Kinetic study of the oxidation of ascorbic acid by aqueous copper(II) catalysed by chloride ion

Two methods of monitoring the chloride-catalysed oxidation of ascorbic acid by aqueous Cu-II have been developed that allow the reaction conditions to be varied more widely than previously and thereby permit a fuller elucidation of the behaviour and rate law for this system. It has been possible to study the reaction over a wide concentration range from 1.6 to 500 mM Cl- and 4 to 100 mM Cu-II. For the first time, it is shown that the ascorbic acid (H(2)asc)-Cu-II-chloride ion-Cu-I-dehydroascorbic acid (dha) system comes to equilibrium, under anaerobic : conditions, with all species present and is driven towards products by chloride complexation of Cu-I. The results are consistent with the following reaction scheme (i = 0-2). The full rate law has been elucidated and values for various k(1i), k(-2i) and k(2)/k(-1i) have been determined. [GRAPHICS] /SUP> = Cd2+. In the cobalt(II) system, [CoL3](4-), where log(beta(3)/dm(9) mol(-3))=25.56 +/- 0.11, was also detected. The pK(a) values for H3L+ are 1.87 +/- 0.10, 3.72 +/- 0.03 and 10.01 +/- 0.02. These data are discussed together with the fluorescence characteristics of HL-, [ZnL] and [ZnL2](2-).