The Effect of Metaphosphoric Acid and Some Other Inorganic Acids on the Catalytic Oxidation of Ascorbic Acid”

The instability of ascorbic acid in weakly acid solutions has recently been shown to be due to the copper content of the solutions (1). By strict elimination of copper, solutions of ascorbic acid are stable below pH 7.6. The preparation of copper-free reagents, however, is not very practical and in analytical procedures would not eliminate copper contamination from the materials analyzed. The use of metaphosphoric acid for the extraction and titration of ascorbic acid has resulted in the stabilization of ascorbic acid solutions so treated (24). Recently diethyldithiocarbamate (5), S-hydroxyquinoline, glutathione (6), and other compounds forming copper complexes (7) have been used for the stabilization of ascorbic acid solutions. The work reported here was undertaken primarily to investigate a possible quantitative relationship of ascorbic acid, copper, and the amount of metaphosphoric acid necessary to inhibit atmospheric oxidation. A few related observations are also reported.