The influence of halides on the oxidation of ascorbic acid

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Abstract
The oxidation of ascorbic acid was studied under standardized conditions in various acid solutions heated to 100[degree] for 30 min. Ascorbic acid was not oxidized at 100[degree] if Cu was reduced to 1 part in 650 X 106 of soln. The oxidation was greatly reduced if chloride, bromide or iodide salts were added in sufficient amt., the extent of the reduction being proportional to the conc. of the halide present. In weaker cones., however, the halides had a slight destructive effect. Both the destructive and protective influences of the salts were correlated with the amount of Cu present. The action of the halide salts was independent of the nature of the cation, though slightly influenced by the valency of the salt. The relative efficiency of the 3 halides in stabilizing ascorbic acid was of the order, iodide > bromide > chloride. Fluoride had no protective action. The protective influence of the halides was operative over a pH range of 2.0-6.0, with a maximum effect at pH 2.3-4.0, but diminished rapidly between 4.0 and 6.0 or below 2.0. At pH 1.20 a high conc. of halide was deleterious. Hydroxy acids antagonized the protective action of the halide salts. The influence of the halides on the rate of oxidation of ascorbic acid at 20[degree] corresponded qualitatively with their influence at 100[degree]. Quantitatively, however. the destructive effect of chloride and bromide at lower concs. was markedly increased, but that of the iodide not significantly affected by the alteration of temp. An increase in the conc. of O2 enhanced the destructive influence of the halides at 20[degree].