Thermodynamic study of Eu3+/Eu2+ redox reaction in aqueous solutions at elevated temperatures and pressures by means of cyclic voltametry

The redox potential of the couple Eu3+/E2+ in aqueous NaCl, NaClO4 and Na2SO4 solutions of different 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 ClO4- 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 SO42- and in Cl- solutions of higher pH (pH 3-5) was obtained. This indicates a negligible complexation of Eu3+ by means of Cl- but a strong one by means of OH- and SO42-. 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 (ΔVel) 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. (orig.)