REDUCTION KINETICS OF TERBIUM IONS IN EQUIMOLAR MELTING SODIUM AND POTASSIUM CHLORIDE

In this study, by means of chronopotentiometry and chronovoltamperometry we evaluated the kinetic parameters (transfer coefficients, heterogeneous charge transfer rates) of electroreduction of terbium ions. We also evaluated the dependence of the kinetic parameters on the concentrations of terbium chloride mass (%): 1, 3, 5, 7, 10 and temperatures from 1073 to 1173 K. The values of the kinetic parameters increase with the increasing temperature and decrease with the increasing concentration of terbium chloride. The experiment was carried out in a three-electrode cell in an atmosphere of purified argon. Based on the results obtained by chronopotentiometric and chronovoltamperometric methods, it was shown that the process of electroreduction of TbCl63- chloride complexes in the equimolar NaCl-KCl melt is irreversible in the whole studied temperature range and terbium chloride concentrations at potential scan rates from 0.2 to 20 V/s (chronovoltamperometry) and current densities from 0.01 to 1.2 A/cm2 and transitional time from 0.01 to 1.5 s (chronopotentiometry). In this study, it was shown that in an equimolar melt of NaCl-KCl, terbium is in the divalent and trivalent states, forming the TbCl63- and TbCl42- complexes. Based on the experimental data, in the studied temperature range and terbium chloride concentrations, we assumed that the reduction of terbium ions occurred in two stages. The electrode electroreduction is complicated by the kinetic current of the preceding chemical reaction. We determined the preceding chemical reaction's rate constants. Based on the data obtained, we could model the TbCl63- complex's reduction in the previously mentioned melt. The reduction process included the previous stage of the complex dissociation. The results obtained by different methods are consistent with each other.