We have been searching for an electrode reaction that displays simple kinetic behavior in solutions containing adsorbing anions. The desire was to use the observed rate of a simple standard "probe reaction" in each electrolyte of interest to establish the value of the effective potential drop, ~b2, across the diffuse layer for that electrolyte. The resulting values of 4)2 could then be used confidently in unravelling the "messier" kinetic behavior typically displayed by many electrode reactions in specifically adsorbing electrolytes. The Eu(III)/Eu(II) couple appeared to be a likely electrode reaction for our purposes on the basis of the report by Sluyters and co-workers ~'2 that, after suitable corrections, the rate constants obtained in perchlorate, iodide, and chloride electrolytes did not differ greatly. We chose to investigate the kinetics of this reaction in thiocyanate supporting electrolytes in anticipation of subsequent measurements on reduction rates for adsorbed reactants which are frequently encountered in thiocyanate electrolytes. While our study was in progress Sluyters and co-workers presented their results for Eu(III)/Eu(II) reactions in thiocyanate 3. However, their measurements were confined to a single thiocyanate concentration (1 M), and the chief conclusions they reached about the mechanism of the electrode reaction differ from those to which we were led by kinetic data obtained at several concentrations of thiocyanate. For these reasons we present our data and analysis here and contrast our conclusions with those of SluYters and co-workers 3.
EXPERIMENTAL
Reagents
Analytical grade reagents and triply distilled water were used throughout. A stock solution containing 0.05 F Eu(III) in 0.1 F HC104 was prepared by dissolving europium(III) oxide (99.9~o, Research Inorganic Chemical Corp.) in a known excess of 60~o HC10 4 and diluting to volume. All other europium solutions were prepared from this stock solution by dilution. Sodium thiocyanate solutions were prepared from the reagent grade salt immediately prior to their use. Sodium perchlorate solutions were prepared by dilution of a stock solution made from sodium carbonate and perchloric acid. All solutions were deoxygenated by means of a stream of prepurified nitrogen which was passed through copper turnings at * Contribution No. 4514.