceeds with an increase in absorbance at wavelengths 350–390 nm. During this stage the pentaamminechromato complex and the ascorbate anion react via an outer-sphere mechanism to form an ion pair. The rate equation for this stage is (i) with [A] _T being total ascorbate, K ₄ and k ₅ are the equilibrium constant for adduct formation and first-order rate constant for its decomposition, respectively. At 25.0 °C, k ₅ and K ₄ have respective values of (3.5 ± 0.4) × 10 ^- s ^-1 and (8.5 ± 1.7) × 10 ² dm ³ mol ^-1 . The second stage involving a decrease in absorbance at similar wavelengths is the reduction of the free chromium(VI) ion which is postulated to have been formed during the first stage of the reaction. The pseudo-first-order rate constant for this stage can be expressed as in equation (ii) where K ₁ and K ₆ are the proton-dissociation constants for ascorbate and the hydrogenchromate anion. At 25 °C, the rate constant for oxidation of ascorbate by HCrO ₄ ^- , k ₇ , is 5.15 ± 0.06 dm ³ mol ^- s ^-1 while k ₈ , the corresponding value for oxidation by CrO ₄ ^2- , is (4.6 ± 1.5) × 10 ^- . Cobalt(III) is reduced to cobalt(II), as >90% cobalt(II) was detected at the end of the second stage of the kinetic reaction. The activation enthalpies lie in the range 20–47 kJ mol ^- , and the ΔS ^‡ values range from -204 J K ^- mol ^-1 for the first stage to -50 J K ^-1 mol ^-1 for the k ₈ step in the second stage. The mechanism is discussed with respect to the oxidising properties of both the cobalt and chromium centres, and a comparison is also made with the reaction of free chromate ion and L-ascorbic acid.