Fused Salt Polarography Using a Dropping Bismuth Cathode

The reduction of Ni(II), Co(II), Cd(II), and Pb(II) was studied at glassy carbon electrodes in molten lithium chloride-potassium chloride eutectic at 450~ using the technique of linear sweep voltammetry. Peak-shaped voltammograms were obtained at scan rates of 10 mV to 1 V/sec. Linear dependence of peak currents on metal ion concentration and the square root of voltage scan rate was obtained for all the cations studied. The diffusion coefficients of Ni(II), Co(II), Cd(II), and Pb(II) were calculated from the experimental data and found to be 1.26, 1.02, 0.98, and 1.29 x 10 -~ cm~/sec, respectively.The polarographic method has been extensively applied to a study of the solute species dissolved in molten salts. These studies have been carried out both at dropping metal electrodes (mercury, bismuth, lead) and solid (platinum, tungsten, graphite) microelectrodes. The subject has been thoroughly covered in a number of reviews (1-4). At elevated temperatures, the polarographic method suffers from the use of large polarization times so that it is extremely difficult to completely eliminate the convective transport of the electroactive species to the electrode surface. Further, the metal ions which result in the deposition of a solid metal at the solid microelectrodes show an "increasing limiting current phenomenon" (1, 2, 5, 6) due to an increase in the electrode area. These difficulties can be eliminated by using the techniques of chronopotentiometry or linear sweep voltammetry (oscillographic polarography) where polarization times of only a few seconds or less are employed. The chronopotentiometric method has been widely used (1, 2) in molten salts in the past whereas the technique of linear sweep voltammetry has attracted attention only recently.Mamantov et al. employed this technique to study the voltammetry of several cations in molten fluorides (7-12) at platinum wire, pyrolytic graphite, or glassy carbon electrodes and in molten NaNO~-KNO3 eutectic (13) at platinum wire electrodes. Francini et al. used the dropping mercury electrode to study the voltammetric behavior of several cations (14, 15), halide ions (16) and oxygen and its ions (17) in low melting nitrates. The reduction of Ag(I) at platinum wire electrodes was studied by Hills, Oxley, and Turner (18) in molten LiC1-KC1 eutectic and by Kawamura (19) in mixtures of molten NaNOa-KNO3. Behl and Gaur (20) studied the reduction of Ag(I) and Pb(II) at platinum wire electrodes in molten NaNO3-Ba(NO3)2 eutectic. Naryshkin and co-workers studied the reduction of several cations at platinum microelectrodes and determined their diffusion coefficients in molten NaC1-KCI (21) and LiC1-KC1 (22) eutectic. They also used this method to study (23) the temperature dependence of diffusion coefficients in the latter solvent.It was the purpose of this study to examine the applicability of the theory (24, 25) of linear sweep voltammetry to the reduction of Ni(II), Co(II), Cd(II), and Pb(II) at glassy carbon electrodes in molten * Electrochemical Society Active Member. Key word...

Section: Ep--ep~contrasting

confidence: 86%

Linear Sweep Voltammetry of Ni(II), Co(II), Cd(II), and Pb(II) at Glassy Carbon Electrodes in Molten Lithium Chloride-Potassium Chloride Eutectic

Behl

1971

“…These values are in good agreement with the literature data. 3,9 The obtained results confirm that the reduction of Pb 2+ ions on the Mo substrate in the LiCl-KCl-PbCl 2 melt over the temperature range of 723-823 K is a one-stage diffusion-controlled reaction. …”

Section: Resultssupporting

confidence: 66%

“…Dependence of i d on the molar fraction of PbCl 2 and PbO difference in a LiCl-KCl-PbCl 2 -PbO melt at 773 K. complex compound: N Pb 2 OCl 2 = N PbO [9] The concentration of electroactive Pb 2+ particles that participate in the electrode process is equal to: [10] where N 0 PbCl2 -the initial molar fraction of PbCl 2 . The molar fraction was then recalculated to the molar concentration according to the equation:…”

Section: Discussionmentioning

confidence: 99%

The Electrodeposition of Lead in LiCl-KCl-PbCl₂and LiCl-KCl-PbCl₂-PbO Melts

Pershin

Khalimullina

Архипов

et al. 2014

The mechanism for the electrode process that occurs on a molybdenum substrate in LiCl-KCl-PbCl 2 and LiCl-KCl-PbCl 2 -PbO melts depending on the concentration of PbO was investigated over the temperature range of 723-823 K using stationary polarization curves and voltammetry techniques. An increase in the concentration of PbO was found to decrease the diffusion-limiting current density for the recovery of lead, which was identified by the appearance of an additional peak in the voltammograms; it can be explained by a decrease in the concentration of Pb 2+ ions decrease due to the formation of Pb 2 O 2+ complex ions in the electrolyte. The evaluation of the equilibrium constant K E and the rate constant of the reaction suggests that this reaction is strongly shifted toward the formation of the Pb 2 O 2+ complex ions. A theoretical polarization curve for the diffusion kinetics was calculated.Investigating the kinetics of electrode processes allows one to determine the kinetic parameters of reactions, to calculate the activation energies of these reactions and to establish the limiting stages of electrode processes. 1 Additionally, data related to the mechanism of the electrode processes is valuable for developing the scientific basis for electrochemical technologies that meet the current ecological and energy saving requirements.In the literature, considerable attention is focused on the electrorecovery of lead from molten salts. The authors 2,3 reported that the electro recovery of Pb 2+ ions from a LiCl-KCl melt is a diffusioncontrolled, single-stage reaction.Haaberg et al. 4 investigated the lead electrodeposition process in LiCl-KCl melts and in pure PbCl 2 and determined that the migration of Pb 2+ ions controls the rate of the cathode process in the pure PbCl 2 melt at temperatures from 823 to 858 K. A diffusion regime of the process was observed in the LiCl-KCl melt; the diffusion coefficient for the Pb 2+ ions was equal to 2 · 10 −5 sm 2 /s. The cathode behavior of lead ions on a molybdenum substrate in LiCl-KCl-PbCl 2 and LiCl-KCl-PbCl 2 -MgCl 2 melts was studied using voltammetry and chronopotentiometry techniques in later works. 5 The extraction of lithium from the initially deposited lead was found to result in the formation of a Li-Pb alloy in LiCl-KCl-PbCl 2 and LiPb-Mg melts with the addition of MgCl 2 . The diffusion coefficient of Pb 2+ ions was determined using various techniques, and its value was found to be 2.26 · 10 −5 sm 2 /s. In a number of studies, there are considerable differences regarding the mechanism of the cathode process. Therefore, Strenberg et al. 6 established a two-stage mechanism for the recovery of Pb 2+ ions through the stage of the formation of monovalent Pb + ions.These works were primarily performed during the second half of the XX th century and were related to dilute solutions of lead in alkali metal chlorides, free of oxide ions. There is no information available regarding the mechanisms of electrode processes in oxide-chloride melts. The structures and compositions of ...

“…It is clear that further measurements, involving other metals, would be useful, and since this work was completed there have been further reports (18,19) of the successful use of liquid electrodes in melts. It is hoped that these electrodes will be used to extend the range of capacity data in molten salt systems and to confirm the tentative values shown in Fig.…”

Section: Ir~teryacial Capacitiesmentioning

confidence: 95%

Impedance Phenomena in Molten Salts

Hills¹,

Johnson

1961

Impedances have been measured of polarized platinum micro-electrodes in the molten salt systems, LiC1-KC1 at 400~ NaNO~-KNO. at 270~ NaBr-A1Br8 at 225~ and PbCL at 530~ Frequency dispersion of both the observed resistance and capacitance was evident in all cases, but apparent double layer capacities obtained by extrapolation to infinite frequency are in reasonable accord with some previously reported data. They are of magnitudes consistent with a simple Helmholtz model although the anodic values tend to be high, perhaps because of specific adsorption. Some measurements of faradaic impedance have also been made, but these were less successful and the exchange current for the Pt/Pt ~ system is high.Metal~melt interfaces are met with in four connections:(i) undergoing steady cathodic electrolysis either as part of an electroanalytical study or as part of an electrowinning process;(ii) at equilibrium in a melt which may or may not contain a significant concentration of the metal ions, e.g., at the internal surface of a metal vessel containing molten salts;(iii) approximately at equilibrium but subject to a small applied voltage changing periodically with time; and (iv) undergoing steady anodic electrolysis where in many cases the metal dissolves smoothly to form ions of its lowest stable valence state.In each case, the composition of the metal-melt interface and the degree of reactivity of the metal should be explicable in terms of the kinetic parameters (particularly exchange currents) of the various feasible electrode reactions. The structure of the melt in contact with the metal is an important related property of such systems, and some conclusions with regard to this may be drawn from values of double layer capacities at metal-melt interfaces.Until recently, only the a-c impedance method has been applied to the measurement of exchange currents and double layer capacities in melts. Double layer capacities of 15-20 ~F cm -~ were observed at carbon electrodes in cryolite-alumina melts (1), and impedance measurements at carbon electrodes in halide melts were interpreted in terms of a slow heterogeneous anode reaction such as C1 + C1 -* C12 (2).Randles and White (3) found that the capacitypotential curves for mercury in molten alkali nitrates, perchlorates, and bisulfates were shallow parabolas with minimum values of ,~20 #F cm -~. In the case of nitrates, a very high concentration of water increased the capacity and produced some semblance of the "hump" found in the curves for aqueous solutions. The rate constant for the discharge of Ni ~ from nitrates was obtained by a straightforward application of the faradaic impedance theory.Hill (4) measured the impedance of Ag in dilute solutions of AgNO3 in molten LiNO3-NaNO~-KNO~, of Ag in solutions of AgC1 in LiC1-KC1, of W in LiC1-KC1 and W in solutions of TiCL and TiCI~ in LiC1-KC1. In all cases the residual capacity and resistance of the solvent were markedly frequency dependent, the double layer capacities obtained by extrapolation to infinite frequency being 300, 500, ...