Y. Castrillejo

Use of electrochemical techniques for the study of solubilization processes of cerium-oxide compounds and recovery of the metal from molten chlorides

Abstract This work presents a study on the chemical and electrochemical properties of CeCl3 in two molten chloride mixtures with different oxoacidity properties, the eutectic LiCl–KCl at 723 K and the equimolar CaCl2–NaCl melt at 823 K. The E–pO2− (potential–acidity) diagram for Ce–O compounds stable in both melts has been constructed by combining both theoretical and experimental data. The stable oxidation states of Ce have been found to be (III) and (0) in both melts; Ce(IV) is only stable in the form of solid CeO2. The standard potential value of the Ce(III)/Ce(0) system has been determined by potentiometry, giving values −3.154±0.006 V and −3.036±0.009 V versus Cl2/Cl− in the eutectic LiCl–KCl at 723 K and the equimolar CaCl2–NaCl melt at 823 K, respectively. On the other hand, from the calculated activity coefficient values it was possible to say that the Ce(III) ions form stronger complexes with the chloride ions in the alkaline melt. This is probably due to the smaller amount of free chloride ions in the calcium molten mixture. Identification of the Ce–O compounds that are stable in the melts as well as the determination of their solubility products was easily carried out by potentiometric titration using an yttria stabilised zirconia membrane electrode (YSZME). The results indicated that Ce2O3 is a strong oxobase and that CeOCl is a solid stable compound in the melts studied. CeO2 is also a stable compound that can exist under oxobasic conditions. The best chlorinating conditions could be extracted from the comparison of the E–pO2− diagram corresponding to the Ce–O compounds and that of some chlorinating mixtures. Experimental solubilization tests allowed us to verify the thermodynamic chlorinating predictions. Moreover the electrochemical behaviour of CeCl3 solutions was also investigated. Transient electrochemical techniques, such as cyclic voltammetry, chronopotentiometry and chronoamperometry were used in order to study the reaction mechanism and the transport parameters of electroactive species on metallic substrates such as tungsten and molybdenum. The results showed that the Ce(III)/Ce(0) system is quasi-reversible with values of the charge transfer rate constant, k°, and transfer coefficient, α, around 10−3.7 cm s−1 and 0.4, respectively. The diffusion coefficient of Ce(III) ions was also determined by different electrochemical techniques, obtaining a value in the order of 1×10−5 cm2 s−1. The validity of the Arrhenius law was also verified by plotting the variation of the logarithm of the diffusion coefficient versus 1/T.

Electrochemical behaviour of magnesium ions in the equimolar CaCl2NaCl mixture at 550 °C

Abstract The electrochemical behaviour of magnesium ions was studied in molten CaCl 2 NaCl at 550 °C by cyclic voltammetry, chronoamperometry and chronopotentiometry. We observed that the electrode process of the reduction of Mg(II) on a tungsten electrode is quasireversible. The values of the transfer coefficient, α, and the charge-transfer rate constant, k 0 , were calculated by means of the logarithmic analysis of the convoluted curves. Those values were compared with the ones obtained from the steady-state current-potential curves. On the other hand, the diffusion coefficient of the Mg(II) ions, has also been determined by different electrochemical techniques, giving values ranging from 7.2 to 9.9 × 10 −6 cm 2 s −1 .

Study of the Contents and Speciation of Heavy Metals in River sediments By Factor Analysis

Abstract Heavy metals Zn, Cd, Pb, Cu, Ni and Co from two spanish river sediments have been determined by voltammetric techniques, along with % CaCO3 and % organic matter, and the whole data set examined by means of Factor Analysis, finding a different behaviour of the metals in each river. Heavy metal speciation in the sediments was carried out by using the Tessiers scheme: Zn and Pb were mainly associated to ‘reducible phases’, Cd and Cu bound to ‘organic matter and sulphides’, and Ni and Co were distributed between the above fractions. The ‘residual’ metals were used as pollution indicators from the rivers, finding a similar behaviour to other industrialized river systems.

Validation of a tubular bismuth film amperometric detector: Determination of diclofenac sodium by multisyringe flow injection analysis

A tubular bismuth film electrode (BFE), installed as part of a multisyringe flow injection system, was used as an amperometric detector to determine the concentration of diclofenac sodium in pharmaceutical formulations. A tubular voltammetric detection cell was employed, in which the hydrodynamic flow conditions were not disturbed. This automated method allows the continuous regeneration of the BFE, preventing passivation of the detector and improving the sensitivity of detection. The influence of several variables on this sensitivity, such as the injection volume, deposition time and flow rate were evaluated; a two-level factorial experimental design was employed for this. In optimal conditions, the linear range of the calibration curve varied from 6.0-50.0 micromol L(-1), with a detection limit of 4.3 micromol L(-1). A sampling rate of 90 determinations/h was achieved; the relative standard deviation of analytical repeatability was <3.5%. After 30 injections the bismuth film on the electrode surface was automatically renewed. The method was validated by comparing the results obtained with those provided by RP-HPLC; no significant difference were seen (p<0.05).

A chemical and electrochemical study of titanium ions in the molten equimolar CaCl2+NaCl mixture at 550°C.

Abstract The chemical and electrochemical properties of solutions of titanium chlorides in the fused CaCl 2 +NaCl equimolar mixture were studied at 550°C using different working electrodes. We determined the stability range of the various oxidation states of titanium and calculated the standard potentials of the Ti(III)/Ti(II) and Ti(II)/Ti(0) redox couples, the solubility products of the titanium oxides, the kinetic parameters of the electrochemical systems at the different electrodes and the diffusion coefficients of the electroactive species. We have also studied the titanium electrodeposition process by potential step measurements which indicated instantaneous nucleation of titanium at tungsten substrates. All these data allowed us to build-up the potential-pO 2− diagram which summarizes the properties of Ti-O compounds in the melt studied and can be used (together with the E-pO 2− diagram of chlorinating gaseous mixtures) to predict operating conditions for the process of industrial production of metallic titanium from molten salt systems.

Electrodeposition of Ho and Electrochemical Formation of Ho–Al Alloys from the Eutectic LiCl–KCl

The electrochemistry of a LiCl-KCl-HoCl 3 melt at inert and reactive electrodes has been studied at temperatures between 673 and 823 K. At a W electrode the electroreduction of Ho(III) proceeds in a single step and electrocrystallization plays an important role. Experimental current-time transients followed the theoretical models based on instantaneous nucleation with three-dimensional growth of the nuclei whatever the applied overpotential. Mass transport toward the electrode is a diffusion process, and the validity of the Arrhenius law was verified. For an Al electrode, the electroreduction of Ho(III) takes place at less cathodic potential values than at the inert W electrode which indicated the formation of Ho-Al intermetallic compounds. Ho-Al alloy films were obtained by potentiostatic electrolysis, and the samples were characterized by X-ray diffraction and scanning electron microscopy. The activity of Ho in the aluminium phase as well as the standard Gibbs energy of formation for Al 3 Ho were estimated from open-circuit chronopotentiometric measurements.

Oxoacidity reactions in equimolar molten CaCl2-NaCl mixture at 575 °C

Abstract The behaviour of an yttria-stabilized zirconia electrode in the CaCl2-NaCl melt has been investigated at 575 °C. The response of the electrode is Nernstian and permits its use in the titration of Mg(II) with oxide ions. The solubility product of MgO was also determined. We have calculated the equilibrium constant of the carbonate ion dissociation as well as those corresponding to the systems HCl H 2 O and Cl 2 O 2 , finding that molten CaCl2-NaCl is more acidic than the LiCl-KCl eutectic melt and the BaCl2-CaCl2-NaCl (23.5:24.5:52 mol%) melt. On the other hand, the mixture can be purified with respect to oxide ions by gaseous reagents such as HC1 and Cl2.

Pseudopolarographic determination of stability constants of labile zinc complexes in fresh water

The formation and distribution of labile zinc species in fresh water were investigated by using a pseudopolarographic approach. Pseudopolarograms were obtained by plotting the anodic stripping voltammetric peak current versus the deposition potential for a series of solutions containing 100 μg/l Zn(II) and increasing amounts of ligand, at the pH of a typical river water sample. Stoichiometry and stability constants of the complexes were evaluated from the shift of the half-wave potential of the pseudopolarogram as a function of the ligand concentration. Four major inorganic ligands occurring in river waters were examined: chloride, hydrogenphosphate, hydrogencarbonate and sulphate, but no shift was obtained for chloride at the concentration levels tested. The species ZnHPO4, Zn(HPO4)4−3, ZnHCO3+, Zn(HCO3)2 and ZnSO4 were found to occur at ligand concentrations ≤ 0.25 M, and their stability constants calculated. The distribution diagram of Zn in the river water investigated showed that Zn species present in concentration ≥ 0.1% of the total amount of metal were: Zn(H2O)2+6 (aquo-ion), ZnHPO4, ZnHCO3+, Zn(HCO3)2, ZnSO4, ZnOH+ and Zn(OH)2. Species of higher coordination index only appear at a ligand concentration superior to that present in fresh water.

Electrodeposition of Magnesium from CaCl2 ­ NaCl ­ KCl ­ MgCl2 Melts

Electrodeposition of magnesium from calcium chloride-based melts was studied on metallic (tungsten and molybdenum) and glassy carbon electrodes. Pure CaCl 2 melt and different alkali chloride solvents including binary CaCl 2 -NaCl, equimolar composition, and ternary CaCl 2 -NaCl-KCl melts with compositions 35:55:10 mol %, containing different MgCl 2 concentrations, were used. In the case of the binary mixture, the operating temperature was both above and below the melting point of metallic magnesium (923 K) so that solid and liquid deposits were obtained. Underpotential deposition of calcium is an important process in melts containing CaCl 2 due to formation of Ca-Mg alloys, which affects the magnesium electrodeposition process. Three-dimensional magnesium deposits that grow macroscopically on the studied substrates were found. This influences determination of the diffusion coefficient of Mg(II) ions. Formation of a magnesium monolayer on both electrodes, and especially on tungsten, was a stable process in the ternary mixture containing MgCl 2 , which was related to the presence of 10 mol % KCl. Moreover, in this melt a remarkable pseudopassivation process was found at high overpotentials.

Acidity and purification of the molten zinc chloride (33.4 mol%)-sodium chloride (66.6 mol%) mixture

Abstract The behavior of an yttria-stabilized zirconia electrode in the ZnCl 2 2NaCl melt has been investigated between 450 and 500°C. The response of the electrode was nernstian and limited in oxobasic media by the solubility of zinc oxide. Although it is more acidic than LiClKCl eutectic melt, the chlorozincate can be purified towards oxide ions by gaseous reagents such as HCl and Cl 2 .