Electrode kinetics and double-layer structure III. The Eu(II)/Eu(III) electrode reaction at the DME in 1 M KCl and 1 M KI base electrolyte solutions

Abstract

A detailed study has been made of the kinetics of the Eu(III)/Eu(II) electrode reaction in 1 M KCl and in 1 M KI at the dropping mercury electrode, by means of impedance measurements, both at equilibrium potentials (varying cox*/cred/* ratio, cox*+cred*=10 mM) and at externally controlled electrode potentials (cox*=10 mM). The data were analyzed according to the complex plane method, i.e. by considering the electrode admittance as a function of frequency. For the correct interpretation of the results it has been necessary to apply a modified version of the Frumkin correction, taking into account the considerable influence of the highly charged electroactive ions on the potential 2 in the outer Helmholtz plane. Careful comparisons between the results at different potential regions revealed that the data are consistent with a potential-independent, “true” transfer coefficient α (0.59 in 1 M KCl and 0.52 in 1 M KI). On the basis of these values the apparent rate constant ksha was determined in various ways, which gave consistent results. It is shown that the potential dependence of ksha is almost entirely due to the Frumkin effect, since values of the “true” rate constant kshf were substantially potential independent. The mean values are: kshf=(1.4±0.2)×10−5 (in 1 M KCl) and kshf=(0.6±0.1)×10−5 cm s−1 (in 1 M KI). Since in the studied potential region the amounts of specifically adsorbed CI− and I− ions are very different and are moreover strongly potential dependent, it can be concluded that, oppositely to some amalgam-forming redox couples studied previously, specific adsorption has no influence on the charge transfer rate of the Eu(III)/Eu(II) couple. From this it can be inferred that the charge transfer takes place outside the inner part of the double layer (probably even at some distance from the outer Helmholtz plane), the reactants remaining hydrated before or during the reaction. This finding might be a general feature of electrode reactions where both the Ox and the Red components are present in the electrolyte solution.