Kinetics and mechanism of the Co(II)-assisted oxidation of L-ascorbic acid by dioxygen and nitrite in aqueous solution

A detailed study of the oxidation of L-ascorbic acid by dioxygen and nitrite in water at pH 5.8 and 7.0, catalyzed by the octasulfophenyltetrapyrazinoporphyrazine complex of cobalt(II), was carried out using conventional spectrophotometric, low-temperature and high-pressure stopped-flow techniques. The Co(II) complex activates L-ascorbic acid through an intramolecular one-electron oxidation step that involves the reduction of the octasulfophenyltetrapyrazinoporphyrazine. The reaction rate strongly depends on pH due to the different redox behaviour of the L-ascorbic acid/ascorbate species present in solution. Kinetic parameters for the different reaction steps of the catalytic process were determined. The final product of the reaction between L-ascorbic acid and nitrite was found to be nitrous oxide.