Role of trace metal ions. Kinetics and mechanism of the copper(II)-catalysed oxidation of ascorbic acid with peroxodiphosphate in acetate buffers

Abstract

The reaction between peroxodiphosphate and ascorbic acid (H2A) in acetate buffers is essentially catalysed by trace metal-ion impurities of CuII and FeIII present in the reagents. Copper(II) has an about ten times larger catalytic activity than FeIII, and the activity is so specific and precise that one can calculate the amount of CuII present in a system as an impurity, from the initial rates of ‘uncatalysed’ reactions. The reactions in the absence and presence of copper(II) have the same characteristics. The rate law (i) is obeyed for the catalysed reaction, where K1d and K2d are –d[H2A]//dt=[H2A]T[CuII](k1[H+]+k2Kd1+k2Kd2/[H+])//(Kd1+[H+])(i) the acid dissociation constants of ascorbic acid, and k1, k2, and k3 are the rate constants for the reactions CuII+ H2A, CuII+ HA–, and CuII and A2– respectively. The values of K1, k2, and k3 were found to be 27 ± 2, (1.24 ± 0.10)× 103, and (1.1 ± 0.08)× 1010 mol–1 dm3 s–1 at 35° and I= 1.0 mol dm–3.