Polarography of some anions in aqueous organic solvent mixtures

The present investigation deals with the polarographic behaviour of iodate ions in various aqueous mixtures of methanol, ethanol, n-propanol, iso-propanol, 1,2-propanediol, 1,3-propanediol, and glycerol at 30°C. 0.1M-NaOH was used as a base electrolyte. The half-wave potential ranges from 1.24 to 1.46 and the diffusion current decreases with increasing percentage of the solvent. The plots ofi _d vs. andi _d vs.c were linear and pass through origin for all the solvent mixtures, showing that the limiting current is diffusion controlled. The plots of logi/(i_d–i) vs.E _d.e. were linear for all the aqueous organic solvent mixtures, but the value of slope suggested that the reductions are irreversible. The electrode kinetics was studied byKoutecky's method for irreversible electrode process.     

Electrochemical Behavior of Electrode System (H2 + H2O + Ar), Pt/BaCe0.9Nd0.1O3–α Near Its Equilibrium Potential

The polarization conductance of the (H₂ + H₂O + Ar), Pt/BaCe_0.9Nd_0.1O_3– electrode system is studied at different partial pressures of hydrogen and water, temperatures, exposure times, and electrode structures. The polarization conductance is proportional to , exponent p depending on the cell exposure time in experimental conditions. The cell exposure lowers the electrode electrochemical activity, increases electrolyte resistance, and decreases effect of on the polarization conductance and electrolyte conductance. To a first approximation the polarization conductance is proportional to the neodymium concentration in the BaCe_{1 – x} Nd _x O_{3 –} phase (0.01 x 0.1). Possible routes of the electrode reactions are discussed. Rate-determining stages in parallel reactions of hydrogen ionization and oxygen ion discharge in the system are suggested.     

Electrodeposition of Lu on W and Al electrodes: Electrochemical formation of Lu–Al alloys and oxoacidity reactions of Lu(III) in the eutectic LiCl–KCl

The electrodeposition of lutetium on inert electrodes and the formation of lutetium–aluminium alloys were investigated in the eutectic LiCl–KCl in the temperature range 673–823 K. On a tungsten electrode, the electroreduction of Lu(III) proceeds in a single step and electrocrystalization plays an important role. Experimental current–time transients are in good agreement with theoretical models based on either instantaneous or progressive nucleation with three dimensional growth of the nuclei, depending on the working temperature. The diffusion coefficient of Lu(III) was determined by chronopotentiometry by applying the Sand equation. The activation energy for diffusion was found to be 31.5 ± 1.3 kJ mol⁻¹. Al₃Lu and mixtures of Al₃Lu and Al₂Lu, characterized by XRD analysis and SEM, were obtained from the LiCl–KCl melt containing Lu(III) by potentiostatic electrolysis using an Al electrode. The activity of Lu and the standard Gibbs energies of formation for Al₃Lu were estimated from open-circuit chronopotentiometric measurements. The E–pO²⁻(potential–oxoacidity) diagram for Lu–O stable compounds in LiCl–KCl at 723 K has been constructed by combining theoretical and experimental data. In this way, the apparent standard potential for the Lu(III)/Lu system has been determined by potentiometry. Potentiometric titrations of Lu(III) solutions with oxide donors, using a yttria stabilized zirconia membrane electrode “YSZME” as a pO²⁻ indicator electrode, have shown the stability of LuOCl and Lu₂O₃ in the melt and their solubility products have been determined at 723 K.     

Electrochemical Properties of Electrode Materials of the Composition (Yb,Y)0.5Ca0.5MnO3

Electrochemical characteristics, such as the parameter of resistance (/d) of an intermediate layer at the electrode material/electrolyte interface and the polarization resistance (R ) of the three-phase boundary gas/solid-electrolyte/electrode under study, are studied as a function of the electrode layer thickness, temperature, and oxygen content in the gas phase for electrode materials of the composition (Yb, Y)_0.5Ca_0.5MnO₃. Established is the similarity in the behavior of /d and R from the above parameters. The determined effect on these characteristics is exerted by the composition of the electrode material. In the temperature range 300–1100 K, at = 10²–10⁵ Pa, the least values of the parameter of resistance and polarization resistance are intrinsic to samples with the thickness of the electrode layer 100 mg cm^–2 of the composition Y_0.5Ca_0.5MnO₃. For systems studied, the least temperature of establishment of electrochemical equilibrium at the three-phase boundary is 875 (±5) 5 K.     

Taylor dispersion measurements at low electrolyte concentrations. I. Tetraalkylammonium perchlorate aqueous solutions

An equipment for the determination of mutual diffusion coefficients using the Taylor's dispersion technique is described. The radius of the capillary was determined with the help of various calibration methods. Diffusion coefficients of aqueous tetraalkylammonium perchlorates, Me₄NClO₄, and Et₄NClO₄, were measured at 25°C in the concentration range 10^–3 to 5×10^–2 mol-dm^–3, and the slightly soluble Pr₄NClO₄ up to 1×10^–2 mol-dm^–3. The slope of linear plots ofD vs. is in agreement with theory, in contrast to the limiting valuesD ₀, which all deviate by about –5% from the Nernst-Hartley values.     

Nonlinear Impedance of Liquid In–Ga Electrode in Aqueous Solutions of a Symmetrical Surface-Inactive Electrolyte

Experimental data related to the potential dependence of nonlinear characteristics of the electrical double layer at a liquid In–Ga electrode in aqueous solutions of a symmetrical surface-inactive electrolyte are obtained for the first time. It is shown that, as opposed to polycrystalline Cd and Pb electrodes, on a liquid (atomically smooth) In–Ga electrode, as on Hg, there is a clear intersection of the potential dependences of a nonlinear signal for different concentrations of a 1–1-valence surface-inactive electrolyte at one point. The intersection point exactly corresponds to the potential of zero charge of an electrode undistorted by specific adsorption of ions. It is established that, when estimating hydrophilicity of metals by a nonlinear impedance method, most information is provided by the region of average negative charges, rather than by the region near zero charge. It is shown that, as opposed to a linear impedance method, the nonlinear impedance method makes it possible to determine, directly from experiment, quantities that directly characterize the metal–solvent chemisorption interaction in a pure form; at the same time, these quantities are criteria of lyophilic nature of metals. Quantities that characterize the metal–solvent chemisorption interaction, obtained by the linear and nonlinear impedance methods are in good agreement, which confirms the validity of the approach we proposed earlier for separating the difference between reciprocal capacitances of the inner part of the electrical double layer on Hg and metal M, ^Hg _M C ^-1 _i= 1/C ^Hg _i– 1/C ^M _iinto physical (^Hg _M C ^–1)_physand chemical constituents. This coincidence also confirms correctness of numerical values obtained earlier for quantities (^Hg _M C ^–1)_phys.     

Electrochemical and kinetic behavior of the MnO<Subscript><Emphasis Type="Italic">X</Emphasis></Subscript>/C–β-PbO<Subscript>2</Subscript> electrode

The MnO_X/C–-PbO₂ electrode was obtained by admixture of -PbO₂ to MnO_X/C. Slow scan voltammetry shows a double reduction peak which is discussed. Galvanostatic intermittent titration technique curves are also used and the value of the proton diffusion coefficient, D(H⁺), is determined. All studies are made in 2 M KOH.Presented at the 3rd International Meeting on Advanced Batteries and Accumulators, 16–20 June 2002, Brno, Czech Republic     

Electroconductivity,Nature of Conduction,Thermodynamic Stability of the BaPr1 –xYxO3 – αCeramics

The electroconductivity and the nature of conduction of vacuum-dense ceramics BaPr_{1 – x} Y _x O_{3 –} (x= 0.05–0.15) is studied at temperatures of 373 to 985°C, of 2.1 × 10⁴to 10^–11Pa, and of 40 to 2400 Pa. The coefficient of linear thermal expansion is measured. The ceramics have a perovskite structure and are practically p-type semiconductors with a maximum conductivity of 0.26 S cm^–1at x= 0.10 and 800°C, in air. The share of ionic (proton) conductivity of the ceramics does not exceed 0.2–0.4%. The conductivity is weakly dependent on the air humidity. In a hydrogen-containing atmosphere, the ceramics undergoes reduction with destruction. Boundaries of thermodynamic stability of BaPr_0.9Y_0.1O_{3 –} at 500–900°C are determined.     

Determination of medecamycin by adsorptive stripping voltammetry with an activated electrode

The adsorptive and electrochemical behaviors of medecamycin were investigated on a glassy carbon electrode (GCE) pretreated by anodic oxidation at +1.8 V for 5 min in 0.025 mol l^–1 NH₃-NH₄Cl (pH 8.6) solution. An adsorptive stripping voltammetric method for the determination of medecamycin at the pretreated glassy carbon electrode has been developed. Medecamycin was accumulated in NH₃-NH₄Cl buffer (pH 9.0) at a potential of –0.7 V (vs. saturated calomel electrode (SCE)) for a certain time, and then determined by second-order differential anodic stripping voltammetry. The second-order differential anodic stripping peak current at +0.72 V was proportional to the concentration of medecamycin in the range 2.0 g ml^–1 to 50.0 g ml^–1. The detection limit (three times the signal-to-noise) was 1.0 g ml^–1 and the relative standard deviation of the results was 3.28% for eight successive determinations of 10.0 g ml^–1 medecamycin. This method has been applied to the direct determination of medecamycin in commercial tablets and spiked urine samples with satisfactory results.     

Linear scan stripping voltammetry of copper(II) at the chemically modified carbon paste electrode

A new chemically modified electrode (CME), -benzoinoxime (CUPRON) modified carbon paste electrode, for determining copper(II) is reported because of its excellent selectivity and sensitivity. The electrode is made by mixing a quantity of CUPRON (25%, w/w) with graphite powder (50%, w/w) and paraffin oil (25%, w/w). The CME preferentially deposits copper from the pH 8.5 NH₃-NH₄Cl buffer solution containing copper(II) under an open circuit and most of metal ions do not interfere with the measurements. The detection limit (S/N of three) for determining Cu(II) is 3 × 10^–10 g/ml after 10 min accumulation in fast linear scan stripping voltammetric measurement. Linear calibration curves are obtained for Cu(II) concentration ranged from 1 × 10^–8 M to 1 × 10^–6 M. The response can be maintained with relative standard deviation of 6.0% in a 5 × 10^–6 M Cu(II) solution after eight accumulation/measurement/ regeneration cycles at the same electrode surface. The effect resulted from carbon paste preparation, reduction potential, electrode renewal, electrolyte and solution pH, preconcentration time, concentration dependence, possible interference and other variables has been evaluated. As for application, the CME demonstrates its high sensitivity and copper-selectivity in complex composition samples, such as anodic mud and polluted water.     

An estimation of the surface ionization constant of oleic acid in aqueous sodium chloride solution

The zeta potential () measurements and the site binding theory were utilized for calculations of the parameters of the electrical double layer (edl), ionization, and complexation constants for oleic acid-aqueous sodium chloride solution interface. Assuming that is equal to the diffuse layer potential ( _d ) of the edl, the charge of the diffuse part of the edl was calculated from the Gouy-Chapman equation. The intrinsic ionizaiton constant was then determined by an extrapolation method to be . Subsequently, the surface potential ( ₀) was calculated, and it was found that ₀ changes by 50 mV per pH unit (50 mV/pH) or 42.5 mV/pH for 10^–3 and 10^–2 M NaCl, respectively. For further calculations, the integral capacity of the outer zone of the compact part of the edl was assumed to be for both ionic strengths. It was established that the intrinsic complexation constant for the binding of Na⁺ ions with the surface of oleic acid ispK _Na ^int = 2.9±0.5 if the integral capacity of the inner zone of the compact edl (K ₁) is 80 for 10^–3 M NaCl, but 280 for 10^–2 M NaCl. The use of the sameK ₁ value for both ionic strengths gives a differentpK _Na ^int for different NaCl concentrations, and also provides unrealistic surface charge ( _o ) values greaterfor 10^–3 M NaCl than for 10^–2 M NaCl, at the same pH of the solution.     

Solid-Solute Phase Equilibria in Aqueous Solution. XI. Aqueous Solubility and Standard Gibbs Energy of Cadmium Carbonate

The solubilities of CdCO₃ (otavite) in aqueous NaClO₄ solutions have been investigated as a function of ionic strength (0.15 I/mol-kg^–1 5.35, 25°C) and temperature (25°C T 75°C, I = 1.00 mol-kg^–1). A new Chemsage optimization routine was employed to simultaneously evaluate solubility data from this work and other sources, as well as standard electrode potentials determined at different ionic strengths. With the Pitzer equations the solubility constants, , were extrapolated to infinite dilution resulting in log and the ternary ion-interaction parameters ^S_Na,Cd = 0.19 and at 25°C. In addition, the following set of thermodynamic quantities can be derived from the present solubility data for otavite: _f G = –674.2±0.6 kJ-mol^–1; _f H = –755.3±3.4 kJ-mol^–1; S = 93±10 J-mol^–1K^–1. However, the present solubility data are also consistent with a recent determination of the standard entropy of otavite which leads to a recommended set of thermodynamic quantities [_f G (CdCO₃) = –674.2±0.6; _f H (CdCO₃) = –752.1±0.6; S (CdCO₃) = 103.9±0.2].     

Diffusion–Migration Currents on Microelectrodes in the Absence of Supporting Electrolyte at an Arbitrary Charge of Ions

Limiting diffusion–migration currents on microelectrodes in systems comprising two types of electroactive ions and one type of inactive ions with arbitrary charges (z _j) and diffusion coefficients (D _j) are calculated. The electric field which induces the ion migration disappears when the condition D ₁₂/D ₂₁ = z ₁/z ₂ is fulfilled for two types of electroactive ions, where _j are their stoichiometric coefficients in the electrode reaction. For a reversible [Fe(CN)₆]^3–/4– system on microelectrodes, the migration effects are more pronounced as compared with the convective reactant delivery, which is explained by the different nature of the dependence of the limiting current on the diffusion coefficient.     

Mass transfer in shell-core inorganic TIP1 ion exchanger

The ion exchange processes of (OAc^–) and (OAc^–) proceeding in shell-core inorganic ion exchanger Ti (HPO₄)₂·1/2H₂O has been studied and the diffusion equation whose boundary conditions are satisfied by a shell-core model was solved. Based on the equation solved and experimental data, the diffusion coefficients corresponding to the exchange process (OAc^–) and Li⁺–H⁺ (OAc^–) at 17°C are found to be 7.7×10^–9 and 6.2×10^–8 cm² s^–1 and the activation energies 3.4×10⁴ and 5.0×10³ J mol^–1, respectively. Compared to the gel type of styrene-divinylbenzene strong acid exchanger with 20% cross linking, it can be concluded that the rate of or exchange is 3.5 times faster than that in the organic exchanger.TIP was obtained from the Salt Lake Institute of the Academy of Science of China.     

IR study of cation effects on the O-D stretching frequencies of isotopically dilute HDO in aqueous salt solutions

IR spectra of 3 normal solutions of 14 different salts [chlorides of Al⁺⁺⁺, Be⁺⁺, Mg⁺⁺, Ca⁺⁺, Sr⁺⁺, Ba⁺⁺, Zn⁺⁺, Cd⁺⁺, Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, N(CH₃) ₄ ⁺ ] in both, 96% H₂O+4% D₂O and 100% H₂O, were measured in the frequency range =2 800–2 100 cm^–1. From up to 18 single measurements for each solution the frequencies and halfwidths of the O-D stretching bands of isotopically dilute HDO were determined with high accuracy. Frequencies in the range =2 510–2 529 cm^–1 and halfwidths in the range =155–205 cm^–1 resulted atT=30°C with standard deviations typical less than ±1 cm^–1 and ±4 cm^–1, respectively. An almost perfect correlation between the O-D stretching band parameters and the polarizing power of the cations was obtained.Herrn Prof. Dr.A. Neckel, Wien, zum 60. Geburtstag gewidmet.     

Group additivity for the standard partial molal heat capacities and volumes of polar compounds in methanol at 25°C

A flow heat capcity calorimeter and a flow vibrating tube densimeter have been used to measure the apparent molal heat capacities and volumes of benzene and 25 polar compounds in methanol at 25°C. These quantities have been extrapolated to infinite dilution to obtain the standard partial molal heat capacities and volumes. The and data have been used in conjunction with an additivity scheme previously determined for alkanes. Group contributions were evaluatd for –OH, –NH₂, –COOH, –C₆H₅, C=O, –COO–, –CONH–, –O–, –S–, and –S₂–. The concentration dependences of _cp and _v of nonelectrolytes in methanol are qualitatively similar but much smaller than in water.     

Structural Study of Methyl Acrylate in a Nematic Liquid Crystal by 1H NMR with 13C Satellites

The molecular structure and conformation of methyl acrylate have been determined by analyzing the ¹H NMR spectrum with ¹³C satellites of the molecules dissolved in the liquid crystal ZLI 1132. In the structural analysis, direct coupling constants have been corrected for molecular vibrations and converted to those in the r structure. The correlation between reorientational motion and internal rotation has been taken into account by using the mean external potential described in terms of bond interaction tensors. The potential for the internal rotation about the bond has been represented as V() = (V ₁/2) (1 – cos ) + (V ₂/2)(1 – cos 2), where is zero for the s-cis configuration, and the determined values of V ₁ and V ₂ are 2.7(1) and 8.4(13) kJ mol^–1, respectively. The relative abundance of the s-cis conformer, 76(1)%, agrees with a gas-phase value, 67(11)%, determined by electron diffraction within experimental errors.     

Mass,size and shape of micelles formed in aqueous solutions of ethoxylated nonyl-phenols

The study was extended to analysis of mass, size and conformation of micelles formed in aqueous solutions of ethoxylated nonyl phenols. The results obtained by ultracentrifugal technique between 293 and 323 K have proved that the slightly ethoxylated nonyl phenols form micelles with high molecular mass and larger size at constant temperature, while the increasing length of the ethylene oxide chain favours formation of micelles of smaller molecular mass and size. The transformation of conformation from oblate to spherical shapes ensues with increasing temperature at constant ethoxy number or with ethoxylation at constant temperature. The second virial coefficient decreases with increasing temperature and decreasing ethoxy number. In accordance with the earlier conclucions, the change of the second virial coefficient relates to enhanced variation of monomer solubility, stabilization of micelle structure and increased deviation from ideal behaviour of a given micellar system.Symbols a major axis of micelle, Å - a _m attractivity factor, cm³ erg molecule² - b minor axis of micelle, Å - c concentration, g dm^–3 - c _b equilibrium concentration at the bottom of the cell, g dm^–3 - c _m equilibrium concentration at the meniscus of the cell, g dm^–3 - c _o initial concentration in the cell, g dm^–3 - c _M critical micellization concentration, mol dm^–3 - e eccentricity - f _IS Isihara-constant - f/f _o frictional ratio of micelle - amount of water in micelle per ethoxy group, mol H₂O/mol EO - n aggregation number, monomer micelle^–1 - n _EO number of ethoxy groups - r distance of Schlieren peak from the axis, cm - r _b distance of cell bottom from the axis, cm - r _m distance of cell meniscus from the axis, cm - R _h equivalent hydrodynamic radius of micelle, Å - s _t sedimentation coefficient, s - reduced sedimentation coefficient, s - reduced limiting sedimentation coefficient, s - ¯v _t volume of micelle, cm³ micelle^–1 - partial specific volume of solute, cm³g^–1 - partial specific volume of solute reduced to 293 K, cm³ g^–1 - B _a, B_e constants, cm³ mol g^–2 - B ₂ second virial coefficient, cm³ mol g^–2 - M _m ^a mass average apparent molecular mass of micelle, g mol^–1 - M _m mass average molecular mass of micelle corrected withB ₂, g mol^–1 - M _m ^cM mass average molecular mass of micelle belonging toc _M, g mol^–1 - M ¹ mass average molecular mass of monomer, gmol^–1 - N _A the Avogadro's number, molecule mol^–1 - R universal gas constant, erg mol^–1 K^–1 - T temperature, K - _t ^o dynamic viscosity of solvent atT temperature, g cm^–1 s^–1 - dynamic viscosity of solvent at 293 K, g cm^–1 s^–1 - _t density of solution atT temperature, g cm^–3 - _t ^o density of solvent atT temperature, g cm^–3 - density of solvent at 293 K, g cm^–3 - angular velocity, rad s^–1 - time, s     

Evaluations of solid electrodes for use in voltammetric monitoring of heavy metals in samples from metallurgical nickel industry

Evaluation of different solid electrode systems for detection of zinc, lead, cobalt, and nickel in process water from metallurgical nickel industry with use of differential pulse stripping voltammetry has been performed. Zinc was detected by differential pulse anodic stripping voltammetry (DPASV) on a dental amalgam electrode as intermetallic Ni–Zn compound after dilution in ammonium buffer solution. The intermetallic compound was observed at –375 mV, and a linear response was found in the range 0.2–1.2 mg L^–1 (r²=0.98) for 60 s deposition time. Simultaneous detection of nickel and cobalt in the low g L^–1 range was successfully performed by use of adsorptive cathodic stripping voltammetry (AdCSV) of dimethylglyoxime complexes on a silver–bismuth alloy electrode, and a good correlation was found with corresponding AAS results (r²=0.999 for nickel and 0.965 for cobalt). Analyses of lead in the g L^–1 range in nickel-plating solution were performed with good sensitivity and stability by DPASV, using a working electrode of silver together with a glassy carbon counter electrode in samples diluted 1:3 with distilled water and acidified with H₂SO₄ to pH 2. A new commercial automatic at-line system was tested, and the results were found to be in agreement with an older mercury drop system. The stability of the solid electrode systems was found to be from one to several days without any maintenance needed.     

Catalytic Activity of Thallium, Lead, and Bismuth Adatoms in the Gold Dissolution Reaction in Cyanide Solutions: A Comparative Characterization

Quartz microgravimetry is used to determine the ratio between coefficients of mass transfer (1 : 0.54 : 0.48), which characterizes relative values of rates of diffusion of hydroxy complexes of thallium, lead, and bismuth in alkaline solutions. The ratio is used when refining the condition under which on a renewable electrode in solutions containing these ions at the concentration c _i the electrode coverage by relevant adatoms _i with increasing duration of contact of the electrode with solution reaches constant values: (c _Tl t) = (0.54c _Pb t) = (0.48c _Bi t). Measured are i,t curves on a renewable gold electrode at E = const in solutions containing 0.1 M KCN, 0.1 M KOH, 0.01 M KAu(CN)₂, and 8 × 10^–6 M compounds of thallium or 1.5 × 10^–5 M, lead, or 1.6 × 10^–5 M, bismuth. Shown is that at (, E) = const the currents of dissolution of gold in these solutions increase in the series Tl < Pb < Bi, which evidences an increase in this series of the catalytic activity of adatoms of these metals. Shown is that at = const the catalytic action of adatoms of thallium and bismuth has an approximately additive character. The obtained data are analyzed with allowance made for the explanation offered earlier for the catalytic effect of adatoms on the anodic dissolution of gold based on the hypothesis about the shift of the potential of the free zero charge in the negative direction after substituting a metal atom for chemisorbed cyanide ions.