The solubilities of different multicomponent lanthanide oxide (Ln2O3) solid solutions including binary (Ln1 and Ln2 = La, Nd, Eu, or Tm), ternary (Ln1, Ln2, and Ln3 = La, Nd, Eu, or Tm), and higher systems (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) were studied after aging for four weeks at 60°C. Our recent study revealed that the phase transformations in binary ((La, Nd) and (La, Eu)) and ternary (La, Nd, Eu) systems are responsible for the formation of (La, Nd)(OH)3, (La, Eu)(OH)3, and (La, Nd, Eu)(OH)3 solid solutions, respectively. The variations in the mole fractions of La3+, Nd3+, and Eu3+ in the sample solutions of these hydroxide solid solutions indicated that a thermodynamic equilibrium might account for the apparent La, Nd, and Eu solubilities. Conversely, the binary and ternary systems containing Tm2O3 as the heavy lanthanide oxide retained the oxide-based solid solutions, and their solubility behaviors were dominated by their congruent dissolutions. In the higher multicomponent system, the X-ray diffraction patterns of the solid phases, before and after contact with the aqueous phase indicated the formation of a stable oxide solid solution and their solubility behavior was explained by its congruent dissolution.

Voltammetry has shown promise as a method to estimate the concentrations of actinides in the molten LiCl-KCl used as an electrolyte in spent nuclear fuel electrorefiners. This salt typically contains several actinides in addition to many active metal fission products (rare earths, Group I & II metals). However, most of the voltammetry studies to date have focused on a single actinide or lanthanide in eutectic LiCl-KCl. This paper examines experimental and analytical techniques that can be used to estimate the concentration of a molten salt mixture containing both lanthanum (III)- and gadolinium(III)-chloride in eutectic LiCl-KCl. The aspects of the experimental procedures and setup that are unique to a multi-lanthanide mixture are briefly discussed. Experimental results from qualitative and quantitative analyses of cyclic voltammetry and open-circuit potentiometry are presented. Due to the close proximity of their standard potentials, extensive analytical work is required to estimate the concentrations. Two approaches are used in this work: peak separation and multivariate analysis. The merits of these two methods will be analyzed and discussed.