Showing papers in "Russian Journal of Electrochemistry in 1996"

Physicochemical principles of testing ion-exchange membranes

Abstract: The results of investigations into concentration dependences of a wide number of transport characteristics pertinent to ion-exchange membranes of different production are generalized. A series of experimental procedures for measuring electrotransport characteristics of membrane materials under comparable experimental conditions is suggested. The nature of concentration dependences of electroconductivity, diffusive and electroosmotic permeability, and potentiometric transport numbers in a wide range of concentrations of equilibrium NaCl solutions is discussed. It is suggested that three concentration zones, which correspond to certain physicochemical conditions of the NaCl solution and membrane, be singled out in order to characterize changes in the electrotransport properties. All the membrane properties studied are found to be controlled by the content and state of water in swollen samples that represent phase-separated conducting systems. Parameters of the two-phase model, which allow one to elucidate the reasons for distinctions in electrotransport of ions and water in ion-exchange membranes, are reported and compared. The research performed lays a physicochemical foundation for testing membranes with the aid of the approaches that are described in the paper

Formation of Ozone and Hydrogen Peroxide during an Electrical Discharge in the Solution-Gas System

Abstract: Hydrogen peroxide formation is studied in an electrodeless electrochemical reaction, which occurs in a liquid during an interaction between water dissociation products and active species formed by an electrical discharge in the electrode/liquid gap. Data are obtained for the corana, spark, and arc discharge (at 0.05-0.1, 0.5, and >2 mA, respectively), separately for positive and negative voltage across the electrode placed over the liquid surface. Measurements are taken in oxygen, helium, and hydrogen environments, at atmospheric pressure and room temperature. Yield of ozone is determined under the same conditions of the electrical discharge between the electrode and the liquid surface. Hydrogen peroxide formation is shown to essential depend on the electrical discharge regime, gas composition in the discharge gap, and the solution acidity. The ozone yield depends solely on the gas composition and the voltage polarity across the discharge gap

Model for the competitive transport of ions through ion exchange membranes with a modified surface

Abstract: A mathematical model for the membrane with a modified surface is considered. The proposed model is based on the Nernst-Planck equations. Surface modification is taken into account by introducing phenomenogical conductivity coefficients. The competitive transport of two counterions is studied. It is shown that the modifying layer affects significantly the permselectivity at low current densities and does not affect the membrane permselectivity at the limiting current. These regularities result in the appearance of an extremum in the dependence of the effective transport numbers on the current density

Electrodialysis of dilute electrolyte solutions : Some theoretical and applied aspects

Abstract: Conventional techniques for enhancing electrodialysis efficiency of dilute solutions are considered. The techniques in question involve inserting separators of different designs in the membrane channels and reducing the intermembrane distance. The possibilities for utilizing conjugated side effects, which accompany electrodifusive transport of salt ions in membrane systems at currents exceeding to overlimiting values, are discussed. Examples of practical electrodialysis application for producing deionized water are reported

The structure of the electrical double layer at the faces of bismuth, antimony, or cadmium single crystals in surface-inactive electrolytes

Abstract: Previously published experimental data on the capacitance of electrical double layer at the faces of bismuth, antimony, or cadmium single crystals in solutions of surface-inactive electrolytes in water, acetonitrile, methanol, and ethanol are analyzed in terms of various models. Potential drops in the near-surface metal layer and contributions, C M , of the capacitance of the metallic phase for the individual faces of Sb, Cd, or Bi single crystals are determined. It is established that the C M values, as determined in terms of both the Leiva-Schmickler-Henderson model and the Amokrane-Badiali model, depend on the solvent nature . It is found that the two models can be applied to describing the regularities that control the structure of the electrical double layer at the faces of Sb, Cd, or Bi single crystals only to the first, rather rough approximation

The underpotential deposition of zinc on iron from an alkali zincate electrolyte in the presence of an organic additive

Abstract: Formation of zinc adatoms on iron at potentials more positive than the equilibrium potential of zinc is studied by stripping voltammetry. Zinc adatoms are shown to deposit on an iron substrate from a 2.5 M KOH+ 0.012 M Zn(II) solution with formation of solid substitutional solution Zn-Fe. Introducing water-soluble rod-like polyheteroarylene based on dianhydride of 1,4,5,8-naphthalenetetracarboxylic acid and 2,2'-disulfobenzidine inhibits the process of zinc adatom formation

Properties of niobium coatings electrodeposited from fluoride melts

Abstract: Properties of niobium coatings, produced by electrolyzing the (Li-Na-K)F eut -K 2 NbF 7 melt, are studied as a function of the deposition conditions. The coatings are shown to have high uniformity at thicknesses exceeding 5 μm. The relation between roughness parameters and the conditions under which the coatings are produced is studied. The electrolysis conditions are found to affect mechanical and electrical characteristics of the coatings

Analysis of electrodiffusion equations in the decomposition form

Abstract: Decomposition of unidimensional steady-state Nernst-Planck and Poisson equations is considered. Order of terms in decomposition equations for the electrical-field strength over the entire diffusion layer at sublimiting and overlimiting currents is estimated. The exception is the boundary-layer region near the membrane surface, where the asymptotic method of boundary-layer functions can be used. The electroneutrality condition is analyzed, and the condition of the quasi-uniform distribution of the charge density (QDC), which generalizes the electroneutrality condition, is discussed. An algorithm for numerical solution of decomposition equations is proposed

The influence of the metal lyophilicity on the structure of the electrical double layer in the solvents with a large dielectric constant : The structure of the electrical double layer on the gallium metals in N-methylformamide

Abstract: The structure of the electrical double layer at La,In-Ga, and Tl-Ga in N-methylformamide (N-MF) in a solution of the same concentration of a surface-inactive electrolyte(NaClO 4 ) is found to differ from that at Hg and to essentially depend on the electrode nature. This phenomenon points to the specific interaction of N-MF with metals, which rises in the series Hg< Tl-Ga

Electroreduction of boron in chloride-fluoride melts

Abstract: Electroreduction of the BF 4 - complexes is studied on the silver and glassy-carbon electrodes in the NaCl-KCl and NaCl-KCl-NaF melts. The boron complexes are shown to reduce in these melts via a single, irreversible stage involving three electrons. The process of cathodic boron incorporation in the glassy-carbon electrode is found to be accompanied by formation by formation of the B 4 C compound. The transfer and diffusion coefficients for boron complexes are measured. The diffusion coefficients obtained when using the silver and glassy-carbon electrodes are shown to be close, because of an insignificant contribution made by the discharge of boron complexes into the overall process

Electrochemical characterization of porous membranes with adsorbed charges from convective diffusion and counter-current electrolysis experiments

Abstract: Experimental methods for the electrochemical characterization of porous membranes with adsorbed charges are described. Convective diffusion and counter-current electrolysis experiments permit the determination of the membrane constant (effective membrane aera/membrane thickness) and the ion transport numbers, respectively. The first one is a key parameter in electrokinetic experiments, while the latter provides information on the membrane selectivity. A simple, theoretical model based on the Nernst-Planck equation and the Langmuir isotherm can used to describe the coupling between ion adsorption and transport

On a generalization of the electroneutrality condition

Abstract: The condition of the quasi-uniform distribution of the space-charge density (The QDC condition), which generalizes the electroneutrality condition, is suggested. It is shown that hte use of the QDC condition, in conjunction with the system of equations consisting of the Nernst-Planck and Poisson equations, allows one to find approximately the distribution of concentrations of cations and anions of a binary electrolyte, as well as the electrical-field strengths both at sublimiting and overlimiting current densities. The concentration distributions obtained coincide with the corresponding exact functions found by means of the Poisson equation over the entire diffusion layer except for a thin, quasi-equilibrium boundary layer with a thickness of the order of the Debye length

Electrodialysis of dilute solutions : The influence of pH on the transport of salt ions

Abstract: The solution pH in the desalination channel of an electrodialysis cell is shown to depend on the nature of membrane system's degree of polarization, and the ratio between the aeras of cation-exchange and the anion-exchange surfaces. A qualitative correspondence is revealed between the exerimental data and the equation derived from the system of the Nernst-Planck-Poisson equations, taking into account the water dissociation at the membrane-solution interface. The appearance of H + or OH - ions in the solution being desalinated is shown to result in decelerated mass transfer of salt ions of the like charge acros the corresponding membranes. This is caused not only by the competitive transport of salt ions and products of water dissociation, but to the weakening of the exaltation effect of the limiting current as well

The process of electroreduction of the niobium chloride complexes in the NaCl-KCl melt

Abstract: Electroreduction of the niobium chloride complexes in the NaCl-KCl melt is studied. Diffusion coefficients for the Nb(IV) and Nb(V) complexes and standard rate constants of charge transfer for the reduction of Nb(V) to Nb(IV) are determined. The electrochemical behavior of Nb 3 Cl 8 in an equimolar mixture of sodium and potassium chlorides is discussed

The mechanism of electrochemical activation of the C-H bond during reduction of organic compounds in the presence of aromatic N-oxides as the intermediates

Abstract: Rate constants for the C-H bond activation during oxidation of cyclohexanol, tetrahydrofuran, and diethyl ether in the presence of phenazine N,N'-dioxide and 2,3,5,6-tetramethyl-pyrazine-N,N'-dixoxide are determined by cyclic voltammetry at the glassy-carbon electrode in a 0.1 N LiClO 4 solution in acetonitrile and by a numerical calculation of the first electrode stage. A novel phenomenon, which involves catalysis of the two-stage electrode process, is discovered. The catalysis is characterized by a catalytic current observed at the second electrode stage. A net two-electron mechanism is suggested for oxidation of organic substance during a catalytic process, in a complex with the radical cation of the intermediate-an aromatic N-oxide

Simulation of electrochemical problems by numerical integration

Abstract: A numerical integration method for the simulation of redox reactions complicated by the reactant and product adsorption at the surface of a dropping mercury electrode and a hanging, or a static mercury drop electrode is described

The use of flicker-noise spectroscopy for studying the mechanism of the overlimiting current in a system with cation-exchange membrane

Abstract: Spectral analysis of flow-frequency fluctuations of the membrane potential, which occur when electrical current passes through a polyamide cation-exchange membrane placed in a NaCl solution, is employed for studying the mechanism of ionic transport at overlimiting current densities. Spectra of noise density of the membrane potential, S v , as a function of the concentration and viscosity of electrolyte as well as their dependence on the current density are investigated in the frequency region 0.002-500Hz. Fluctuations of the membrane potential are shown to become discernible when the current density j is approaching the limiting value j lim . When j is approximately equal to j lim , the fluctuation spectra include reproducible, discrete lines at frequencies 20-250 Hz. At jj lim the S v function becomes smooth and corresponds to the flicker noise. Parameters of the S v spectra depend on the electrolyte viscosity. The spectral regularities observed are interpreted in terms of the model that suggests the appearance of hydrodynamic instabilities in the δ N -Nernst layer, at the membrane boundary, at j≥j lim . The mechanism leading to turbulent pulsations and the role played by turbulence in the process of ionic transport at overlimiting currents are discussed

Electrochemical synthesis employing sacrificial anodes

Abstract: Potentialities of the preparative electrolysis in a diphragmless electrolyzer equipped with a sacrificial anode are considered. The feasibility of synthesizing organometallic compounds, alcoholates, and transition-metal complexes is stated. The pratical implementation of electrochemical synthesis in nonaqueous media is considerably expanded. The processes that employ sacrificial anodes for electrochemical carboxylation, direct- and cross-dimerization of organic halides, the synthesis of alcohols by coupling halides with aldehydes and ketones, and the synthesis of ketones from halohydrides of carboxylic acids are sucessfully realized. The processes resulting in the formation of compounds with carbon-phosphorus, carbon-silicon, and carbon-sulfur bonds are described. Carbocyclic compounds have been synthesized by coupling dihalides with activated olefins. The data are given pertaining to conducting the synthesis of organic substances on an expanded scale

Diffusion of hydrogen and deuterium atoms in water

Abstract: Diffusion coefficients for hydrogen and deuterium atoms in aqueous electrolyte solutions are measured using the laser photoemission from metal into solution. Corrected for the solution ionic strength, the coefficients found are as follows: D H =(7.7±1.0)×10 -5 cm 2 S -1 at 25°C in H 2 O and D 2 O, respectively. The activation energy is equal to 12±1.5 kJ mol -1 . On the basis of comparing these diffusion coefficients with those for muonium, H 2 , and the noble gases, a conclusion is drawn that, similar to the helium atom, the hydrogen and deuterium atoms generate around themselves clathrate structures of type I (the channel diameter of about 0.54 nm), whereas atoms and molecules with the larger Van der Waals radius are localized in cavities of type II, with larger diameters. The neon atom and hydrogen molecule occupy an intermediate position. The anomalously high values of the diffusion coefficients (≥7cm 2 s -1 ), which are typical for light impurities, are attribuable to jumps between neighboring cavities of type I, caused by thermal fluctuations. The considerable isotopic effect, D H (H 2 O)/D D (D 2 O), is assigned to the participation of isotope-sensitive libration motion of water in the thermoactivated reorganization of the cavity

The electroreduction mechanism of copper(II) citrate complexes

Abstract: The mechanism involved in electroreduction of citrate complexes of copper(II) is analyzed in the pH interval from 3.5 to 11 using the technique of isopotential solutions and polarization resistances determined under forced convection. In acidic media, the CuLH - species is the prevailing electroactive complex; in alkaline media, it is the CuL 2- complex that predominantly discharges

Kinetic and thermodynamic aspects of equilibriums in low-temperature gas sensors

Abstract: The operation principles of low-temperature, potentiometric gas sensors based on solid electrolytes are considered, along with the basic detection processes. The shape of the adsorption isotherm of the component being analyzed at the working electrode is shown to be related to the shape of the concentration dependence of emf generated by potentiometric sensors. The reasons for the deviations of sensor emf observed experimentally from the theoretical values are discussed. The conditions pertinent to the formation and removal of products of electrochemical processes from triple-phase boundaries solid electrolyte/ working electrode/ gas are determined

The binary mediator system consisting of organic electron carrier and metal ions in the reactions of electrochemical reduction of bromo- and chloroorganic compounds

Abstract: Electrochemical reduction of organohalogen compounds involving a double mediator system of metal ions and organic electron carrier is studied. The catlytic action of Ni(II), Co(II), Pd(II), Pt(II), and Rh(III) ions on the kinetics of homogenous reduction of aliphatic, alicyclic, and aromatic bromo- and chloro- organic compounds by radical-anions of organic compounds in aprotic media containing tetraalkylammonium salts as supporting electrolytes is found. In all cases, the is a synergistic effect increasing with the increase in concentration of metal ions and with the decrease in the efficiency of organic carrier. By the example of binary mediator reduction involving Ni(II) and Co(II) ions, the reaction is shown to be of the second order by metal ions

The electrolyte ionic composition and formation of vanadium-containing anodic films

Abstract: The effect of the polyphosphate concentration, time, and electricity passed through the solution on the ionic composition of aqueous solutions of Na 6 P 6 O 18 and NaVO 3 .2H 2 O is studied by 31 P NMR. It is shown that formation of vanadium-containing anodic films under conditions of microplasma discharges is connected with the presence of heteropolyoxo anions [PV 14 O 42 ] 9- in the phosphate-vanadate electrolyte

Electrochemical reduction of oximes : the mechanism of the process

Abstract: The effect of the structure of aromatic and aliphatic oximes on the mechanism of their electrochemical reduction in aqueous media is considered

Electrochemical synthesis of anodic films, containing divalent cations, on aluminum and titanium

Abstract: For aqueous electrolytes based on polyphosphate and salts of Mg 2+ , Ca 2+ , Ba 2+ , Cd 2+ , Zn 2+ , Ni 2+ , and Cu 2+ , it was shown that films containing these cations may be synthsized on valve metals (Al, Ti) under the conditions of microplasma anodizing. Element composition of the surface structures produced was studied. It was concluded that the processes of complex formation in aqueous solutions show promise for developing the electrolytes, which allow one to produce anodic films of complex chemical composition

The numerical modeling of the Ag+ and I- ion hydration in a wide temperature interval

Abstract: The process of water nucleation on single-charged ions of silver and iodine is modeled by the Monte Carlo method. The molecule cluster formed is in thermal and material contact with the vapor at a specified temperature and pressure. The 52 numerical runs performed span the temperature region 200

Cathodic processes in the electrosynthesis of tantalum borides

Abstract: The mechanism of cathodic processes involved in the electrochemical synthesis of tantalum borides in molten NaCl-KCl and NaCl-KCl-NaF mixtures containing potassium heptafluorotantalate and potassium tetrafluoroborate as electroactive components is investigated by linear voltammetry. Tantalum borides are found to form at the cathode during discharge of heteronuclear complexes