Kinetics of the Oxidation of Ascorbic Acid by the Copper (II) Ion in an Acetate Buffer Solution

The kinetics of the oxidation of ascorbic acid by the copper(II) ion has been studied spectrophotometrically by a stopped-flow technique. At pH 4–6, the initial rate of the decrease in the monohydroascorbate anion is of the second-order with respect to the total concentration of the copper(II) ion and of the first-order with respect to the concentrations of the monovalent and divalent ions of ascorbic acid: -(d[HA^-]/dt)_t=0=k_1[Cu(II)]_T^2[HA^-]+k_2[Cu(II)]_T^2[A^2-] where HA− and A2− are the monovalent and divalent ions of ascorbic acid respectively. k1, k2, and the activation parameters are obtained as follows: k1=3.1×104, k2=6.1×1010 M−2 s−1, ΔH=17 kcal/mol, ΔS=8.3 cal/deg mol, and ΔG=15 kcal/mol. The EPR signal from the ascorbate radical is not observed in the kinetic run by either a flow technique or a quenching technique. Some discussions of the mechanism of the reaction are given.