Eutectic LiCl-KCl Research Articles

The thermodynamic and transport properties of GdCl3 in molten eutectic LiCl-KCl derived from the analysis of cyclic voltammetry signals

Pyroprocessing technology is a promising tool for recycling nuclear fuel and producing high purity gadolinium for industrial applications. An efficient implementation of pyroprocessing entails a careful characterization of the electrochemical and transport properties of lanthanides in high temperature molten salts. In this work, the cyclic voltammetry signals of Gd in molten LiCl-KCl salt were recorded for a combination of three temperatures (723 K, 773 K, and 823 K) and three concentration levels (3 wt. %, 6 wt. %, and 9 wt. %) including concentration levels higher than previously reported and relevant for a realistic application of pyroprocessing for molten salt recycle, and the concentration effects were investigated. Four scan rates (200 mV/s to 500 mV/s) were used for each condition, and the signals were examined using conventional Cyclic Voltammetry (CV) analysis equations and by utilizing a two-plate Brunauer, Emmett, and Teller (BET) model accounting for mass diffusion, kinetics, adsorption, and the evolution of electrode morphology via a nonlinear least squares procedure for fitting the model to the experimental signals. It was determined that the redox process is quasi-reversible for the scan rates being used. Furthermore, the applicability of the conventional equations for CV analysis was shown to be problematic for the conditions used, and this is thought to be due to the fact that these equations were derived under the assumption of reversible conditions. The model-derived values for diffusivity are consistent with the literature and are shown to decrease with increasing concentration. This may be due to increased interactions at higher concentration levels. It was also shown that the formal redox potential increased with a concentration and was slightly more positive on the covered electrode.

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

The electrochemical behavior of MgCl2 in molten LiCl-KCl eutectic was investigated to evaluate its suitability as a surrogate for PuCl3 in studies related to the eletrorefining of used nuclear fuel. The reduction of Mg2+ was found to be electrochemically reversible up to 300 mV s−1 at 773 K. The diffusion coefficient for Mg2+ was calculated to be 1.74 and 2.17 × 10−5 cm2 s−1 with and without U3+ present, respectively, at 773 K using cyclic voltammetry (CV). Upon comparison to literature data, the diffusion coefficient of Mg2+ differs by only 8.8% (with U3+ present) from that of Pu3+ and the difference in peak potentials was only 79 mV. Binary-analyte mixtures of UCl3 and MgCl2 in eutectic LiCl-KCl were further investigated using CV, normal pulse voltammetry (NPV), chronoamperometry (CA) and open-circuit potential (OCP) measurements for the purpose of comparing each technique's accuracy in measuring U3+ and Mg2+ concentrations. Of all the techniques tested, NPV resulted in the lowest error which was, on average, 11.4% and 9.81% for U3+ and Mg2+, respectively.

Electrochemical formation and thermodynamic properties of Tb–Bi intermetallic compounds in eutectic LiCl–KCl

The electrochemical formation and thermodynamic properties of Tb–Bi intermetallic compounds were investigated on a Bi film electrode in eutectic LiCl–KCl.

Electrochemical Extraction of Holmium and Thermodynamic Properties of Ho-Bi Alloys in LiCl-KCl Eutectic

The electrochemical reduction of Ho (III) on W electrode and liquid Bi film electrode was investigated in LiCl-KCl eutectic by various electrochemical techniques. The reduction reaction of Ho (III) was found to be diffusion-controlled process with three electrons exchanged, which was considered as quasi-reversible. The diffusion coefficient of Ho (III) was determined by cyclic voltammetry and chronopotentiometry. The results of cyclic voltammetry and square wave voltammetry obtained on Bi film electrode exhibited two new reduction peaks at less negative potential than that on W electrode due to the formation of different Ho-Bi intermetallic compounds. Furthermore, the thermodynamic properties on the formation of Ho-Bi intermetallic compounds were measured using open circuit chronopotentiometry in the temperature range from 773 K to 863 K. The electroextraction of holmium was performed on liquid Bi electrode by potentiostatic and galvanostatic electrolysis. The extractive products were characterized by scanning electron microscopy and energy dispersive spectrometry (SEM-EDS) and X-ray diffraction (XRD). It was showed that Ho-Bi alloys consisted of HoBi and Bi phases. The concentration of Ho (III) was monitored by square wave voltammetry and analyzed by inductive coupled plasma atomic emission spectrometer (ICP-AES). The calibration curve was obtained on linear relationship of peak currents and concentration of Ho (III). Meanwhile, the extraction efficiencies were calculated by ICP-AES and calibration curve, respectively, the highest extraction efficiency could reach 95.9% by potenostatic electrolysis at −1.6 V for 27 h.

Preliminary Study of Optimization of Normal Pulse Voltammetry for Actinide Measurement in Molten Salt Electrorefiners Using LiCl-KCl-UCl3-MgCl2 as a Surrogate Salt

Normal pulse voltammetry has been applied and optimized to develop correlations between diffusional current and concentration for UCl3 and MgCl2 in molten eutectic LiCl-KCl. This method was selected to mitigate working electrode area growth in concentrated molten salt mixtures in which a fairly wide potential range is needed to analyze all actinides. MgCl2 has been selected as a surrogate for PuCl3 based on similar standard reduction potential. Diffusional current plateaus were successfully attained by adjusting the step size to 0.03 V. Relaxation time of 15 seconds was found to be sufficient to recover after each pulse. A matrix of mixtures were prepared and tested with UCl3 concentration varying between 1 to 9 wt% and MgCl2 concentration varying between 0.3 and 1.5 wt%. The measured diffusional currents correlated well with concentration and matched a model that includes both diffusion and migration from the bulk molten salt to the working electrode surface. Concentrations of MgCl2 do not appear to affect the UCl3 diffusional current and vice versa. This is, thus, a promising method for simultaneous analysis of UCl3 and PuCl3 in molten salt electrolytes used for spent nuclear fuel electrorefining.

Thermodynamic evaluation of LiCl-KCl-PuCl3 system

The present study focuses on developing the phase diagram of LiCl-KCl-PuCl3 system based on the CALPHAD (CALculation of PHase Diagram) method. Sub-binary systems of LiCl-KCl, LiCl-PuCl3, and KCl-PuCl3 were developed first by optimizing the Gibbs energy parameters according to the experimental data from literature using the two-sublattice model. Then the enthalpy of mixing of the LiCl-KCl-PuCl3 system was estimated, to explore the ternary interactions, using an empirical correlation derived from the surrounded-ion model for the asymmetric salt system due to the scarcity of available data for the ternary system. Based on the phase diagram developed, the solubility of PuCl3 in the eutectic LiCl-KCl melt at different temperatures was obtained, as well as the Gibbs energy of formation of PuCl3 in the salt as a function of PuCl3 concentration. The present study extends the experimental data for dilutions to concentrated solutions for which no experimental data have been well reported.

Determination of activity coefficient of lanthanum chloride in molten LiCl-KCl eutectic salt as a function of cesium chloride and lanthanum chloride concentrations using electromotive force measurements

The thermodynamic behavior of lanthanides in molten salt systems is of significant scientific interest for the spent fuel reprocessing of Generation IV reactors. In this study, the apparent standard reduction potential (apparent potential) and activity coefficient of LaCl3 were determined in a molten salt solution of eutectic LiCl-KCl as a function of concentration of LaCl3. The effect of adding up to 1.40 mol % CsCl was also investigated. These properties were determined by measuring the open circuit potential of the La—La(III) redox couple in a high temperature molten salt electrochemical cell. Both the apparent potential and activity coefficient exhibited a strong dependence on concentration. A low concentration (0.69 mol %) of CsCl had no significant effect on the measured properties, while a higher concentration (1.40 mol %) of CsCl caused an increase (become more positive) in the apparent potential and activity coefficient at the higher range of LaCl3 concentrations.

Exchange Current Density and Diffusion Layer Thickness in Molten LiCl-KCl Eutectic: A Modeling Perspective for Pyroprocessing of Metal Fuels

The DIFAC (DIFfusion of Actinides in EleCtrorefiner) computer code for pyroprocessing, developed earlier by the authors, is modified in the present work to model electrorefining at the liquid cadmium electrode. The modeling of electrorefining of metal fuels requires accurate knowledge of two important kinetic parameters: exchange current density io and diffusion layer thickness δ. These are estimated in the present work by polarization methods and employing Tafel and Allen-Hickling analysis for Gd3+/Gd, U3+/U, and Zr2+/Zr couples in LiCl-KCl eutectic at 773 K for an inert cathode and compared with literature data, wherever possible. The equilibrium potentials for these couples at an inert electrode are found to be –1.94, –1.52, and –1.22 V, respectively, at 773 K. Electrochemical studies are also carried out in LiCl-KCl eutectic to estimate io and δ for the anodic dissolution of Na-bonded U-Zr and Gd-U-Zr alloy and are compared with the anodic dissolution of U-Pu-Zr alloy. The equilibrium potential of Na-bonded U-Zr alloy in LiCl-KCl-UCl3 was found to be –1.46 V, and those for Gd-U-Zr alloy in blank LiCl-KCl and LiCl-KCl-UCl3 were –1.56 and –1.34 V, respectively, at 773 K. The exchange current densities of Na-bonded U-Zr and Gd-U-Zr alloy were found to be in the range of 40.1 to 46.5 mA · cm−2 and 16.8 to 27.3 mA · cm−2 at 773 K, respectively.A preliminary design of the liquid cadmium electrode suitable for laboratory-scale experiments on uranium- and plutonium-based systems is also reported in the present work. The io and δ of gadolinium, uranium, and zirconium are subsequently estimated at the liquid cadmium electrode at 773 K. The equilibrium potentials for Gd3+/Cd6Gd, U3+/[U]Cd, and Zr2+/Cd3Zr couples in LiCl-KCl eutectic at 773 K for the liquid cadmium electrode are found to be –1.35, –1.13, and −1.12 V, respectively. Finally, a few algorithms are proposed for modeling electrorefining data at the liquid cadmium electrode for multicomponent systems.

Methods for Determining the Working Electrode Interfacial Area for Electroanalytical Measurements of Metal Ions in Molten LiCl-KCl

Voltammetry is a promising approach to determining the concentration of actinides and rare earths in molten chloride salts for real time or near real time monitoring of nuclear fuel electrorefining. In this process, actinides and rare earths accumulate in eutectic LiCl-KCl from reaction of UCl3 with spent nuclear fuel. Monitoring the salt composition is key in order to maintain optimal process conditions, to determine when to treat or discard salt, and to safeguard the process against nuclear material diversion. Voltammetry methods such as cyclic voltammetry (CV), chronopotentiometry (CP), chronoamperometry (CA), normal pulse voltammetry (NPV), and square wave voltammetry (SWV) have been extensively reported in the literature for application to molten LiCl-KCl typically containing one rare earth or actinide chloride. Our studies have expanded the salt matrices to include quaternary salts with two electrochemically active metal chlorides. We believe this to be the first step in determining if these basic electrochemical analysis methods can be applied to salts containing many components with similar standard reduction potentials. In the simpler ternary salt systems, classic diffusion limited rate equations such as those derived by Randles-Sevcik and Berzins-Delahay have been applied to correlate CV peak height to concentration. Application of these equations requires knowledge of both the diffusion coefficient of the salt compound in the LiCl-KCl and the interfacial area between the working electrode and salt. In our experimental investigations, we have determined that both of these quantities can be difficult to establish a priori. Diffusion coefficients for metal chlorides in molten LiCl-KCl appear to vary with concentration. And the high temperature electrochemical cells located in insulated furnaces make it challenging to precisely determine the electrode-salt interfacial area. In this paper, we focus on empirical evaluation of methods for determining this area—ranging from the simplest approach of dipping a cold electrode into the molten salt to measure the film height to more elaborate approaches such as using a vertical electrode actuator, coating the electrodes with glass, and integrating a pre-peak which may represent formation of a monolayer of metal on the electrode. Salt mixtures investigated include LiCl-KCl-UCl3, LiCl-KCl-UCl3-MgCl2, LiCl-KCl-ThCl4-LaCl3, LiCl-KCl-LaCl3-GdCl3, and LiCl-KCl-LaCl3-MgCl2. It will be shown that with accurate determination of the electrode-salt interfacial area that concentration correlations can be developed with error as low as 2-3%. Both using differential insertion depth with the vertical electrode translator and using glass-coated working electrodes are promising approaches—the latter of which is believed to be more easily implemented in a production operation scenario.

Electrochemical Behavior of Samarium in Molten LiCl-KCl

The mass transport behavior of samarium was investigated in molten eutectic LiCl-KCl in order to develop real-time concentration monitoring capabilities for the pyrochemical reprocessing of used nuclear fuel. Pyrochemical reprocessing relies on electrolysis in molten LiCl-KCl to recycle fissile material from used nuclear fuel. Without the capabilities to monitor concentrations of various actinides and fission products, pyrochemical reprocessing cannot be commercialized due to strict material accountability requirements set by the International Atomic Energy Agency (IAEA) [1]. Therefore, physical electrochemical properties of these species in molten salts is extremely important. Cyclic voltammetry was utilized to investigate electrochemical behavior of samarium in LiCl-KCl. By establishing valid relationships between these parameters and the peak current, the diffusion coefficient of samarium in our system can be reported with confidence [2]. Experiments were conducted in a high-purity argon atmosphere glove box using a three-electrode cell contained in an alumina crucible inside a resistive furnace. A high-resolution translation stage equipped with ceramic electrode mounts was used to accurately control electrode positions inside the melt. Tungsten was used as the counter and working electrodes, and a silver wire was used as a quasi-reference (Ag|AgCl2). The variation in peak current that resulted from changing the concentration, scan rate and electrode surface area will be reported, along with calculated diffusion coefficient values for samarium in molten LiCl-KCl at 500°C. The effects of using molybdenum as a working electrode will also be reported and compared to results obtained using tungsten. A comparison between reported diffusion coefficient values obtained by separate research organizations and results obtained in these studies will also be discussed. Acknowledgements: This work was performed under the auspices of the Department of Energy (DOE) under contracts DE-NE0008262 and DE-NE0008236 as well as the US Nuclear Regulatory Commission (USNRC) under contracts NRCHQ-11-G-38-0039. Dr. Kenny Osborne serves as the program manager for the DOE award and Ms. Nancy Hebron-Isreal serves as the grants program officer for the NRC awards.

Effect of H2O on Corrosion of Ni in Molten Eutectic LiCl-KCl

In the absence of water and oxygen, it is known that molten chloride salts are relatively non-corrosive to a wide range of metals. Corrosion rates are expected to increase when water is present, however. To understand the threshold at which very small concentrations of water can cause corrosion of metals such as nickel and iron, a series of electrochemical measurements were made in molten eutectic LiCl-KCl and LiCl-KCl-UCl3. The presence of UCl3 is relevant for the use of this electrolyte in nuclear fuel electrorefining systems. Such systems are normally operated in inert atmosphere glove boxes or hot cells, where water concentration ranges from 1 to 50 ppm. In this study, argon gas containing up to 900 ppm H2O was bubbled into molten LiCl-KCl or LiCl-KCl-UCl3. Either a nickel or iron rod was inserted into the salt along with a Ag/AgCl reference electrode encased in a close-ended ceramic tube. The open circuit potential (OCP) was measured between the metal rod and the reference electrode as the gaseous Ar+H2O was bubbled into the salt. The temperature was held constant at 773 K. This is expected to result in formation of HCl, which will corrode the metal and cause a metal chloride to form and partition into the salt phase. Based on the Nernst Equation, this should cause a positive rise in the OCP. Based on changing OCP as a function of time, the rates of corrosion were compared for different concentrations of H2O in the gas stream. Cyclic voltammetry scans and samples of the salt were taken to verify the concentration of nickel and iron chloride in the salt at the end of each experiment. Noticeable corrosion was achieved in a time frame of 4 hours with water concentration as low as 120 ppm. This result demonstrates the importance of achieving a very high level of dryness in the salt prior to heating and melting it. Water concentration in the contacting atmosphere should also be well below 100 ppm to ensure long-term stability of the steels and similar metal alloys in molten LiCl-KCl or LiCl-KCl-UCl3. Implications and strategies for long term operation of electrorefiners using LiCl-KCl will be discussed.

Methods for Determining the Working Electrode Interfacial Area for Electroanalytical Measurements of Metal Ions in Molten LiCl-KCl

In this paper, we present an empirical evaluation of methods for determining the interfacial area between a working electrode and molten salt—specifically demonstrated using the eutectic LiCl-KCl system. Approaches studied include dipping a cold electrode into the molten salt, using a vertical translator to adjust the height of the electrode, partially coating the electrodes with an insulating material, and integrating a pre-peak which may represent formation of a monolayer of metal on the electrode. Salt mixtures investigated include LiCl-KCl-UCl3, LiCl-KCl-UCl3-MgCl2, LiCl-KCl-LaCl3-GdCl3, and LiCl-KCl-LaCl3-MgCl2. With accurate determination of the electrode-salt interfacial area, concentration correlations can be developed with error below 2%.

Investigation of concentration-dependence of thermodynamic properties of lanthanum, yttrium, scandium and terbium in eutectic LiCl-KCl molten salt

Thermodynamic properties of rare earth metals in LiCl-KCl molten salt electrolyte are crucial to the development of electrochemical separation for the treatment of used nuclear fuels. In the present study, activity coefficient, apparent potential, and diffusion coefficient of lanthanum, yttrium, scandium, and terbium in the molten salt (58 at% LiCl and 42 at% KCl) were calculated by the method of molecular dynamics simulation up to a concentration around 3 at% at temperatures of 723 K and 773 K. It was found that the activity coefficient and the apparent potential increase with the species concentration while diffusion coefficient shows a trend of increase followed by decrease. The calculated results were validated by available measurement data of dilution cases. This research extends the range of data to a wide component and would provide further insight to the pyroprocessing design and safeguards.

Estimation of key physical properties for LaCl3 in molten eutectic LiCl–KCl by fitting cyclic voltammetry data to a BET-based electrode reaction kinetics model

Understanding the electrochemical properties of rare earth elements is important for developing efficient techniques for separating rare earth elements from actinides recovered during the electrodeposition process. In this study the cyclic voltammetry for lanthanum in molten LiClKCl eutectic was recorded at 773 K for different scan rates and different bulk concentrations. A model accounting for mass transport, kinetics and adsorption was applied to analyze the experimental data via performing a nonlinear least squares fit. The results of the simulation are compared against the results of a conventional analysis of the cyclic voltammograms and against the existing literature. At the scan rates used, the reduction/oxidation process is quasi-reversible. The values of diffusivities derived from simulation were larger than the ones derived commonly using equations for diffusion-limited processes. However, those equations were derived based on an assumption of reversibility. This simulation-based approach may provide a more accurate option for analyzing systems that do not exhibit reversibility.

First-principles based computational study on nucleation and growth mechanisms of U on Mo(110) surface solvated in an eutectic LiCl-KCl molten salt

We utilize first principles density functional theory (DFT) calculations and ab-initio molecular dynamic (AIMD) simulations to identify underlying mechanisms elucidating the initial stage of electrocrystallization process of U on Mo(110) surface in a eutectic LiCl–KCl molten salt at T = 773 K. Our results clearly unveil surprisingly different principles on the nucleation of U in the media from that under vacuum conditions. U nanoclusters exposed to vacuum completely collapse into flat atomic layers on Mo(110) surface similar to an electrodeposition process. On the other hand, Cl ions in eutectic molten salt thermodynamically drive crystallite formation consisting of UCln (n = 3–6) through agglomeration of U atoms. Those crystallite gradually grows into bigger nuclei by adsorbing on Mo(110) surface. We propose that those behaviors are understandable only with revised conventional theories and that atomic level interactions among U, LiCl–KCl molten salt and Mo(110) surface play a key role to describe the atomic-scale dendrite formation of U in the electrorefining process. Our study can be one of the basic steps to design efficient electrorefining systems by identifying the fundamental cause of the experimentally observed uranium nucleation phenomena. Copyright © 2016 John Wiley & Sons, Ltd.

Voltammetric Analysis of Mixtures of Molten Eutectic LiCl-KCl Containing LaCl3 and ThCl4 for Concentration and Diffusion Coefficient Measurement

Several electrochemical techniques, including cyclic voltammetry (CV), chronopotentiometry-open circuit potential (CP-OCP), chronoamperometry (CA) and normal pulse voltammetry (NPV) were applied to the eutectic LiCl-KCl melt containing LaCl3 and ThCl4 at 773K. The dependence of diffusion coefficient of La(III) on concentration of LaCl3 was investigated by using CV, CP and CA. As the concentration of LaCl3 increases, the DLa(III) value obtained from CV gradually decreases, while the values derived from CA and CP exhibit a dome-shaped variation trend. In the LiCl-KCl-LaCl3-ThCl4 system, the diffusion coefficients of La(III) and Th(IV) are both approximately constant regardless of changes in the composition of LaCl3 and ThCl4. Concentration correlations were developed for LaCl3 and ThCl4 using the normalized CV peak heights in addition to the diffusion-limited relationships for CV, CA and NPV. NPV was found to be superior for the concentration prediction over the range studied in this work.

Direct preparation of Al-base alloys from their oxides/metal precursors in the eutectic LiCl–KCl melt

A study was carried out on the preparation of Al–Cu–Li alloy from their oxides/metal precursors using the method of electro-deoxidation in the eutectic LiCl–KCl melt at 648 K. Cyclic voltammetry was used to characterize the system. The samples were prepared by potentiostatic electrolysis at −1.0 V to −2.0 V (vs. Ag+/Ag) for 5 h. XRD analysis shows that Li2O is not electrochemically reduced to Li at −1.0 V (vs. Ag+/Ag) or more negative potential. During the preparation process of Al–Cu–Li alloy, lithium peroxide is formed as an intermediate compound. Al–Cu–Li alloy is chemically prepared through the reaction between aluminum and lithium peroxide by heating of Al–Cu–Li2O precursors in KCl–LiCl–LiF melt at 1023 K. Eelectro-deoxidation in LiCl–KCl melt can increase the lithium content in the final alloy product. Al–Mg and Al–Nd alloy were also prepared by using the same method from their mixture of aluminum and corresponding oxide, respectively. Al–Nd alloy can only be obtained at the temperature above 773 K. Al–Li alloy could not be obtained in eutectic CaCl2–LiCl melt because of formation of calcium aluminates.

Evaluation of the thermochemical properties of Ho–Cd intermetallic compounds using electrochemical techniques

The electrochemical formation of HoCd6, HoCd45/11, HoCd3, HoCd2 and HoCd, on a Cd coated tungsten electrode in the eutectic LiCl–KCl has been evaluated using electrochemical techniques over the temperature range of 673–823 K.

Application of Voltammetry for Electroanalytical Measurement of Concentrations in LaCl3-MgCl2 Mixtures in Eutectic LiCl-KCl

For electroanalytical measurement of molten chloride salts, various voltammetry methods have been used on mixtures of MgCl2 and LaCl3 in eutectic LiCl-KCl. These salt mixtures are of primary interest to spent nuclear fuel electrorefining systems. MgCl2 is used as a surrogate for PuCl3, while LaCl3 represents rare earth fission product salts. Cyclic voltammetry scans with these salt mixtures exhibited extra peaks, presumably due to underpotential deposition of La(III) ions onto Mg. Quantitatively, the current contribution from each peak was best-determined using normal pulse voltammetry and fit to the Cottrell equation. This yielded linear correlations for concentration versus diffusional current with very high correlation factors. Applying this method, it was determined that concentration measurement error was less than 4% for both MgCl2 and LaCl3.

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

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%).