The role of buffers on the kinetics of L-ascorbic acid oxidation catalyzed by copper(II)

The chemical role of copper chelates on ascorbic acid oxidation was examined. The kinetics of uncatalyzed and Cu(II) ion- catalyzed oxidation of ascorbic acid in aerated and buffered (e.g., acetate, citrate, tartrate and phthalate) solutions were studied at 20degreesC. The rate of the oxidation reaction in the presence of cupric ion as the catalyst, in the pH range investigated (pH 3.2-6.5), was found to be a first - order reaction with respect to ascorbic acid. The effects of pH and cupric ion concentration on the reaction rate suggest a mechanism that may involve the formation of a transition complex between monohydrogenascorbate ion and Cu(II) - buffer chelate. The differences in the reaction rates in the various buffers are due to the different complexing abilities of Cu(II) with the various buffer salts. Coordinatively unsaturated Cu(II) - buffer complexes which are comparatively less stable than the higher chelates were more active in the Cu(II) - induced oxidation of ascorbic acid.