Effect of photo-oxidation of ascorbic acid on the determination of hydrogen peroxide and 3-hydroxybutyric acid using the titanium(iv)–porphyrin reagent

Abstract

The Ti-TPyP reagent, i.e. an acidic aqueous solution of oxo[5,10,15,20-tetra(4-pyridyl)-porphyrinato]titanium(IV) complex, denoted as TiO(tpyp), was developed as a highly sensitive reagent for determining hydrogen peroxide. The Ti-TPyP reagent exhibits a sharp absorption peak at 432 nm. Following the addition of hydrogen peroxide to the reagent, a new absorption peak appeared at 450 nm. Its absorbance was proportional to the hydrogen peroxide concentration with an apparent molar absorptivity of 1.1 × 10⁵ L mol⁻¹ cm⁻¹. In the determination of hydrogen peroxide using the Ti-TPyP reagent, ascorbic acid concurrently present in the samples caused a positive error in the analytical results. In the present paper, the source of the error was revealed to be due to the production of hydrogen peroxide through the oxidation of ascorbic acid by singlet oxygen arising from the energy transfer between the dissolved oxygen molecule and the photo-excited porphyrin moiety of the TiO(tpyp) complex. The reaction mechanism was proposed for the photo-oxidation of ascorbic acid caused by the TiO(tpyp) complex to produce dehydroascorbic acid and hydrogen peroxide. The effect of ascorbic acid is really a serious problem in the practical uses of the Ti-TPyP reagent in food and biochemical analysis. However, use of ascorbic acid oxidase as a scavenger for ascorbic acid was found to be quite effective to remove its effect in the flow injection analysis of 3-hydroxybutyric acid in human serum.