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 Cu^II and Fe^III present in the reagents. Copper(II) has an about ten times larger catalytic activity than Fe^III, and the activity is so specific and precise that one can calculate the amount of Cu^II 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 K¹_d and K²_d are –d[H₂A]//_dt=[H₂A]_T[Cu^II](k₁[H⁺]+k₂K_d¹+k₂K_d²/[H⁺])//_{(Kd¹+[H⁺])}(i) the acid dissociation constants of ascorbic acid, and k₁, k₂, and k₃ are the rate constants for the reactions Cu^II+ H₂A, Cu^II+ HA^–, and Cu^II and A^2– 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.