Studies on the Influence of Parallel Processes on Hydrogen Evolution from Weak Acids at a Normal and Hydrodynamically Modulated Rotating Disk Electrode

The simultaneous reduction of water and a weak acid, HA, and the coupled homogeneous reaction of OH− and HA effect the response of an ordinary rotating disk electrode and one subjected to sinusoidal hydrodynamic modulation in very different ways. The ordinary rotating disk “limiting” current increases as the electrode potential becomes more negative, while the “limiting” modulated current response shows a rapid decrease from its “limiting” value. The distortion in the limiting rotating disk electrode plateauand modulated current responses are due to the reaction of hydroxyl ions with the electro-inactive weak acid. This increases theaverage flux of the weak acid to the reaction layer and also increases the total production of hydronium ions by dissociation of HA. The convective-diffusion problem has beensolved for both kinds of rotating disk electrodes. Expressions for the distorted kinetic limiting plateau current and modulated current as a function of the dissociation, equilibrium and rate constants for the weak acid, applied potential, rotation speed, diffusion coefficients of the species, and Tafel parameters for the reduction of water, are give.