Abstract.
The redox couple between ascorbic acid (vitamin C) oxidation and reduction of the
semiquinoid form of p-phenylenediamine (PPDA)is the basis of a new chronometric assay of
vitamin C.Commercial horseradish peroxidase or citrus peroxidase isolated from seeds catalyzes the hydroperoxidation of PPDA through a reversible semiquinoid form to a colored condensation product. Ascorbic acid interrupts formation of the colored product by coupling its oxidation to reduction of the semiquinoid. After all the ascor
bic acid is oxidized, oxidation of the semiquinoid proceeds to the dye form. By using a standard peroxidase preparation, ascorbic acid content of orange juice was shown to be inversely proportional to the time lag from initiation of the reaction to first appearance of colored product. A rapid "yes-no" method for testing juices for label
ing compliance and a simple method for quantitating ascorbic acid levels in citrus juices were developed.