Electrode kinetics and double layer structure IV. The Eu(II)/Eu(III) electrode reaction at the DME in 1 M KSCN; Influence of complex formation

Abstract

A detailed study has been made of the kinetics of the Eu(III)/Eu(II) reaction at the DME in 1 M KSCN by means of impedance measurements. The data were analyzed according to the complex-plane method. Because of the considerable complexation of the europium ions in this medium, it appeared to be necessary to calculate the influence of complex formation on the apparent rate constant of the electrode reaction. Therefore an expression to correct for this effect was derived, assuming that the reaction takes place via the uncomplexed ions.

In order to apply Frumkin corrections, φ2 potentials were calculated for the systems concerned, accounting for the presence of the electroactive species. It could be shown that even 0.5 mM concentrations of trivalent cations have a considerable effect.

The data are consistent with a “true” transfer coefficient α=0.61. With this value the apparent rate constant was determined at different potentials and different europium concentrations. It was shown that the true rate constant was potential independent, with a mean value kshf=(3.2±0.2)×10−4 cm s−1. As in the potential region studied the amount of specifically adsorbed SCN− ions is strongly potential dependent, it can be concluded that, just as in KCl and KI, specific anion adsorption has no influence on the charge transfer of the europium couple. The rather large value of the rate constant in KSCN can be explained by assuming that the reaction plane is somewhat inside the outer Helmholtz plane. Another possibility is that the reaction takes place via the Eu(SCN) complex. In this case the true rate constant is also potential independent, with a mean value kshf=(12±0.4)×10−4 cm s−1. Analysis of the data of Anson et al.5 on the Cr3+/Cr2+ reaction with our way of correcting for complex formation shows that also in this case the amount of specifically adsorbed anions has no influence on the change transfer of this reaction. This confirms our hypothesis that it holds for each electrode reaction where both the Ox and the Red components are present in the electrolyte solution.