Photoelectrochemical analysis of ascorbic acid

Channel electrode voltammetry is used to analyze for ascorbic acid via its photochemical reaction with toluidine blue. On formation by light of wavelength similar to 630 nm, the excited state of the dye undergoes an irreversible redox reaction with ascorbic acid producing the reduced form of the dye and oxidized ascorbic acid. Voltammograms recorded on an exhaustively preirradiated solution containing known amounts of ascorbic acid show a current flowing as a result of the reoxidation of the reduced form of the dye, whereas no current flows under dark conditions. Analysis of this current as a function of now rate leads to a direct relationship between oxidation current and the amount of ascorbic acid present initially. This, together with the observation that the phenomenon is unaffected by the presence of glucose, is used to obtain a value for the ascorbic acid content of the fruit drink Ribena which is in excellent agreement with independent nonelectrochemical measurements.