Thermodynamic Study of Eu³⁺ /Eu²⁺ Redox Reaction in Aqueous Solutions at Elevated Temperatures and Pressures by Means of Cyclic Voltammetry

Abstract

Abstract The redox potential of the couple Eu 3+ /Eu 2+ in aqueous NaCl, NaClO 4 and Na 2 SO 4 solutions of different ionic strength and various pH values has been determined by means of cyclic voltammetry up to 458 K and 1 kbar. In all cases reversible voltammograms were obtained. Compared to the redox potential in C1O 4 - solutions of pH 2, no significant shift was observed in Cl-solutions of the same pH, whereas a drastic shift to more negative potentials in solutions of SO 2- 4 and in Cl- solutions of higher pH (pH 3-5) was obtained. This indicates a negligible complexation of Eu3* by means of CP but a strong one by means of OH and SO 2- 4 . An isothermal pressure increase up to 1 kbar led to a shift of only few mV more negative, indicating a small pressure dependence of the change of the partial molar volume (ΔV e1 ) accompanying the redox reaction, which results in this case only due to the different degrees of electrostriction. A more drastic shift of the redox potential (in the positive direction) results with increasing temperature. The isobaric temperature dependence of the redox potential is described by a two parameter equation which remains valid up to the saturation pressure at 458 K, due to the small pressure effect. ΔS and ΔH of the redox reaction has been determined