Electroanalytical Measurements of Binary-Analyte Mixtures in Molten LiCl-KCl Eutectic: Gadolinium(III)- and Lanthanum(III)-Chloride

Abstract

Electroanalytical measurements, such as cyclic voltammetry and chronopotentiometry, have been applied by other researchers to many eutectic LiCl-KCl mixtures containing a single analyte, typically a rare earth or actinide ion. These measurements have demonstrated the applicability of electroanalytical measurements of concentration in molten LiCl-KCl eutectic, which is prevalently used as an electrolyte in the electrorefining of used nuclear fuel (UNF). However, UNF electrorefiners contain multiple actinide and rare earth ions, necessitating the ability to make electroanalytical measurements in multi-analyte mixtures. The presence of multiple analytes creates interferences and overlapping signals. Chronoamperometry, cyclic voltammetry (CV), chronopotentiometry and open-circuit potential measurements were performed in eutectic LiCl-KCl mixtures containing a range of GdCl3 and LaCl3 concentrations to explore the issues associated with applying electroanalytical techniques in multi-analyte molten salt mixtures and to develop possible solutions. CV was found to be most promising for separating and analyzing each ion's signal. CV measurements were semi-differentiated and correlated to concentration using peak heights and principal component regression (PCR) to determine the concentration from electrochemical signals. The lowest average relative error was obtained using PCR for Gd3+ (6.21%) and using peak height correlations for La3+ (10.4%).

Semi-differential analysis of irreversible voltammetric peaks

“…The most accurate way to express asymmetry of a peak is by application of two separate mathematical equations for ascending and descending sections of the signal (split functions) [48,74,75]. This procedure, however, is not very convenient since requires a very careful selection of the fitting constraints.…”

“…Various electrochemical processes, such as redox reactions with reduced and oxidised species present in the same phase [6,14,[21][22][23][24][25][26][27][28][29][30][31][32][33][34][35], adsorption on surfaces of electrodes [36][37][38], electrodeposition [39][40][41][42][43][44][45][46][47][48] or electrodissolution [49][50][51] were analysed by means of semi-differentiation or semi-integration. The most extensive mathematical treatment related to these methods was applied to the electrochemical reactions were both oxidised and reduced form are soluble in the electrolyte phase and their transport proceeds under conditions of semi-infinite linear diffusion [4].…”

Electroanalytical measurements of binary-analyte mixtures in molten LiCl-KCl eutectic: Uranium(III)- and Magnesium(II)-Chloride

“…Recently, quaternary molten salt mixtures have been examined to elucidate and resolve some of the challenges associated with using electroanalytical techniques to measure the concentration of key elements in a used nuclear fuel (UNF) electrorefiner [1][2][3][4][5]. These salt mixtures were comprised of a matrix salt, eutectic LiCl-KCl, and two analytes.…”

“…These studies identified signal overlap, interaction of deposited metals and working electrode (WE) area growth as key issues associated with a binary-analyte mixture. Semi-differentiation of cyclic voltammetry (CV) measurements was used to reduce the interference of a background current from another analyte [2,5]. The deposition of alloying metals has been found to affect the potentials at which the more active metal deposits and to create additional signals.…”

“…The deposition of alloying metals has been found to affect the potentials at which the more active metal deposits and to create additional signals. However, the inclusion of these signals in the correlation of current to concentration has had a minimal effect on accuracy [3][4][5]. Square wave voltammetry was identified to be suitable for only low concentrations (<0.1 wt% Pu) due to metal deposition augmenting the WE area, but normal pulse voltammetry (NPV) was found to be suitable at higher concentrations [1].…”