Dixon, D.,; Dasgupta, Tara, P.; Sadler-McKnight, N.
Author Affiliation, Ana.
Chemistry; Faculty of Pure and Applied Sciences
Mechanism of the oxidation of L-ascorbic acid by the pentaamminechromatocobalt(III) complex ion in aqueous solution
Journal of the Chemical Society. Dalton Transactions
Date of Publication
Also published in Transition Metal Chemistry, vol. 20 : 295-99
Oxidation of L-ascorbic acid by pentaamminechromatocobalt(III) nitrate has been investigated over the ranges 6.1£ pH £ 8.7,21.0 £ q £ 30.0°C, in excess of L-ascorbic acid at a constant ionic strength 0.50 mol dm-3 (NaC1O4). 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) 1/kobs= (1/k5K4[A]T) + 1/k5) (i) with [A]T being total ascorbate, K4 and k5 are the equilibrium constant for adduct formation and first-order rate constant for its decomposition, respectively. At 25.0°C, k5 and K4 have respective values of (3.4 ± 0.4) x 10-2 s-1 and (8.5 ± 1.7) x 102 dm-3 mol-1. The second stage involving a decrease in absorbance at similar wavelengths in the reduction of the free chromium(VI) ion which is postulated to have 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 k1 and K6 (k7[H+] + k2K6)K1[A]T kobs= _______________________________ (ii) [H+] + K6)(K1+ [H+]) are the proton-dissociation constants for ascorbate and the hydrogenchromate anion. At 25.0°C, the rate constant for oxidation of ascorbate by HCrO4-, k7, is 5.15 ± 0.06 dm3 mol-1 s-1 while k8, the corresponding value for oxidation by CrO42-, is (4.6 ± 1.5) x 10-2. 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-1, and the DS‡ values range from -204 J K-1 mol-1 for the first stage to -50 J K-1 mol-1 for the k8 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....