Formal Standard Potentials Research Articles

Assessing the viability of chromium (II) as a corrosion inhibitor for 316H stainless steel in NaF-BeF2 molten salt

The redox behavior of Cr(II)/Cr(III) and Cr(II)/Cr in an 873 K NaF-BeF2 (FNaBe) melt are electrochemically investigated using cyclic voltammetry (CV), square wave voltammetry (SWV), stripping chronopotentiometry (SCP), and chrono amperometry (CA). The formal standard potential for these two redox reactions is determined to be 1.457 and 1.486 V vs Eeq,Be(II)/Be. The potential of utilizing Cr(II) as a corrosion inhibitor for 316H stainless steel (316HSS) in FNaBe melt is evaluated from thermodynamic perspective by setting Esalt≤Eeq,Cr(II)/Cr316. The results indicate that effective inhibition of 316HSS corrosion can be achieved when CCr(II) ≥ 0.429CCr(III)2/3. Experimental studies are conducted to explore the corrosion of 316HSS in the FNaBe-Cr(III)/Cr(II) melt, and the results align with the thermodynamic evaluation.

Relationship between the Thermodynamic Quantities and Coordination Structure of Oxide Ions in CaCl2-Based Chloride Melts.

Measuring the thermodynamic quantities and coordination structure of oxide ions in CaCl2-based melts is essential for comprehensively understanding the relationship between the thermodynamic and microscopic behaviors of high-temperature molten salts. In this study, the standard formal chemical potentials of oxide ion, μO2- o', and activity coefficient, γO2- , in NaCl-CaCl2, NaCl-KCl-CaCl2, and LiCl-KCl-CaCl2 melts at 873 K, were evaluated by measuring the dependence of potential of O2/O2- on the oxygen partial pressure by using non-consumable ceramic electrodes; the μO2- o' was -527 ± 0.3, - 535 ± 0.1, and -538 ± 0.2 kJ mol-1 and the γO2- was 0.10, 0.30, and 0.45 for each melt. In addition, the coordination structure of oxide ions in each melt was investigated by combining high-temperature Raman spectroscopy and density functional theory calculations. The coordination structures of oxide ions were identified as [NaO2]3- and [CaOCl3]3- in the NaCl-CaCl2 melt, [CaOCl2]2- in the NaCl-KCl-CaCl2 melt, and [LiCaO]+ and [Li3KO]2+ in the LiCl-KCl-CaCl2 melt, revealing that the stable structure was significantly different depending on the melt composition. The activity coefficients showed a tendency to depend on the nearest-neighbor cation coordinated with the oxide ion. The reported data will provide insights into the physicochemical properties of high-temperature melts and contribute to controlling the compatibility of materials and melts in pyrochemical engineering processes.

Kinetic and Thermodynamic Properties of Ytterbium Chloride and Fluoride Complexes in Chloride Melts

The electrochemical behavior of trichloride ytterbium in NaCl-KCl, KCl, and CsCl melts was studied in the temperature range 973–1173 K by different electrochemical methods. The diffusion coefficients (D) of Yb(III) and Yb(II) were determined by linear sweep voltammetry, chronopotentiometry and chronoamperometry methods. The standard rate constants of charge transfer (k s) for the Yb(III)/Yb(II) redox couple were calculated on the basis of cyclic voltammetry data using Nicholson’s equation. The formal redox potentials E * Yb(III)/Yb(II) in alkali chloride melts were obtained by the cyclic voltammetry and the Gibbs free energy for the reaction: YbCl2(sol.) + 1/2 Cl2(g.) ↔YbCl3(sol.) in alkali chloride melts was calculated. The electrochemical behavior of the Yb(III)/Yb(II) redox couple in the NaCl-KCl- NaF (5 wt%) melt was studied. A comparative analysis of electrochemical behavior of ytterbium chloride complexes in the NaCl-KCl melt and ytterbium fluoride complexes in the NaCl-KCl- NaF (5 wt%) melt was performed. It was shown that the formation of the stronger fluoride complexes reduced diffusion coefficients, standard rate constants of charge transfer for the Yb(III)/Yb(II) redox couple, and shifted the formal standard redox potentials to more electronegative values.

Immobilization of poly(o-aminophenol) film on Pt surface: A robust sensor for detecting As(III) in an acidic medium

Poly(o-aminophenol) film was immobilized onto a platinum surface to develop a arsenic senor in acidic medium via oxidation of As(III). The surface of the electrode was characterized using X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS), and the linear polarization techniques. The fabrication of the polymeric film on the platinum surface created noble catalytic sites pertaining to As(III) oxidation in acidic medium. The fact is ascertained from the observation of the lowering of onset and peak potential along with the increment of oxidation current in reference to a platinum electrode. The As(III) oxidation reaction follows first-order kinetics with a mixed mechanistic pathway following the conversion into As(V) with two electron transfers. From the assessment of the hydrodynamic voltammogra, the standard rate constant and formal potential were determined to be 1.65×10−3 cm−1, and 0.62 V, respectively. Comparatively, the rate constant in the rotational mode is almost three times greater than in the static mode, implying a mass transfer-controlled process. From the amperometric investigation, the sensitivity, LOD, and LOQ values were found to be 0.0021 mA.cm−2μM−1, 1.73±0.031 µgL−1, and 4.59±0.043 µgL−1, respectively. The stability and the real time assessment render excellent applicability with firm validation for arsenic detection in water with the PoAP film modified Pt electrode.

Electrochemistry of Neodymium in an Equimolar NaCl-KCl Melt without and with Addition of Fluoride Ions

An investigation of compounds of those few elements in the oxidation state (II), by opposition to the general trivalent lanthanides is of special interest. The present work follows our previous investigations on samarium, europium ytterbium chloride systems and, in particular, the electrochemical behavior of the Sm(III)/Sm(II), Eu(III)/Eu(II) Yb(III)/Yb(II) redox couples. It deals with neodimium compounds.The electrochemical behavior of NdCl3 in molten salts was studied in few works, but only melts with a low melting point LiCl-KCl and LiCl-KCl-CsCl were investigated.The electroreduction of NdCl3 in an equimolar NaCl-KCl mixture was studied in the temperature range 973-1123 K by different electrochemical methods. It was shown that the discharge of Nd(III) to Nd(II) is complicated by the disproportionation reaction (DPP):3Nd(II) ↔ 2Nd(III) + Nd (1)Ignoring of reaction DPP can lead to incorrect determination of values diffusion coefficients and formal standard redox potentials of the Nd(III)/Nd(II) redox couple. The relatively narrow interval of polarization rate was found when the recharge process was reversible and not accompanied by the DPP reaction. At these scan rates the diffusion coefficients and formal standard redox potentials of the Nd(III)/Nd(II) redox couple were determined by cyclic voltammetry.The standard rate constants for the redox reaction Nd(III) + e- ↔ Nd(II) were calculated on the basis of cyclic voltammetry at sweep rates corresponding to the quasy-reversible process. The nature of working electrode on the rate of charge transfer for the Nd(III)/Nd(II) redox couple was studied. It was found that the values of k s determined at a molybdenum electrode were higher than those at a glassy carbon electrode.The introduction of fluorine anions into a chloride melt at a molar ratio of F-/Nd(III) = 5 leads to the disappearance of the recharge wave and only an ascending section associated with joint discharge of neodymium complexes and sodium and potassium cations was observed. Thus, the introduction of fluorine anions leads to stabilization of the highest oxidation state of neodymium in chloride-fluoride melt and neodymium in the oxidation state +2 does not exist in the melt.Introducing of fluoride ions to the melt NaCl-KCl containing Nd(II) accompanied by reaction of disproportionation:3NdCl4 2- + 12F- ↔ 2NdF6 3- + Nd + 12Cl- (2)The standard rate constants of charge transfer for the Nd(III)/Nd(II) redox couple were compared with the values obtained for other redox couples of lanthanides and these results were discussed in connection with the strength and stability of complexes.

(Digital Presentation) Electrode Potentials of Gallium in Fused Alkali Chlorides

Electrochemical properties of gallium were investigated in 3LiCl–2KCl and 6NaCl–9KCl–5CsCl eutectic based melts at 430–700 and 500–760 oC, respectively. Zero current potentiometry was employed for studying the electrochemical behavior of gallium. Gallium ions in two oxidation states, Ga(III) and Ga(I), were present in the melt in contact with the metal. Formal standard electrode and red-ox potentials of gallium, E*(Ga/Ga(III), E*Ga/Ga(I) and E*Ga(I)/Ga(III), in these molten alkali chloride mixtures were determined.

Influence of Hydrotropes on the Solubilities and Diffusivities of Redox-Active Organic Compounds for Aqueous Flow Batteries.

In this study, we explored the extent to which hydrotropes can be used to increase the aqueous solubilities of redox-active compounds previously used in flow batteries. We measured how five hydrotropes influenced the solubilities of five redox-active compounds already soluble in aqueous electrolytes (≥0.5 M). The solubilities of the compounds varied as a function of hydrotrope type and concentration, with larger solubility changes observed at higher hydrotrope concentrations. 4-OH-TEMPO underwent the largest solubility increase (1.18 ± 0.04 to 1.99 ± 0.12 M) in 20 weight percent sodium xylene sulfonate. The presence of a hydrotrope in solution decreased the diffusion coefficients of 4-OH-TEMPO and 4,5-dihydroxy-1,3-benzenedisulfonate, which was likely due to the increased solution viscosity as opposed to a specific hydrotrope–solute interaction because the hydrotropes did not alter their molecules’ hydraulic radii. The standard rate constants and formal potentials of both 4-OH-TEMPO and 4,5-dihydroxy-1,3-benzenedisulfonate remained largely unchanged in the presence of a hydrotrope. The results suggest that using hydrotropes may be a feasible strategy for increasing the solubilities of redox-active compounds in aqueous flow batteries without substantially altering their electrochemical properties.

Thermodynamic Properties of Hafnium Chlorides Dissolved in the NaCl-KCl Melt Obtained by Electrochemical Transient Techniques

The electroreduction of HfCl4 in the NaCl-KCl melt was studied in the temperature range 973–1123 K by cyclic voltammetry and impedance spectroscopy methods. It was determined that electrochemical reduction of Hf(IV) in the NaCl-KCl melt occurs via two successive stages involving transfer of two electrons at each stage. The formal standard potentials of E*Hf(II)/Hf, E*Hf(IV)/Hf and the formal standard redox potentials of E*Hf(IV)/Hf(II) were determined from the cyclic voltammetry and the values of the standard rate constants of charge transfer obtained by impedance spectroscopy method. The thermodynamic properties of the dilute solutions of hafnium di- and tetrachloride formation from the elements in an equimolar mixture of molten NaCl-KCl were determined. The free Gibbs energy changes for the reaction HfCl4(sol.)↔ HfCl2(sol.) + Cl2(g.) and the equilibrium constants of the metal-salt reaction Hf(IV) + Hf ↔ 2Hf(II) were calculated.

Thermodynamic Properties of Hafnium Chlorides Dissolved in the NaCl-KCl Melt Obtained by Electrochemical Transient Techniques

The electroreduction of HfCl4 in the NaCl-KCl melt was studied in the temperature range 973-1123 K by cyclic voltammetry and impedance spectroscopy methods. It was determined that electrochemical reduction of Hf(IV) in the NaCl-KCl melt occurs via two successive stages involving transfer of two electrons at each stage. The formal standard potentials of E * Hf(II)/Hf, E * Hf(IV)/Hf and the formal standard redox potentials of E * Hf(IV)/Hf(II) were determined from the cyclic voltammetry and the values of the standard rate constants of charge transfer obtained by impedance spectroscopy method. The thermodynamic properties of the dilute solutions of hafnium di - and tetrachloride formation from the elements in an equimolar mixture of molten NaCl-KCl were determined. The free Gibbs energy changes for the reaction HfCl4(sol.) « HfCl2(sol.) + Cl2(g.) and the equilibrium constants of the metal-salt reaction Hf(IV) + Hf ↔ 2Hf(II) were calculated.

Thermodynamic Properties of Hafnium Chlorides Dissolved in the NaCl-KCl Melt Obtained by Electrochemical Transient Techniques

The electrolysis of molten salts is an effective method for production of hafnium. The knowledge of thermodynamic properties and electrochemistry of hafnium provides data for optimization of electrolytic process. In previous studies, the thermodynamic properties of HfCl2 and HfCl4 in an equimolar mixture NaCl-KCl were determined by EMF method. However, in these investigations oxide materials were used in the construction of the electrochemical cell and the reference electrode. Our experiments showed that oxide materials interacted with the NaCl-KCl-HfCl2, NaCl-KCl-HfCl2-HfCl4 and NaCl-KCl-HfCl4 melts. EMF measurements are unsuitable for the abovementioned systems due to reactions of the next types: HfCl4 + O2- ↔ HfOCl2↓ + 2Cl- (1) 2HfCl2 + O2- ↔ HfOCl2↓ + Hf + 2Cl- (2) This reaction leads to decreasing of the concentration Hf(IV) and Hf(II) complexes and to change the ratio of lower and higher oxidation states. In addition, measurements of equilibrium potentials in melts containing Hf(II) are complicated by their tendency toward disproportionation: 2Hf(II) ↔ Hf(IV) + Hf (3) The reaction of disproportionation of the type (2) occurs due to corrosion of oxide materials because compounds of hafnium have a high affinity to oxygen. For the elimination of the abovementioned difficulties for the determination of thermodynamic properties hafnium in molten systems electrochemical transient techniques was used. In our study, cyclic voltammetric curves and impedance spectra were recorded at different electrodes (glassy carbon, several metals) with respect to a glassy carbon quasi-reference electrode. The glassy carbon ampoule served as the counter electrode. While the potential of this quasi-reference electrode does not constitute a thermodynamic reference, the use of this electrode was preferred in order to avoid any contact between the melt and oxygen-containing material as used in classical reference electrodes. A Ag/NaCl-KCl-AgCl (2 wt.%) reference electrode was used in order to obtain more reliable potential values. At the final stage of each experimental set, this reference electrode was immersed in the melt for a short time for determination of the potential peaks, the melt being no longer used after these measurements. In this study, the thermodynamic properties of hafnium chloride solutions in an equimolar mixture NaCl-KCl were obtained with utilization of this approach. The electroreduction of HfCl4 in the NaCl-KCl melt was studied in the temperature range 973-1123 K by cyclic voltammetry and impedance spectroscopy methods. It was determined that electrochemical reduction of Hf(IV) in the NaCl-KCl melt occurs via two successive stages involving transfer of two electrons at each stage. The formal standard potentials of E * Hf(II)/Hf, E * Hf(IV)/Hf and the formal standard redox potentials of E * Hf(IV)/Hf(II) were determined from the cyclic voltammetry and the values of the standard rate constants of charge transfer obtained by impedance spectroscopy method. The free Gibbs energy changes for the reaction HfCl2(sol.) + Cl2(g.) ↔ HfCl4(sol.) and the equilibrium constants of metal-salt reaction Hf(IV) + Hf ↔ 2Hf(II) were calculated. The thermodynamic properties of the dilute solutions of hafnium di-and tetrachloride formation in the NaCl-KCl melt were determined.

An Electrochemical Study of Divalent Ytterbium Species in NaCl–KCl and NaCl–KCl–Cscl Based Melts

A usual oxidation state of rare earth metals in chloride melts is +3. A number of lanthanides (particularly europium, ytterbium, samarium, and thulium) can also form thermodynamically stable ions in the oxidation state +2. Ln(II)/Ln(III) redox processes are normally investigated employing transient electrochemical techniques where the formation of Ln(II) ions is limited to the near electrode surface layer. The present work was aimed at studying the reduction of Yb(III) ions in the bulk of the melt and assessing the stability of Yb(II) chloro species in molten chloride media. The experiments were carried out in the melts based on NaCl–KCl–CsCl eutectic and NaCl–KCl equimolar mixtures at 550–850 and 700–850 oC, respectively, employing stationary and transient electrochemistry and high temperature spectroscopy methods. Concentration of ytterbium in the melt was varied from 1 to 10 wt. %. Ytterbium(II)/(III) formal standard redox potentials were obtained from the results of electrochemistry measurements (cyclic voltammetry and potentiometry). Electronic absorption spectra of Yb(II) species were recorded both in molten and quenched electrolytes. Reduction of Yb(III) to Yb(II) can be achieved by shifting the equilibrium of the reaction YbCl6 3– + Cl– ↔ YbCl6 4– + ½ Cl2 to the right by lowering partial pressure of chlorine in the atmosphere above the melt. Here, zirconium, as a metal with high affinity for chlorine, was used as a getter. The reduction process was followed by measuring the absorption spectra. As the reaction proceeded, the color of the melt changed from colorless of YbCl6 3– to brown. Analysis of the resulting melts showed that the mean oxidation state of ytterbium was below three, the value depended on temperature, duration of the reduction and melt composition. Increasing temperature or reduction time expectedly resulted in a greater degree of reduction. Ytterbium(III) can also be reduced with a suitable reductant and hydrogen was chosen here as an example. Sparging the melt containing Yb(III) ions with hydrogen resulted in decreasing mean oxidation state of ytterbium: YbCl6 3– + Cl– + ½ H2 ↔ YbCl6 4- + HCl. The course of the reduction process was followed by measuring the redox potential. The efficiency of hydrogen as a reducing agent was comparable to the thermal decomposition in the presence of zirconium getter. Hydrogen reduction was also a very convenient way of preparing the melts with a low Yb(II) content for spectroscopy measurements. Another way of reducing Yb(III) to Yb(II) is electrolysis: YbCl6 3– + e– ↔ YbCl6 4-. In a series of preliminary experiments Yb(II)/Yb(III) redox potentials were determined from the results of cyclic voltammetry measurements. Reduction of Yb(III) to Yb(II) was carried out by potentiostatic electrolysis under an inert (argon) or a reducing (hydrogen) atmosphere with and without agitating the melt. The lowest mean oxidation state of ytterbium in the melt thus obtained was 2.33 showing that two thirds of Yb(III) could be reduced to Yb(II). Acknowledgement. This work was supported by the Ministry of Education and Science of the Russian Federation (project No. 4.5062.2017/8.9).

Determination of Distribution Equilibrium-Potential Differences Based on Extraction with Several Crown Ethers by Nitrobenzene, 1,2-Dichloroethane and Dichloromethane

Extraction constants (Kex± & Kex) were determined at 298 K for the extraction of sodium picrate (NaPic) by nitrobenzene (NB), 1,2-dichloroethane (DCE) and dichloromethane using 3m-crown-m ethers and their benzo-derivatives (m = 5, 6; abbreviated as L) together with the determination of conditional distribution constants (KD,Pic) of picrate ion, Pic-, into these diluents. The K1,org (= [NaLPic]org/[NaL+]org[Pic-]org) values at the organic (org) phases, such as NB & DCE, were calculated from the relation Kex/Kex± = K1,org. Distribution equilibrium-potential differences (Dfeq) at extraction equilibria were evaluated from the equation Dfeq = -0.05916´(log KD,Pic - constant) at 298 K. Correlations of the above equilibrium constants, particularly Kex±, with Dfeq were examined. Furthermore, the standard formal potentials for Na+ transfers across the interfaces were briefly evaluated from calculated Dfeq, [Na+] and [Pic-]org. The above extraction systems were characterized by K1,org and the complex formation constants of Na+ with L in the org phases.

Measurement of europium (III)/europium (II) couple in fluoride molten salt for redox control in a molten salt reactor concept

The fluoride molten salt such as FLiNaK and FLiBe is one of the coolant candidates for the next generation nuclear reactor concepts, for example, the fluoride salt cooled high temperature reactor (FHR). For mitigating corrosion of structural materials in molten fluoride salt, the redox condition of the salts needs to be monitored and controlled. This study investigates the feasibility of applying the Eu3+/Eu2+ couple for redox control. Cyclic voltammetry measurements of the Eu3+/Eu2+ couple were able to obtain the concentrations ratio of Eu3+/Eu2+ in the melt. Additionally, the formal standard potential of Eu3+/Eu2+ was characterized over the FHR's operating temperatures allowing for the application of the Nernst equation to establish a Eu3+/Eu2+ concentration ratio below 0.05 to prevent corrosion of candidate structural materials. A platinum quasi-reference electrode with potential calibrated by potassium reduction potential is shown as reliable for the redox potential measurement. These results show that the Eu3+/Eu2+ couple is a feasible redox buffering agent to control the redox condition in molten fluoride salts.

Corrosion of Austenitic Steels and Their Components in Molten Nacl–Kcl–UCl3

The fused halides can be used for nuclear fuel reprocessing and as working media for molten salt nuclear reactors. However practical implementation of such technologies is limited by the problem of finding suitable corrosion resistant materials capable of long-term working in contact with molten salts. From the economical point of view the application of stainless steels as construction materials for the molten salt media is one of the most prospective ways. In the present study the corrosion behaviour of metallic iron, nickel, chromium, molybdenum, AISI 316L and 12Kh18N10T (analogue of AISI 321) austenitic steels was studied in NaCl–KCl–UCl3(1 wt.% U) melts at 750 ºC. Gravimetric measurements served as a basis for estimating corrosion resistance of studied materials. Quenched melts were analyzed using ICP-AES method. The surface of the alloy samples after corrosion tests was characterised using XRD, SEM and Х-ray microanalysis. The foils of corroded materials were examined by TEM. The corrosion rates of studied metals in NaCl–KCl–UCl3melt decrease in the following order: Cr>Fe>Ni>Mo, which correlates well with the formal standard potentials of metals in chloride melts indicating electrochemical nature of the corrosion processes. Molybdenum is the only corrosion resistant metal in the uranium-containing chloride melts and it can be used as a construction material for molten salt reactors. The major products of stainless steel corrosion in chloride melts are iron, chromium and manganese species. The surface of the corroded samples of austenitic stainless steels was depleted in manganese and chromium and enriched in nickel and molybdenum. Intergranular corrosion was observed for all types of austenitic steels. In terms of intensity and depth of the corrosion layer in NaCl–KCl–UCl3 the studied steels can be ranged in the order AISI 321>AISI 316L that correlates with the amount of excessive phases formed (figure). Figure 1

Electrode and Redox Potentials of Molybdenum and Stability of Molybdenum Chloro-Species in Alkali Chloride Melts

Molybdenum electrode and Mo(III)/(IV) redox potentials were measured in fused alkali chlorides. Experiments were performed in individual salts (LiCl, NaCl, KCl, RbCl and CsCl) and in several binary and ternary eutectic or low-melting mixtures between 633 and 1173 K (depending on the melting point of the solvent salt). Formal standard electrode potentials E*Mo/Mo(III) and redox potentials E*Mo(III)/Mo(IV) in respect to Cl−/Cl2 couple and Gibbs free energy change of formation of molybdenum(III) chloride in alkali chloride melts were calculated. Electronic absorption spectra of Mo(III) ions were recorded, and spectroscopic parameters of MoCl63− complex ions determined. High temperature spectroscopy measurements were used to study the stability of Mo(III) chloro-species in fused chlorides and the reaction rates of Mo(III) ions disproportionation were also determined. Diffusion coefficients of molybdenum ions in LiCl-KCl-CsCl and NaCl-CsCl eutectic based melts were determined from the results of electrochemical measurements.

Electrochemical and Thermodynamic Properties of Lanthanum in a Chloride Melt – Liquid Metal System

The behavior and properties of lanthanum in a “molten salt – liquid metal” system in relation to pyrochemical reprocessing of spent nuclear fuels was considered. Temperature dependencies of lanthanum formal standard electrode potential E* La/La(III) and Gibbs free energy change of lanthanum trichloride formation in LiCl–KCl–CsCl eutectic based melts were obtained between 668–976 K. Also the behavior of lanthanum was studied in Ga–Al (1.6 wt. % Al) and Ga–Zn (3.64 wt. % Zn) eutectic based metallic alloys, and activity, solubility and activity coefficients of lanthanum determined.

Computational Model of Magnesium Deposition and Dissolution for Property Determination via Cyclic Voltammetry

The development of a practical magnesium-anode battery requires electrolytes that allow for highly efficient magnesium exchange while also being compatible with cathode materials. Here, a one-dimensional continuum-scale model is developed to simulate cyclic plating/stripping voltammetry of a model magnesium-based electrolyte system employing magnesium borohydride/dimethoxyethane [Mg(BH4)2/DME] solutions on a gold substrate. The model is developed from non-electroneutral dilute-solution theory, using Nernst-Planck equations for the mass flux and Poisson's equation for the electrostatic potential. The electrochemical reaction is modeled with multistep Butler-Volmer kinetics, with a modified current/overpotential relationship that separately accounts for the portions of the current responsible for nucleating new deposits and propagating or dissolving existing ones. The diffusivities of the electrolyte species, standard heterogeneous rate constant, charge-transfer coefficient, formal potential, and nucleation overpotential are determined computationally by reproducing experimental voltammograms. The model is computationally inexpensive and therefore allows for broad parametric studies of electrolyte behavior that would otherwise be impractical.

Brief Determination of Standard Formal Potentials for the Transfers of Several Pairing Anions across the Nitrobenzene/Water Interface by Na(I) Extraction with 18-Crown-6 Ether

Extraction constants (Kex±) for the extraction of sodium salts (NaA) with 18-crown-6 ether (18C6) from water (w) into nitrobenzene (NB) were determined at 298 K, together with the determination of individual distribution constants (KD,A) of several pairing anions A−. The symbols Kex± and KD,A were defined as [NaL+]NB[A−]NB/([Na+][L]NB[A−]) at L = 18C6 and [A−]NB/[A−], respectively; the subscript “NB” shows the NB phase. Also, ion-pair formation constants for sodium 2,4-dinitrophenolate (NaDnp) and its ion pair complex with 18C6 in water were determined by potentiometry with a Na+-selective electrode. Standardized (S) KD,A values were briefly calculated from the following thermodynamic cycle: KD,A S = Kex±/KD,Na SKNaL,NB. Here, KD,Na S and KNaL,NB denote the standardized individual distribution-constant of Na+ into and a complex formation constant for NaL+ in the NB phase, respectively. Moreover, equilibrium potential differences (Δφeq) at the NB/w interface were estimated from the relation Δφeq = 0.05916log (KD,A S/KD,A) at 298 K. The Δφeq values of A− = F3CCO2 −, MnO4 −, ReO4 − and Dnp− were determined for comparison with the value of picrate ion. The plot of log Kex± versus Δφeq gave a positive correlation at (correlation coefficient)2 = 0.748.

An electrochemical study of uranium behaviour in LiCl–KCl–CsCl eutectic melt

Electrochemical behaviour of uranium was studied in the low melting ternary LiCl–KCl–CsCl eutectic at 573–1073 K employing potentiometry, cyclic voltammetry and chronopotentiometry. Uranium electrode potentials were measured directly and U(III)/U(IV) red-ox potentials were determined from the results of cyclic voltammetry measurements. Formal standard electrode and red-ox potentials of uranium, and thermodynamic properties of uranium chlorides in the studied melt were calculated. Diffusion coefficients of U(III) and U(IV) ions were determined using cyclic voltammetry and chronopotentiometry.

Electrode potentials of uranium in the LiCl–KCl–CsCl eutectic melt

The electrode potentials of uranium in the melt of the eutectic mixture of lithium, potassium, and cesium chlorides are measured in the temperature range 573–1073 K. Formal standard potentials E U * (III)/U and the main thermodynamic characteristics of uranium trichloride in the LiCl–KCl–CsCl melt are calculated, and the electronic absorption spectra of UCl 6 3– ions are measured.