Electrooxidation of ascorbic acid on a polyaniline-deposited nickel electrode: Surface modification of a non-platinum metal for an electrooxidative analysis

The electrooxidation of ascorbic acid (H(2)A), which does not occur on a bare Ni electrode, has been shown to take place on a polyaniline (PANI)-coated Ni electrode in aqueous electrolytes of a wide pH range. The characteristic voltammetric peak of PANI in 0.1 M H2SO4 at 0.2 V vs SCE corresponding to the transformation of leucoemeraldine to emeraldine gradually diminishes with an increase in concentration of H(2)A as a result of adsorption. This peak disappears before the appearance of another peak corresponding to the oxidation of H(2)A at a concentration of 1 mM. The irreversible oxidation current of H(2)A exhibits a linear dependence on the concentration. The effect of adsorption of H(2)A on PANI has been shown to increase the voltammetric peak current. A study on the variation of the PANI thickness and its influence on the voltammetric oxidation of H(2)A has led to an optimum thickness of 1.6 mum. The oxidation currents on the porous PANI/Ni electrode have been found to be several times higher at lower potentials in comparison with the data of a Pt electrode. The reaction has also been studied by ac impedance spectroscopy. In alkaline electrolytes, the Nyquist impedance plot is characterized by two semicircles instead of a single semicircle in acidic electrolytes. Thus, Ni, which is a non-platinum metal, has been found to be useful, by surface modification with PANI, for electrooxidadon of H(2)A. The data are reproducible in the electrolytes of a wide pH range, thus suggesting a good stability, reusability and a long life for the PANI/Ni electrodes.