Oxidation of ascorbic acid by nitrous acid: conditions where autoxidation of nitric oxide is rate determining

In air-saturated aqueous acid (0.01–0.1 mol dm–3) with equimolar (1 × 10–4 mol dm–3) ascorbic acid and nitrous acid, the second-order reaction has a first-order kinetic form due to rapid regeneration of nitrous acid by autoxidation of the nitric oxide produced, and the rate constant takes the form k=a[H+]+b[H+][Cl–], typical of a rate-determining nitrosation, whereas in 0.5–1 mol dm–3 HCl with a deficiency of nitrous acid autoxidation of nitric oxide is for the most part rate-determining.