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 $(H_2A)$, 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 $0.1 M H_2SO_4$ at 0.2 V vs SCE corresponding to the transformation of leucoemeraldine to emeraldine gradually diminishes with an increase in concentration of $H_2A$ as a result of adsorption. This peak disappears before the appearance of another peak corresponding to the oxidation of $H_2A$ at a concentration of 1 mM. The irreversible oxidation current of $H_2A$ exhibits a linear dependence on the concentration. The effect of adsorption of $H_2A$ 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 $H_2A$ has led to an optimum thickness of $1.6 \mu m$. 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 electrooxidation of $H_2A$. 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