Square-Wave Voltammetric Determination of Hydrogen Peroxide Generated from the Oxidation of Ascorbic Acid in a Model Wine Base

The stoichiometry of the formation of hydrogen peroxide from ascorbic acid in a model wine system has been examined by square-wave voltammetry (SWV) on a hanging mercury drop electrode. Both ascorbic acid and hydrogen peroxide could be determined in the same test sample by first employing an anodic scan for ascorbic acid and then a cathodic scan for hydrogen peroxide. Linear calibration plots were obtained for ascorbic acid (up to 235 mg/L; 1.335×10-3 M) and hydrogen peroxide (up to 1.2×10-4 M), both ranges being appropriate given the concentrations likely to occur in white wine. The reaction stoichiometry for the oxidation of ascorbic acid to the production of hydrogen peroxide showed significant deviation from the expected value of 1 : 1. Evidence is presented to suggest that this deviation from the expected stoichiometry is due in part to the generated hydrogen peroxide reacting with residual ascorbic acid. The generation of ascorbic acid in model wine base solutions containing dehydroascorbic acid was observed but hydrogen peroxide was not a critical component of this interconversion reaction. The implication of these observations for the browning of white wines is discussed. The advantages of square-wave voltammetry for the real time measurement of the reaction sequences involving ascorbic acid are highlighted.