The Adsorption of Vanadium(III) on Mercury from Thiocyanate Solutions and Its Electrochemical Consequences

Abstract

The adsorption of V (III) on mercury electrodes in thioeyanate electrolytes has been measured chronocoulometrically. The adsorption is not strongly dependent on potential except in the vicinity of the V (III)/V (II) half-wave potential. The adsorbing species is a V(III)-thiocyanate complex containing one or more thiocyanates. The adsorption of V(II) also occurs, but is much weaker than that of V(III). A minimum value for the exchange current for the V(III)/V(II) couple in the adsorbed state has been estimated coulostatically. Although this exchange current is quite large, 0.5 A/cm 2, its magnitude can be largely accounted for by the enhancement of the reactants' surface concentrations produced by their adsorption.The spontaneous adsorption of electrochemical reactants on the surface of electrodes often produces complications in the study of the kinetics of their electrode reactions. An electrode process that can proceed by more than one pathway, e,g., via both the adsorbed and unadsorbed reactants, corresponds to a complex equivalent circuit which contains elements that cannot be evaluated in separate experiments in the absence of the reactants. As a result, a larger number of kinetic parameters must be extracted from the rate data, and confidence in the accuracy and uniqueness of the resulting kinetic parameters is weakened. Under these circumstances, it is advantageous to have independent quantitative information on the extent of adsorption of the reactants because the kinetic data can then be analyzed with one less adjustable parameter.The technique of chronocoulometry (1) is well suited to provide the desired quantitative adsorption data. It has been applied in the present study to evaluate the adsorption of V(III) and V(II) on mercury electrodes in thiocyanate solutions. The V(III)/V(II) system was chosen for several reasons. It offered an opportunity to compare the adsorption of a transition metal redox couple with the extensively studied anioninduced adsorption of white metal cations (2-6) [Cd(II), Zn(II), Pb(II), In(III)]. Second, the equilibrium adsorption of V(III) and V(II) could be measured in poised solutions containing both halves of the redox couple, thus allowing the potential dependence of the adsorption in the vicinity of the half-wave potential to be determined. Analogous data for the white metal cations are less straightforward to obtain because of the difficulties associated with the preparation and storage of metal amalgam electrodes. Finally, the V(III)/V(II) system was attractive because the electrode kinetics of the couple have been extensively studied (7-14) in electrolytes where no reactant adsorption occurs. It was hoped that a comparison of these rates with those measured with adsorbed reactants would contribute to an understanding of the effect of adsorption on the rate of electrochemical charge transfer for a simple redox reaction. ExperimentalTechniques.--The adsorptions of V(III) and V(II) (which shows very little adsorption) were evaluated at potentials removed from the half-wave...

Senderos de lucha. Las izquierdas mexicanas durante la época de la Guerra Fría

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The Effects of Adsorbed Ions on Simple Electrode Reactions. II. The Reduction Of Chromium(III) Ion in The Presence of TolueneP-Sulfonate Anions

“…In recent years considerable effort has been directed to examining the catalytic effect of adsorbed anions on the kinetics of simple redox reactions involving cations (1). The systems studied have included the Cr(III)/Cr(II) (2)(3)(4)(5), Eu(III)/Eu(II) (6)(7)(8), and V(TII)/V(II) couples (9). However, unambiguous analysis of the kinetic data is often difficult because the nature of the species reacting at the electrode is not always clearly defined, and because the electron transfer process is sometimes accompanied by ligand exchange.…”