Redox electrochemistry and formal standard redox potentials of the Eu(III)/Eu(II) redox couple in an equimolar mixture of molten NaClKCl

Abstract

The electrochemical redox process Eu(III)+e −⇔Eu(II) in an equimolar NaClKCl melt in the temperature range 973–1123 K on glassy carbon electrodes was studied by linear sweep voltammetry, cyclic voltammetry, chronopotentiometry and reversal chronopotentiometry. It was determined that at a sweep rate of ν≤0.1 V s −1, the electroreduction of Eu(III) to Eu(II) is reversible, but at 0.1< ν≤0.3 V s −1, mixed diffusion and electron–transfer control is observed. Further increase of polarization rate, ν≥0.5 V s −1 results in electron–transfer control. The diffusion coefficients of Eu(III) and Eu(II) were determined by linear sweep voltammetry and chronopotentiometry methods. The values found by these methods are in a good agreement with each other. The diffusion coefficients of Eu(III) and Eu(II) in the NaClKCl melt are discussed in connection with the strength and stability of these complex ions. The standard rate constants for the reduction of Eu(III) to Eu(II) were calculated on the basis of cyclic voltammetry data. The sluggish kinetics of this reaction is discussed in terms of substantial rearrangement of the europium coordination sphere. The formal standard redox potentials of E* Eu(III)/Eu(II) were determined from linear sweep voltammetry and cyclic voltammetry.