Kinetic investigation of the reactions connected to the system Ascorbate + O2 by amperometric detection of H2O2 at a modified platinum electrode

A Pt electrode modified by an electrochemically produced bilayer polymeric membrane [polypyrrole/poly(o-phenylenediamine)] entrapping the enzyme glucose oxidase proved able to detect (response time of few seconds) amperometrically (at +0.7 V vs Ag/AgCl) very low concentrations of hydrogen peroxide (micromolar range) in the presence of much higher amounts of ascorbate. The currents due to ascorbate, also electroactive at the given potential, were negligible under any conditions due to its almost complete rejection by the electrode-modifying membrane system. The very peculiar properties of the device setup were exploited to undertake a kinetic study of the reactions connected to the system ascorbate + O2, following the concentration of H2O2 produced in the reaction mixture at 27 °C, pH ) 7. The reaction between ascorbate and H2O2 was also considered; however, different kinetic models based on the two consecutive reactions proved unable to fit the data. An investigation on the single processes by the same experimental approach was then undertaken, leading to two explanations for the inadequacy of simple kinetic models. First, the presence of metal ion traces in the reaction mixture proved to be responsible for the nonlinear dependence of the rate of both reactions on the ascorbate concentration: a mechanism involving the role of ascorbate-metal complexes as the reactants was hypothesized to explain this result. Second, the influence of the reactivity of dehydroascorbic acid, the product of ascorbate oxidation, on the kinetics was ascertained.