Stability of Solutions of Pure Ascorbic Acid and of Dehydroascorbic Acid

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Abstract
Ascorbic acid is stable in 0.5% oxalic acid-10% acetic acid soln. at 4[degree]C for at least 12 days ([double dagger]2.5% for 12 days measured by the Rose method and [double dagger]2.0% for 14 days measured by the Bessey method). In 5% metaphosphoric acid-10% acetic acid soln., at 4[degree]C, ascorbic acid is stable for at least 8 days ([double dagger]2.8% for 12 days by the Roe method and [double dagger]2.3% for 8 days by the Bessey method). Oxalic acid is preferable as a preservative for ascorbic acid because it is more effective, more stable, less expensive, and more convenient. Dehydroascorbic acid in oxalic or metaphosphoric acid solns. with 10% acetic acid is stable when measured by the Roe method, but unstable when measured by the Bessey method. The Roe method measures more than the biologically active forms. Dehydroascorbic acid is unstable in neutral soln. at room temp. when assayed by either method. The Roe method gives the higher values. Both metaphosphoric acid and oxalic acid stabilize ascorbic acid, but neither prevents transformation of dehydroascorbic acid into derivatives which cannot be converted back to ascorbic acid by H2S. The Roe method measures ascorbic acid, dehydroascorbic acid, and other derivatives of ascorbic acid. It is useful to estimate the original ascorbic acid content of fresh food slurried with oxalic acid and stored before analysis. Metaphosphoric acid is too unstable to prepare such slurries.

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