PH-DEPENDENT PROCESSES AT NAFION(R)-COATED CARBON-FIBER MICROELECTRODES

Cyclic voltammetry has been used to examine pH-dependent processes in aqueous solution at carbon-fiber electrodes with scan rates above 100 V s(-1). Specifically, the reduction of p-benzoquinone and the oxidation of dopamine have been investigated. The mechanism of electrolysis of quinoidal compounds has been extensively characterized at slow scan rates with conventional electrodes in prior work. Digital simulations based on the reported rates and E(o) values show that the electrode reaction mechanisms can be diagnosed at high rates of scan by examination of the pH dependence of the peak potentials of cyclic voltammograms. However, the experimentally determined pH dependences of the voltammetric peak potentials for these compounds differ from those obtained by simulation suggesting the electrode mechanism differs at carbon-fiber electrodes. The altered mechanism may reflect surface catalysis since the data show a correlation between the position of the surface voltammetric waves found at carbon-fiber electrodes and the voltammetric peak positions of the compounds. When the electrodes are coated with Nafion(R), the pH dependence of the voltammetric waves is further altered. The surface waves are found to shift linearly with pH, and can be used to diagnose alterations in the pH of the solution.