On the electrochemical behaviour of quinine at the mercury electrode in aqueous solutions

Abstract

It is shown that the so-called “prewave” displayed by quinine in alkaline solutions is a misnomer. It has been identified as the narrow part of the polarization curve where the potential is such that quinine adsorption or reduction is absent, corresponding to an interphase whose inner layer is only populated by water molecules. This conclusion is supported by the absolute values of the charge densities, the shape of the current-time polarographic transients and the fact that extension of its potential domain is favoured by increasing the pH and/or lowering of the quinine concentration. Double-potential programmes have been used to investigate the kinetics of the adsorption-desorption process of quinine. It appears that desorption is controlled by a nucleation and growth mechanism of the Avrami type, while adsorption gives monotonic falling transients. The electroreduction of quinine has been investigated in part and some of the properties of the reduction product assessed (electro-oxidizability, adsorbability). Potential-step chronocoulometry gave a value of the superficial excess of quinine, which has been used to propose a model for the film structure. Inhibition experiments with quinine and its reduction product indicate that both surfactants act efficiently, but to a lesser extent than smaller aromatic bases, and the reduction product is a more active inhibitor than quinine itself.