Oxidation of biological substrates by chromium(VI). Part 1. Mechanism of the oxidation of L-ascorbic acid in aqueous solution

Abstract

The kinetics of oxidation of L-ascorbic acid (H₂A) by potassium chromate has been studied under aerobic and anaerobic conditions over the range 0.002 ⩽[A]_T⩽ 0.040 mol dm^–3, 3.50 ⩽ pH ⩽ 8.70, 17.8 ⩽T⩽ 35.1 °C, 0.06 ⩽l⩽ 0.50 mol dm^–3(NaClO₄) and 0.05 ⩽[O₂]⩽ 0.12 mmol dm^–3. The experimental rate constants obtained in the presence of oxygen are about 10 times smaller than those obtained in its absence. A mechanism involving the formation of a chromium(VI)–ascorbate–oxygen intermediate is discussed in an attempt to explain this. The disappearance of chromium(VI) under aerobic conditions follows the rate law (I) where k_f=(k₁K_a1[H⁺]+k₂K_a1K_c+k₃[H⁺]²)/(K_a1+[H⁺])(K_c+[H⁺]). –d[Cr^VI]/dt=k_f[A]_T[Cr^VI]_T(I) At 25 °C and l= 0.50 mol dm^–3(NaClO₄), k₁= 59.4 ± 0.5, k₂= 0.10 ± 0.02 and k₃= 115 ± 3 dm³ mol^–1 s^–1.