Voltammetry in Weakly Supported Media: The Stripping of Thallium from a Hemispherical Amalgam Drop. Theory and Experiment

Abstract

A theoretical model for electrochemical processes in resistive media is applied to interpret the current measured for the stripping of thallium from a Tl/Hg amalgam. Chronoamperometry is presented for a potential step experiment at a 12.5 µm radius hemispherical mercury drop in which thallium is first deposited and then stripped. Unusual features are observed in the transient stripping current, and it is proposed that these occur when the concentration of thallium(I) cations in solution is so great that the inert electrolyte salt is no longer in excess and the system is only partially supported. The theoretical model uses the Nernst-Planck-Poisson equations, which take into account the effects of the electrical potential in the aqueous phase, and avoids making the approximation of electroneutrality. The numerically simulated current accurately predicts the unusual experimental behavior.

The impact of high Zn° concentrations on the application of AGNES to determine free Zn(II) concentration

“…The similarity of the sluggish stripping of Zn and Cd at comparable high concentrations also lends support to the origin of acb in electroneutrality. This explanation for the sluggish stripping (observation 3) matches theoretical and experimental results (on Tl) recently published [16,17] and could also imply that the potential relevant for the Nernstian equilibrium be different from the nominal applied potential. However, this mechanism of low medium conductivity cannot explain (at least straightforwardly) the current enhancement of observation 4 (for Zn) or why longer deposition times reduce them (observation 5) given that, for the stripping stage, we impose diffusion limited conditions inside the amalgam where migration cannot have any effect on Zn°t ransport.…”

“…Moreover, a long tradition of literature [12][13][14][15] has pointed out that there might be ''kinetic" problems: even at reduced metal concentration globally lower than the solubility limit, during the deposition there might be spots (in the amalgam, close to the surface of the interphase) where the local concentration exceeds the solubility limit and forms deposits. More recently, experimental and theoretical evidences (with Tl) indicate that the stripping currents are strongly influenced by low supporting electrolyte concentrations [16,17].…”

Theory of linear sweep voltammetry with diffuse charge: Unsupported electrolytes, thin films, and leaky membranes

“…The additional species in their work are sometimes neutral [71,72] and sometimes charged [66,70], depending on the reaction being modeled. Modeling of uncharged and supporting ionic species has also been a feature of other models in the literature [58][59][60].…”

Cyclic voltammetry in weakly supported media: The reduction of the cobaltocenium cation in acetonitrile – Comparison between theory and experiment

“…Rather, the assumption of a negligibly small electrical double layer is adopted (see below) which offers computational accuracy and simplicity as well as physical insight. In previous papers we have shown that this approach can be used quantitatively to simulated chronoamperometry for a variety of systems [4][5][6][7]. However, most electrochemists use cyclic voltammetry rather than chronoamperometry and so, in this paper, we demonstrate that cyclic voltammetry of the reduction of the cobaltocenium cation in acet onitrile solution:…”

“…Also, complexation of the redox species of interest with the added electrolyte can occur, while for the case of many non-polar solvents it is impossible to dissolve the required levels of supporting electrolyte needed for rigorous elimination of migrational effects. For these reasons we have recently developed theoretical and simulation procedures aimed at permitting the quantitative prediction of voltammetry at micro and macroelectrodes (but not nanoelectrodes) under weakly supported conditions [3][4][5][6][7][8].…”