The copper-catalysed oxidation of ascorbic acid by dioxygen. Part 4. The effect of chloride ions on the kinetics and mechanism

The effect of chloride ions on the kinetics and mechanism of the copper-catalysed oxidation of ascorbic acid by dissolved oxygen has been examined over the pH range 2.0-3.5 and chloride concentrations up to 0.2 mol dm^-3. The empirical rate law has been established as - d[O₂]/dt = k_obs.[O₂]^1/2 over the whole range. At constant total ascorbate, total copper, and hydrogen ion concentrations, the general relationship (1) was obtained. The constants k_obs. = A + B[Cl^-]/C + D[Cl^-] + E[Cl^-]² (1) A to E were evaluated using data obtained in pure nitrate media to fix A and C, and the dependence of all the constants on pH, total copper concentration, and total ascorbate concentrations established experimentally. It is also pointed out that a mechanism based on the involvement of copper(I) rather than the copper(III) invoked by us cannot be entirely ruled out.