MECHANISM OF OXIDATION OF L-ASCORBIC-ACID BY THE PENTAAMMINECHROMATOCOBALT(III) COMPLEX ION IN AQUEOUS-SOLUTION

Oxidation of L-ascorbic acid by pentaamminechromatocobalt(III) nitrate has been investigated over the ranges 6.1 less than or equal to pH le ss than or equal to 8.7, 21.0 less than or equal to theta less than or equal to 30.0 degrees C, in excess of L-ascorbic acid at a constant i onic strength of 0.50 mol dm(-3) (NaClO4). Overall the reaction occurs in two distinct stages. The first, involving saturation kinetics, pro ceeds with an increase in absorbance at wavelengths 350-390 nm. During this stage the pentaamminechromato complex and the ascorbate anion re act via an outer-sphere mechanism to form an ion pair. The rate equati on for this stage is (i) 1/k(obs) = (1/k(5)K(4)[A](T)) + (1/k(5)) (i) with [A](T) being total ascorbate, K-4 and k(5) are the equilibrium co nstant for adduct formation and first-order rate constant for its deco mposition, respectively. At 25.0 degrees C, k(5) and K-4 have respecti ve values of (3.5 +/- 0.4) x 10(-2) s(-1) and (8.5 +/- 1.7) x 10(2) dm (3) mol(-1). The second stage involving a decrease in absorbance at si milar wavelengths is the reduction of the free chromium(VI) ion which is postulated to have been formed during the first stage of the reacti on. The pseudo-first-order rate constant for this stage can be express ed as in equation (ii) where K-1 and K-6 k(obs) = (k(7)[H+] + k(8)K(6) )K-1[A]T/([H+] + K-6)(K-1 + [H+]) (ii) are the proton-dissociation con stants for ascorbate and the hydrogenchromate anion: At 25 degrees C, the rate constant for oxidation of ascorbate by HCrO4-, k(7), is 5.15 +/- 0.06 dm(3) mol(-1) s(-1) while k(8), the corresponding value for o xidation 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 secon d stage of the kinetic reaction. The activation enthalpies lie in the range 20-47 kJ mol(-1), and the Delta S-double dagger values range fro m -204 J K-1 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 a nd L-ascorbic acid.