The electrochemical behaviour of the Pr(III)/Pr redox system at Bi and Cd liquid electrodes in molten eutectic LiCl-KCl

The electrode reactions of LiCl-KCl-PrCl3 solutions at the surface of liquid electrodes, i.e. Cd and Bi, were investigated by electrochemical techniques. The redox potential of the Pr(III)/Pr couple at the liquid electrodes were observed at more positive potential values than those at one inert electrode. This potential shift was thermodynamically analysed by a lowering of activity of Pr in the metal phase. Cyclic voltammetry, using Cd and Bi pools as working electrodes, was conducted in order to study the reaction mechanism. The results suggest a quasi-reversible behaviour of the Pr(III)/Pr electrochemical system at the liquid electrodes, and the values of the kinetic parameters, k0 and α as well as the reversible half wave potential, E1/2r, have been obtained. The interdiffusion coefficients of Pr in the metal phases seem to be similar to the diffusion coefficient of Pr(III) in solution. From comparison between the equilibrium potential adopted by a praseodymium electrode immersed in a solution containing Pr(III) ions, and the calculated E1/2r of the system Pr(III)/Pr-M (being M = Cd or Bi) it has been possible to estimate the activity coefficients of Pr in the Cd and Bi phases. Open circuit chronopotentiometry using Cd or Bi film electrodes was also conducted. Electromotive force, emf, measurements for various intermetallic compounds in two-phase coexisting states were carried out in the temperature range of 673-823 K. The activities and relative partial molar Gibbs energies of Pr were obtained from the measured emf for various Pr-Cd and Pr-Bi intermetallic compounds, PrCd11, PrCd6, PrCd4.46, PrBi2 and PrBi. The relative partial molar entropies and enthalpies of Pr were also calculated from the temperature dependence of the emf. The standard Gibbs energies of formation for the Pr-Cd and Pr-Bi intermetallic compounds were calculated.