Electrochemical behaviour of praseodymium (III) in molten chlorides

The electrochemical behaviour of praseodymium was studied in two molten chlorides with different oxoacidity properties, the eutectic LiCl-KCl and the equimolar CaCl2-NaCl melt at different temperatures. It has been found that Pr(III) is the only stable oxidation state in both molten chlorides, which was less solvated by the chloride ions in the calcium-based melt due to the stability of CaCl42- ions. Transient electrochemical techniques were used in order to study the reaction mechanism and the transport parameters of electroactive species on an inert electrode. The results showed that the Pr(III)/Pr(0) electrochemical system is quasi-reversible, and the values of the kinetic parameters, k0 and α have been obtained. Mass transport towards the electrode is a simple diffusion process, and the diffusion coefficient of Pr(III) ions has been also calculated. The validity of the Arrhenius law was verified by plotting the variation of the logarithm of the diffusion coefficient vs. 1/T. In addition, the electrode reactions of the LiCl-KCl-PrCl3 solutions at an Al wire were investigated by cyclic voltammetry and open circuit chronopotentiometry. The redox potential of the Pr(III)/Pr couple at the Al electrode was observed at more positive potential values than those at the inert electrode. This potential shift was analyzed thermodynamically by a lowering of the activity of Pr in the metal phase due to the formation of intermetallic compounds.