The kinetics of photosensitized decomposition of ascorbic acid and the determination of hydrogen peroxide as a reaction product

In this work, the kinetics of photosensitized decomposition of ascorbic acid in citrate buffer solution at 25 °C, 35 °C, 45 °C under anaerobic conditions have been investigated. Riboflavin was used as a sensitizer. With increasing temperature it was observed that the initial rate of reaction also increased. However, with increasing concentration of citrate ions, the initial rate decreased. Reactions were studied at ascorbic acid concentrations of 10−3 M, 2 × 10−3 M and 4 × 10−3 M concentrations and in 0.1 M, 0.2 M and 0.4 M citrate ions at pH 4.5. A 100 W tungsten lamp was used as a light source. The amount of decomposed ascorbic acid was determined by a polarographic technique. The formation of hydrogen peroxide (H2O2) as a reaction product was followed quantitatively by recording the current of the H2O2 wave at a half-wave potential of about −0.8 V. This wave was the second polarographic wave of oxygen. The formation of H2O2 was confirmed by an infrared technique. An almost 1:1 molar relationship was found between decomposed ascorbic acid and H2O2 formed. In this reaction riboflavin is excited first by visible light and (through an excitation transfer) ascorbic acid is then oxidized. A reaction mechanism which includes H2O2 is proposed.