Ionic Transport in Electrodialysis of Ammonium Nitrate

During the electrodialysis of ammonium nitrate solution, the fluxes of salt ions pass through the maximum, which is observed near the limiting current, with increasing current density. A decrease in the flux of ammonium ions at the overlimiting current densities is caused by the effect of competitive transport of solution ions and by the formation of weak NH₃ ? H₂O electrolyte due to the alkalization of solution layer adjacent to the cation-exchange membrane in the desalination channel. A decrease in the flux of nitrate ions in the overlimiting current modes is caused by a change in the composition and catalytic activity of the functional groups of anion-exchange membrane towards the dissociation of water molecules due to the effect of ammonium ions.     

Mass transfer examination in electrodialysis using limiting current measurements

The impact of electrodialysis module characteristics on mass transfer was examined using the limiting current method. The current-voltage curves of different electrodialysis modules were measured and limiting currents were determined using the derivative method. The mass transfer coefficients were calculated and the parameters of their dependence on linear flow velocity were estimated. From these the impact of spacer thickness, spacer net type, membrane type, and module geometry were evaluated. It was found that the impact of spacer thickness was almost negligible within the examined range, but a decrease in the mass transfer coefficient could be expected in the case of thicker spacers. By contrast, the spacer net type and type of membrane were found to be very important parameters able to significantly influence the mass transfer. By modifying the module geometry, the mass transfer coefficient could also be altered and, only in this case, the exponential parameter of the dependence was changing. The parameters thus determined may be used to calculate the limiting current in a wide range of operational conditions and may help predict the performance of different electrodialysis module types.     

Hydrodynamic voltammetry at channel electrodes: Part IV. Theory of chronopotentiometry for reversible electrode reactions

The theory of chronopotentiometric measurements at channel electrodes is developed for reversible electrode reactions, by rigorously solving the corresponding time-dependent boundary value problem, where the non-uniform accessibility of the channel electrode surface is taken into consideration. The theoretical equations of the transition time and the potential-time curve are derived as functions of (I_d/I), where I denotes the applied current intensity and I_d the limiting diffusion current obtained at steady-state. Finally, the time variation of the current density distribution at the electrode surface is given.     

Prediction of permselectivity of nitrate and acetate ions in the electrodialysis of aqueous solutions

Electrodialysis of aqueous solutions of acetic and nitric acids encountered in the industrial practice of deacidification of raw glyoxal solutions has been carried out to investigate the permselectivity of the system. The dependence of the permselectivity coefficient τ on experimental parameters such as flow velocity (V), current density (I), concentration of anions (C_i), etc. has been evaluated. A correlation between τ and the critical parameters has been obtained. Well-known theoretical equations have been used to calculate τ at limiting current density I_lim and as I approaches zero. Calculated results are compared with observed values. Limitations of theoretical procedures when applied to a system containing a weak acid are explained.     

Electrodialysis purification of 5-aminolevulinic acid hydrochloride

The possibility of using electrodialysis for purifying 5-aminolevulinic acid hydrochloride from pollutants, ammonium chloride, formed in the course of synthesis was studied. Electrodialysis conditions: current density, linear velocity of a solution in desalting chambers, concentration of ammonium chloride and 5-aminolevulinic acid hydrochloride, were optimized.     

A Study of Concentration Polarization on Ion Exchange Membrane Electrodialysis

The concentration polarization phenomena in ion exchange membrane electrodialysis have been studied with single exchange membrane cell. The limiting current densities of Asahi ion-permselective membranes CK-1 and CK-2, Selemion ion-exchange membranes CMV, AMV, DMV and ASV have been measured with Ag-AgCl reversible electrode in various electrolyte solutions under 25°C and constant flow rate. In sodium chloride solution, the cation exchange membrane is easier to occur concentration polarization than the anion exchange membrane. The limiting current density increases as the concentration of solution increases for the same kind of ion exchange membrane. The experimental limiting current densities of Selemion CMV and AMV in NaCl, KCl, MgCl₂, CaCl₂, BaCl₂, Na₂SO₄, NaOH and HCl aqueous solutions are measured. The results show that the limiting current density increases as the ion mobility and diffusivity increase, and is affected by the transference number of ion. For the mixture of electrolyte solution, there are linear relationship between limiting current density and equivalent fraction of electrolytes.     

Limiting diffusion currents in hydrodynamic voltammetry: II. Wedge electrodes in laminar uniform flow

A precise theoretical equation, including the numerical coefficient, for the limiting diffusion currents at the wedge electrodes immersed in a laminar uniform flow has been derived. The theoretical prediction for the dependence of the limiting diffusion currents on various experimental variables, i.e., the bulk concentration of depolarizer, the linear flow velocity, the electrode length, the wedge angle and the distance between the leading edge of the electrode and the apex of the wedge, has been verified experimentally. Finally, the validity of the numerical coefficient in the theoretical equation has been examined.     

Gas dynamics in channels of a gas-feed direct methanol fuel cell: exact solutions

The gas dynamics in channels on both sides of a gas-feed direct methanol fuel cell (DFMC) are considered. The basic equations for the flow velocity and density are derived, taking into account the mass and momentum transfer through the channel/backing layer interface. For the practical case of small inlet velocities the analog of the Bernoulli equation is formulated and the exact solution of nonlinear gas dynamics equations is obtained. It is shown that the flow in both the cathode and anode channels is incompressible (its density is constant) and electrochemical reactions affect only the flow velocity v. Simple formulae for v as a function of local current density and effective water drag coefficient are derived.     

Electrodialysis for chloride removal from the chemical recovery cycle of a Kraft pulp mill

The selective removal of chloride from a solution with high ionic strength containing multiple divalent anions, using electrodialysis with monovalent selective ion-exchange membranes is described. The process is characterized in terms of limiting current density, selectivity, and current efficiency. The correlation between limiting current density, chloride concentration and average fluid velocity is discussed. Using actual electrostatic precipitator dust from the chemical recovery cycle of a Kraft pulp mill the fouling behavior of the membranes was investigated in batch desalination experiments. Long term behavior over nearly 100 h was studied. The feed contained organic contamination and a large number of inorganic components at low concentrations. The process performed excellently, despite the fact that no pre-treatments were applied to the feed.     

Reduction kinetics of glycinate complexes of palladium(II) on a dropping-mercury electrode in acid perchlorate solutions

Polarograms for the reduction of glycinate complexes of palladium(II) (5 × 10^?5 M) are obtained in equilibrium solutions of pH 0.8–3.0 with different protonated-glycine concentrations c _Hgly (supporting electrolyte, 0.5 M NaClO₄). It is established that the irreversible wave of reduction of complexes Pd(gly)₂ corresponds to the diffusion limiting current I _d ⁽²⁾ . A similar wave at pH 1.5 and c _Hgly = 0.005 M, as well as at pH 1.0 and c _Hgly = 0.05–0.5 M is preceded by the diffusion limiting current I _d ⁽¹⁾ . Values of the I _d ⁽²⁾ /I _d ⁽¹⁾ ratio are close to the ratio between equilibrium concentrations of Pd(gly)₂] and [Pdgly⁺], calculated using the step stability constant for Pd(gly)₂. This fact testifies to the reduction of complexes Pdgly⁺ in the vicinity of I _d ⁽¹⁾ and complexes Pd(gly)₂, in the vicinity of I _d ⁽²⁾ . At pH 0.8–1.2 and [H₂gly⁺] = 1 × 10^?4 to 5 × 10^?3 there is observed the diffusion-kinetic limiting current of the first wave I ₁ ⁽¹⁾ , which increases with increasing [H⁺] and decreasing [H₂gly⁺]. The nature of the slow preceding chemical stage that occurs during the reduction of complexes Pdgly⁺ is discussed.     

Morphology and microtopology of cation-exchange polymers and the origin of the overlimiting current

In electrodialysis desalination processes, the operating current density is limited by concentration polarization. In contrast to other membrane processes such as ultrafiltration, in electrodialysis, current transport above the limiting current is possible. In this work, the origin of the overlimiting current at cation-exchange polymers is investigated. We show that, under certain experimental conditions, electroconvection is the origin of the overlimiting conductance. The theory concerning electroconvection predicts a shortening of the plateau length of membranes with increased conductive or geometrical heterogeneity. We investigate the influence of these two parameters and show that the creation of line undulations on the membrane surface normal to the flow direction, having distances in the range of approximately 50-200% of the boundary-layer thickness, lead to an earlier onset of the overlimiting current. The plateau length of the undulated membranes is reduced by up to 60% compared to that of a flat membrane. These results verify the existence of electroconvection as a mechanism destabilizing the laminar boundary layer at the liquid-membrane interface and causing ionic transport above the limiting current density.     

Coulometric generation of molybdenum(v)

Molybdenum (V) was generated at a platinum cathode from 0.7 M molybdenum(VI) in 4 M sulfuric acid. A current efficiency of 99.9% was attained. A limiting current density of 0.05 mA/cm²/mM was found. The formal potential of the Mo(VI)–Mo(V) couple in 4 M sulfuric acid was determined to be ca. 0.55 V vs. N.H.E. Chromium(VI) solutions were titrated over a wide range of sample size and generating current. Amperometric titration curves were interpreted from current-voltage curves. Titrations could be performed in the presence of oxygen at the 1μeq. level. The effect of nitrate, perchlorate, orthophosphate, and chloride ions on the titration was determined.     

Effect of convective channel-to-channel mass flow in a polymer electrolyte fuel cell

The effect of convective channel-to-channel mass flow on the local performance of a polymer electrolyte fuel cell (PEFC) air cathode is determined experimentally by using submillimeter resolved current density distribution measurements in channel and land areas. A special cell is employed, where the two parallel channels of the cathode flow field can be operated at different pressure. For isobaric operation of the channels (Δp = 0 mbar), the lateral current density distribution shows a distinct minimum in the land area between the channels as diffusive mass transport becomes limiting at a higher cell polarization. Toward higher Δp, the local cell performance in the land area improves initially as a result of an improving convective channel-to-channel mass flow. However, as the pressure difference exceeds a value of 10 mbar, no noteworthy additional benefit is observed with further increasing Δp. Under these conditions, the convective mass flow provides an abundant reactant supply in the land area and, since reactant depletion is no longer limiting, the lateral current density distribution is primarily governed by the local ohmic resistance. As a result, the current density exhibits a maximum in the land area, where the local ohmic resistance shows a minimum.     

Apparent molar and partial molar volumes of aqueous ceric ammonium nitrate solutions at 20, 25, 30, and 35°C

Present paper reports the measured densities (ρ) and refractive indices (n _D) of aqueous solutions of ceric ammonium nitrate (CAN) at 20, 25, 30, and 35°C in different concentrations of solution. Apparent molar volumes (φ_v) have been calculated from the density data at different temperatures and fitted to Massons relation to get limiting partial molar volumes (? _v ⁰ ) of CAN. Refractive index data were fitted to linear dependence over concentration of solutions and values of constant K and n _D ⁰ for different temperatures were evaluated. Specific refractions (R _D) of solutions were calculated from the refractive index and density data. Concentration and temperature effects on experimental and derived properties have been discussed in terms of structural interactions.     

Electrokinetically driven micro flow cytometers with integrated fiber optics for on-line cell/particle detection

This paper presents an innovative micro flow cytometer which is capable of counting and sorting cells or particles. This compact device employs electrokinetic forces rather than the more conventional hydrodynamic forces technique for flow focusing and sample switching, and incorporates buried optical fibers for the on-line detection of cells or particles. This design approach results in a compact microfluidic system and an easier integration process. The proposed cytometer integrates several critical modules, namely electrokinetic-focusing devices, built-in control electrodes, buried optical fibers for on-line detection, and electrokinetic flow switches for bio-particle collection. A linear relationship exists between the focused stream width (d) and the focusing ratio (F/φ), which is estimated to be D≈134.5−53.8F/φ. The relationship between the particle velocity (U) and the applied voltage (V) is also investigated. Numerical and experimental data confirm the effectiveness of the device when applied to the counting and sorting of 10 μm diameter particles and red blood cells.     

Volumetric properties of ammonium nitrate in N,N-dimethylformamide

The densities of the ammonium nitrate in N,N-dimethylformamide (DMF) mixtures were measured at T = (308.15 to 348.15) K for different ammonium nitrate molalities in the range from (0 to 6.8404) mol·kg⁻¹. From the obtained density data, volumetric properties (apparent molar volumes and partial molar volumes) have been evaluated and discussed in the term of respective ionic and dipole interactions. From the apparent molar volume, determined at various temperatures, the apparent molar expansibility and the coefficients of thermal expansion were also calculated.     

Effect of Amino Acids and Sodium Chloride on <Emphasis Type="SmallCaps">d</Emphasis>-Sorbitol in Aqueous Solutions at Different Temperatures: Volumetric and Acoustic Approach

Apparent molar volumes and apparent molar compressibilities for d-sorbitol in (0.05, 0.1, 0.2 and 0.3) mol·kg^?1 aqueous solutions of l-alanine, l-cysteine and l-histidine and NaCl have been determined from measurements of solution density at T?=?(288.15, 298.15, 308.15 and 318.15) K and sound velocity at T?=?298.15 K, as a function of the concentration of the sugar alcohol. The data were used to obtain the limiting apparent molar volumes, limiting apparent molar compressibilities and the corresponding transfer parameters. Limiting apparent molar expansibilities and their second order derivatives and volume interaction coefficients were also estimated. These parameters are discussed in terms of d-sorbitol and co-solute (amino acid or sodium chloride) interactions in aqueous solutions.     

Bactericidal effect of an electrodialysis system on E. coli cells

Using a new electrodialysis system with both cation- and anion-exchange membranes, the bactericidal effect on Escherichia coli has been investigated in detail from the standpoint of electrochemistry. Various electrolyte solutions containing E. coli (10⁸ cells/cm³) were passed through a desalting chamber at a flow rate of 3 cm³/min under varying current densities, and the viability of the cell (%) and the pH changes in the effluents were measured. When a 0.1 M NaCl aqueous suspension was used, a disinfection effect emerged in the vicinity of the limiting current density (LCD 0.81 A/dm²) and increased with an increase in the current density. The pH value of the suspensions decreased owing to the dissociation of water to H⁺ and OH⁻ ions by the well-known “neutrality disturbance phenomenon” in the region beyond the LCD. These tendencies were also observed when other electrolyte suspensions were used. Concerning the effect of the various species on the disinfection of E. coli cells, ionic systems in which a LCD was easily attained were found to have a strong effect.The germicidal effect may be due to a synergistic effect of acidic H⁺ and basic OH⁻ ions which are produced on the anion-exchange membrane and cation-exchange membrane, respectively, of the desalting chamber.     

Implication of volume changes in uranium oxides: A density functional study

In severe nuclear accident scenarios (in air environments and high temperatures) UO₂ fuel pellets oxidise to produce uranium oxides with higher oxygen content, e.g., U₄O₉ or U₃O₈. As a first step in investigating the microstructural changes following UO₂ oxidation to hexagonal high temperature phase of U₃O₈, density functional quantum mechanical calculations of the structure, elastic properties and electronic structure of U₃O₈ have been performed. The calculated properties of hexagonal phase of U₃O₈ are compared to those of the orthorhombic pseudo-hexagonal phase which is stable at room temperature. The total energy technique based on the local density approximation plus Hubbard U as implemented in the CASTEP code is used to investigate changes in the lattice constants. The first-principles calculations predict a 35–42% increase in volume per uranium atom as a result of the transformation from UO₂ to U₃O₈, in agreement with experimental data. The implications of this prediction on the linear expansion and fragmentation of fuel are discussed.