Electrochemical formation of Mg-Li alloys at solid magnesium electrode from LiCl-KCl melts

This work presents a study on electrochemical formation of Mg-Li alloys on solid magnesium electrode in a molten LiCl-KCl (50:50, wt.%) system at 753 K. Cyclic voltammetry and open circuit chronopotentiometry were employed to investigate the electrode reaction. For an Mg electrode, the electroreduction of Li(I) takes place at more positive potential values than at the inert W electrode indicating the formation of Mg-Li alloys. X-ray diffraction and scanning electron microscopy (SEM) analysis of the deposits indicated that α, α + β and β phases Mg-Li alloys with the thickness of 182, 365 and 2140 μm were obtained by potentiostatic electrolysis at −2.26, −2.30 and −2.39 V (vs. Ag/AgCl), respectively. The results showed that formation of α, α + β and β phases Mg-Li alloys could be controlled by applied potential. Lithium contents of Mg-Li alloys can be decreased via electrolysis at low temperature followed by thermal treatment at higher temperature. Mg-Li alloys with excellent mechanical properties can be produced by this novel method.     

Electrochemical behaviour of dysprosium in the eutectic LiCl-KCl at W and Al electrodes

The electrochemical behaviour of DyCl₃ was studied in the eutectic LiCl-KCl at different temperatures. The cathodic reaction can be written:

Dy(III) + 3e ↔ Dy(0)     

Electrochemical synthesis of Ni-Sn alloys in molten LiCl-KCl

The electrochemical formation of Ni-Sn was investigated in molten LiCl-KCl in the temperature range 380-580 °C. Before, an electrochemical study of the Ni²⁺/Ni⁰, Sn⁴⁺/Sn²⁺ and Sn²⁺/Sn⁰ redox couples was performed by cyclic voltametry and chronopotentiometry in a wide temperature range. It had been pointed out that in the case of the Sn⁴⁺/Sn²⁺ redox couple, an insoluble compound is probably formed for T < 460 °C. For higher temperature, this compound becomes soluble and then, the shape of the cyclic voltammogram is analogue to the one usually observed when a diffusion-controlled process is involved. The diffusion coefficient values of Ni²⁺ and Sn²⁺ ions were determined. For instance, D_Ni(II) and D_Sn(II) values deduced from chronopotentiometry were about 2.1 × 10⁻⁵ and 2.7 × 10⁻⁵ cm² s⁻¹ at 440 °C, respectively. Then, Ni-Sn alloys have been formed in potentiostatic mode. The electrochemical route proposed in this paper leads to the formation of crystallized alloys with a well-defined composition depending on the operating conditions.     

Electrochemical behaviour of erbium in the eutectic LiCl-KCl at W and Al electrodes

The electrochemical behaviour of Er(III) was studied in the molten LiCl-KCl eutectic for temperatures ranging from 653 to 823 K. 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 at a tungsten electrode. The results showed that in the eutectic LiCl-KCl, the reduction of Er(III) occurs in a step with a global exchange of three electrons, and that electrocrystallization plays an important role in the electrodeposition process. Chronoamperometric studies indicated instantaneous nucleation of erbium with three-dimensional growth of the nuclei whatever the applied overpotential.Mass transport towards the electrode is a simple diffusion process, and the temperature dependence of the diffusion coefficient of the electroactive specie Er(III) was written in the following equation:

     

Electrochemical studies on the redox mechanism of uranium chloride in molten LiCl-KCl eutectic

The reduction and oxidation processes on platinum and glassy carbon electrodes in molten LiCl-KCl eutectic containing UCl₃ were investigated by cyclic voltammetry (CV), chronopotentiometry and electrochemical impedance spectroscopy (EIS) in the temperature range 660-780 K. Two redox peaks have been observed in the cyclic voltammograms corresponding to the two redox reactions U(IV)/U(III) and U(III)/U which are found to be reversible and quasi-reversible, respectively. The reduction potentials of U(IV)/U(III) and U(III)/U are −0.325 and −1.490 V versus reference electrode (0.1 mol% AgCl in the LiCl-KCl), respectively at 700 K. Chronopotentiometric measurements confirm the three-electron transfer during the reduction of U(III) to U metal. Electrochemical impedance spectroscopy data at various potentials were interpreted either as diffusion, adsorption or reduction process by nonlinear fit of the impedance data to the simulated equivalent circuits.     

Electrochemistry of terbium in the eutectic LiCl-KCl

The electrochemical behaviour of terbium at a tungsten electrode, in the eutectic LiCl-KCl molten was investigated in the temperature range 673-823 K, by means of cyclic voltammetry, chronopotentiometry and chronoamperometry. It was found that during cathodic polarization, deposition of metallic Tb from the chloride mixture onto the tungsten surface proceeds in a single step, and electrocrystallization plays an important role in the whole process. Experimental current-time transients followed the theoretical models based on instantaneous nucleation with three-dimensional growth of the nuclei whatever the applied overpotential. From chronopotentiometric measurements, the diffusion coefficient of the Tb(III) ions was determined by applying the Sand equation and modifying the immersion dept of the working electrode in stages. The validity of the Arrhenius law was also verified and the activation energy for diffusion was found to be 33.4 ± 1.5 kJ mol⁻¹.The standard apparent potential value of the Tb(III)/Tb(0) system has been determined by potentiometry at several temperatures, and the activity coefficient of Tb(III) in the molten chloride media based on a pure supercool reference state has also been calculated.     

Electrochemical mechanism and kinetics studies of haloperidol and its assay in commercial formulations

The kinetics and mechanism for electrochemical reduction of haloperidol, a psychotherapeutic drug used in the treatment of schizophrenia, were studied using square wave and cyclic voltammetries allied to a hanging mercury drop electrode. The experimental and voltammetric parameters were optimized at 0.04 mol L⁻¹ Brinton–Robinson buffer (pH 10), with a pulse potential frequency of 100 s⁻¹, a pulse amplitude of 30 mV and scan increment of 2 mV. Two well-defined peaks were observed, which exhibited properties of fast electron transfer with a strong adsorption process of reactants and products on the electrode surface. The first peak was related to a fast and reversible anion-radical formation originating from the reduction of the carbonyl group, and the second was related to the irreversible reduction of the anion-radical previously formed. Analytical parameters such as: linearity range, equation of the analytical curves, correlation coefficients, detection and quantification limits, recovery efficiency, and relative standard deviation for intraday and interday were compared to similar results obtained by use of the UV–vis spectrophotometry technique, and the analytical results obtained in commercial formulations show that the voltammetric procedure using a hanging mercury drop electrode is suitable for analyzing haloperidol in complex commercial formulation samples.     

Characterization of the surface redox process of adsorbed morin at glassy carbon electrodes

The thermodynamic and kinetics of the adsorption of morin (MOR) on glassy carbon (GC) electrodes in 0.2 mol dm⁻³ phosphate buffer solutions (PBS, pH 7.00) was studied by both cyclic (CV) and square wave (SWV) voltammetries. The Frumkin adsorption isotherm was the best to describe the specific interaction of MOR with GC electrodes. The SWV allowed to characterize the thermodynamic and kinetics of surface quasi-reversible redox couple of MOR, using the combination of the “quasi-reversible maximum” and the “splitting of SW net peaks” methods. Average values obtained for the formal potential and the anodic transfer coefficient were (0.27 ± 0.02) V and (0.59 ± 0.09), respectively. Moreover, a value of formal rate constant (k_s) of 87 s⁻¹ for the overall two-electron redox process was calculated. The SWV was also employed to generate calibration curves, which were linear in the range MOR bulk concentration from 1.27 × 10⁻⁷ to 2.50 × 10⁻⁵ mol dm⁻³. The lowest concentration experimentally measured for a signal to noise ratio of 3:1 was 1.25 × 10⁻⁸ mol dm⁻³ (3 ppb).     

Electrochemical study of the Eu(III)/Eu(II) system in molten fluoride media

The electrochemical behaviour of the Eu(III)/Eu(II) system was examined in the molten eutectic LiF–CaF₂ on a molybdenum electrode, using cyclic voltammetry, square wave voltammetry and chronopotentiometry. It was observed that EuF₃ is partly reduced into EuF₂ at the operating temperatures (1073–1143 K). The electrochemical study allowed to calculate both the equilibrium constant and the formal standard potential of the Eu(III)/Eu(II) system. The reaction is limited by the diffusion of the species in the solution; their diffusion coefficients were calculated at different temperatures and the values obey Arrhenius’ law. The second system Eu(II)/Eu takes place out of the electrochemical window on an inert molybdenum electrode, which inhibits the extraction of Eu species from the salt on such a substrate.     

Adsorption of ochratoxin A (OTA) anodic oxidation product on glassy carbon electrodes in highly acidic reaction media: Its thermodynamic and kinetics characterization

We study the thermodynamics and kinetics of the adsorption of a redox couple having quinone nature on glassy carbon electrodes. This couple is produced by the anodic oxidation of mycotoxin ochratoxin A in 10% acetonitrile + 90% 1 M HClO₄ aqueous solution. The quasi-reversible redox couple was studied by both cyclic (CV) and square wave (SWV) voltammetric techniques. The Frumkin adsorption isotherm best described the specific interaction of the redox couple with carbon electrodes. By fitting the experimental data, we obtained values of −28.4 kJ mol⁻¹ and 0.70 ± 0.02 for the Gibbs free energy of adsorption and the interaction parameter, respectively. SWV fully characterized the thermodynamics and kinetics of the adsorbed redox couple, using a combination of the “quasi-reversible maximum” and the “splitting of SW peaks” methods. Average values of 0.609 ± 0.003 V and 0.45 ± 0.06 were obtained for the formal potential and the anodic transfer coefficient, respectively. Moreover, a formal rate constant of 10.7 s⁻¹ was obtained. SWV was also employed to generate calibration curves. The lowest concentration of mycotoxin was 1.24 × 10⁻⁸ M (5 ppb), measured indirectly with a signal to noise ratio of 3:1.     

Electrochemistry in large unilamellar vesicles. The distribution of 1-naphthol studied by square wave voltammetry

1-Naphthol (NPh) behavior in water and in large unilamellar vesicles (LUVs) formed with the phospholipid 1,2-di-oleoyl-sn-glycero-3-phosphatidylcholine (DOPC) was studied by square wave (SWV) and cyclic voltammetry (CV) techniques and, compared with emission spectroscopy studies. In DOPC LUV media, the redox behavior of NPh shows that the molecule undergoes a partition process between two pseudophases, the water and the LUV bilayer. The NPh electrochemical responses allow us to propose a model to explain the electrochemical experimental results and, in conjunction with our measurements, to calculate the NPh partition constant (K) value between the pseudophases. The K value coincides with the one obtained through emission spectroscopy. Beside the K value, our electrochemical model allows us to calculate the diffusion coefficient (D_LUV) for DOPC LUV which coincides with the D_LUV value obtained through dynamic light scattering (DLS). Thus, our data clearly show that the electrochemical measurements could be a powerful alternative approach to investigate the behavior of non-ionic electroactive molecules embed in a confined environment such as the LUV bilayer.     

The redox thermodynamics and kinetics of flavonoid rutin adsorbed at glassy carbon electrodes by stripping square wave voltammetry

The adsorptive accumulation of rutin (RU) at glassy carbon (GC) electrodes in 10% ethanol + 90% 1 mol dm⁻³ HClO₄ aqueous solution is studied by using cyclic (CV) and square wave (SWV) voltammetries. The Frumkin adsorption isotherm best described the specific interaction of rutin with carbon electrodes. By fitting the experimental data, values of −31.9 kJ mol⁻¹ and 0.54 ± 0.02 were obtained for the Gibbs free energy of adsorption and the interaction parameter, respectively. SWV fully characterized the thermodynamics and kinetics of the surface redox process, using a combination of the “quasi-reversible maximum” and the “splitting of SW peaks” methods. Average values of 0.644 ± 0.003 V and 0.44 ± 0.02 were obtained for the formal potential and the anodic transfer coefficient, respectively. Moreover, a formal rate constant of 6.1 × 10² s⁻¹ was obtained. SWV was also employed to generate calibration curves. The lowest concentration of RU experimentally measured for a signal-to-noise ratio of 3:1 was 2 × 10⁻⁸ mol dm⁻³ (12 ppb).     

Cyclic voltammetry investigation of diffusion of ferrocene within propylene carbonate organogel formed by gelator

Propylene carbonate organogel containing LiClO₄ was formed in the presence of gelator bis-(4-stearoylaminophenyl) methane (BSAPM). The electrochemical behavior and diffusion of ferrocene (Fc) and ferricenium (Fc⁺) entrapped within the organogel was investigated by cyclic voltammetry. The Fc molecules still show redox activity within the organogels in comparison with corresponding solutions of propylene carbonate containing LiClO₄. The shape of the cyclic voltammograms of the Fc electrooxidation in organogel was similar to that in corresponding solutions. The results indicated that redox reactions of Fc/Fc⁺ were a quasi-reversible process of diffusion-controlled single electron transfer in organogels. The diffusion coefficients of Fc and Fc⁺ in organogels decreased with an increase of the concentration of gelator BSAPM, but increased with an increase of temperature. The temperature dependence of the diffusion coefficient in organogels followed classical Arrhenius equation. The activation energy in organogels was found of no difference from that in corresponding solutions.     

Studies of a novel conducting polymer by cyclic and square wave voltammetries: Its synthesis and characterization

A novel conducting polymer of polynaphthidine, poly(NAP), was synthesized electrochemically by direct anodic oxidation of naphthidine in aqueous media. The yield of the electropolymerization reaction depends on the temperature and pH of the solution. It was possible to differentiate two working regions: I (for pH < 0.5 and all temperatures) where the film yield tends to zero and II (for approximately 2.0 < pH < 2.8 and temperatures >15 °C) where the film production is maximum. Therefore, the naphthidine electrooxidation mechanism was studied under experimental conditions of region I by cyclic (CV) and square wave voltammetries (SWV) as well as by controlled potential electrolysis.The experimental conditions of region II were chosen to obtain the poly(NAP). The electrochemical response of the film was investigated in pH 1 HClO₄ + 0.1 M NaClO₄ electrolyte solution by CV and SWV. A plot of I_p,n/fvs. f from SW voltammograms showed the so-called “quasi-reversible maximum”. Formal potential, formal rate constant and anodic transfer coefficient for the surface redox process were also evaluated from the SWV.The poly(NAP) is insoluble in common organic solvents and shows electrochromic behaviour. Its probable structure was determined by FTIR spectroscopy.     

Investigations on Structural and Electrochemical Properties of Some Disulfides Prepared by Schiff Base Reactions

Four different disulfides, [2,2′‐dithiobis‐(2‐mercaptoacetophenone)]‐4‐triphenylmethylthiosemicarbazone ( 1 ), [5,5′‐dithiobis‐(4‐formyl‐3‐methyl‐1‐phenylpyrazole)]‐4‐triphenylmethylthiosemicarbazone ( 2 ), bis[1‐(2‐mercaptophenyl)‐2‐(4‐(1‐phenyl‐3‐methyl)pyrazole)‐azaethene]di‐sulfide ( 3 ) and bis[1‐phenyl‐2‐(4‐(1‐phenyl‐3‐methyl‐5‐mercapto) pyrazole)‐azaethene]disulfide ( 4 ) were synthesized by Schiff base reactions. Their electrochemical behaviour was examined by cyclic voltammetry. The results show low potentials for the disulfide reduction so that these compounds are suitable for the syntheses of tridentate thiolate ligands from disulfides by electrochemical cleavage. In addition compounds 2 and 4 were characterized by X‐ray structure determination. The structures show significant differences of the S—S bonds and angles as compared to other disulfides without bulky substituents.     

Electrochemical behaviour of a new triiron-substituted polyoxomolybdate

A new complex of the Keggin trilacunary polyoxomolybdate (PMo₉) with Fe³⁺ ions, having the formula (PFe₃Mo₉), has been synthesized and characterized by chemical analysis, FT-IR, Raman, UV-VIS-NIR and EPR spectroscopy, as well as by magnetic susceptibility measurements. Cyclic voltammetry performed at different scan rates, pH and supporting electrolyte composition, was used to investigate the electrochemical behaviour of the PFe₃Mo₉ complex in acidic medium and its electrocatalytic effect on H₂O₂ reduction. The voltammetric waves were assigned, and the enhanced electrocatalytic efficiency of PFe₃Mo₉ relative to PMo₉ was attributed to the presence of Fe atoms.     

Redox behaviour of cerium oxychloride in molten MgCl2-NaCl-KCl eutectic

The electrochemical behaviour of cerium oxychloride in MgCl₂-NaCl-KCl ternary eutectic was investigated by cyclic voltammetry at 823 K. The cyclic voltammogram of UO₂Cl₂-CeOCl in MgCl₂-NaCl-KCl eutectic shows two peaks during the cathodic sweep as well as anodic sweep. The reduction of UO₂²⁺ is by a single step two-electron transfer and that of CeO⁺ is by a single step one-electron transfer. The reduction of CeO⁺ was found to be quasi-reversible.The reduction potentials of UO₂²⁺/UO₂ and CeO⁺/CeO versus Ag(I)/Ag reference electrode at 823 K are 0.103 and −0.299 V, respectively. The diffusion coefficient of CeO⁺ at 823 K is in the range of (1.7-1.9) × 10⁻⁵ cm² s⁻¹. The cyclic voltammogram for 0.015 mol% CeOCl shows an additional peak during the anodic sweep at −0.056 V, which is being attributed to monolayer dissolution of CeO at the glassy carbon working electrode. Electrochemical impedance data of 0.015 mol% CeOCl in MgCl₂-NaCl-KCl eutectic at the open circuit potential was fitted to a Randles cell from which the heterogeneous rate constant was estimated. X-ray photoelectron spectroscopy was used to confirm that the oxidation state of cerium in the eutectic is +3.     

Electrochemical behaviour of some 3-methylthio-1,2-dithiolium cations in dimethylformamide solution

Substituted 3-methylthio-1,2-dithiolium cations were prepared by alkylation of the corresponding 1,2-dithiole-3-thiones. These compounds were easily reduced in DMF at a mercury electrode, following two one-electron waves. Electrolysis, carried out at a mercury pool electrode and followed by an alkylation or an acylation, provided the corresponding alkyl (or acyl) dithioesters and by-products. At a platinum or glassy carbon electrode the reduction was an irreversible process, which was dependent upon the cathode nature. Whatever the electrode, the reduction was a combination of two reactions, a two-electron reduction and a one-electron one involving a dimerization. The electrochemical behaviour of 3-methylthio-1,2-dithiolium cations exhibited many analogies with the cathodic behaviour of the corresponding 1,2-dithiole-3-thiones. Moreover, identical intermediates were implicated in the reduction of 3-methylthio-1,2-dithiolium cations and in the reductive alkylation of 1,2-dithiole-3-thiones.     

Electrochemical properties of double wall carbon nanotube electrodes

Electrochemical properties of double wall carbon nanotubes (DWNT) were assessed and compared to their single wall (SWNT) counterparts. The double and single wall carbon nanotube materials were characterized by Raman spectroscopy, scanning and transmission electron microscopy and electrochemistry. The electrochemical behavior of DWNT film electrodes was characterized by using cyclic voltammetry of ferricyanide and NADH. It is shown that while both DWNT and SWNT were significantly functionalized with oxygen containing groups, double wall carbon nanotube film electrodes show a fast electron transfer and substantial decrease of overpotential of NADH when compared to the same way treated single wall carbon nanotubes. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.