Aqueous-electrolyte systems

Fig. 2-2 Simplified potential-pH diagram for an iron/aqueous electrolyte system at 25°C c(Fe ) + c(Fe ) = 10" mol L (explanation in the text).

Mn02 is used for the same purpose as the cathode active material in lithium-manganese dioxide (Li - Mn02) batteries it has been used for a long time in zinc-carbon and alkaline-manganese dioxide batteries, which are aqueous-electrolyte systems. 

This new NEMCA process underlines the potential importance of electrochemical promotion in industrial aqueous electrolyte systems. 

I. The parameter 2Fr0/Io ( A ) must be larger than unity (Chapter 4). Catalysis at the metal/gas interface must be faster than electrocatalysis. This is easy to satisfy in solid electrolyte systems and more difficult to satisfy in aqueous electrolyte systems. 

Table 1.2. Nonmetal electrode potentials in aqueous electrolyte systems. )...

However, in most aqueous electrolyte systems of industrial interest, not only strong electrolytes but also weak electrolytes and molecular nonelectrolytes are present. While the modified Pitzer equation appears to be a useful tool for the representation of aqueous strong electrolytes including mixed electrolytes, it cannot be used in the form just presented to represent the important case of systems containing molecular solutes. A unified thermodynamic model for both ionic solutes and molecular solutes is required to model these kinds of systems. 

To test the validity of the extended Pitzer equation, correlations of vapor-liquid equilibrium data were carried out for three systems. Since the extended Pitzer equation reduces to the Pitzer equation for aqueous strong electrolyte systems, and is consistent with the Setschenow equation for molecular non-electrolytes in aqueous electrolyte systems, the main interest here is aqueous systems with weak electrolytes or partially dissociated electrolytes. The three systems considered are the hydrochloric acid aqueous solution at 298.15°K and concentrations up to 18 molal the NH3-CO2 aqueous solution at 293.15°K and the K2CO3-CO2 aqueous solution of the Hot Carbonate Process. In each case, the chemical equilibrium between all species has been taken into account directly as liquid phase constraints. Significant parameters in the model for each system were identified by a preliminary order of magnitude analysis and adjusted in the vapor-liquid equilibrium data correlation. Detailed discusions and values of physical constants, such as Henry s constants and chemical equilibrium constants, are given in Chen et al. (11). 

In general, data are fit quite well with the model. For example, with only two binary parameters, the average standard deviation of calculated lny versus measured InY of the 50 uni-univalent aqueous single electrolyte systems listed in Table 1 is only 0.009. Although the fit is not as good as the Pitzer equation, which applies only to aqueous electrolyte systems, with two binary parameters and one ternary parameter (Pitzer, (5)), it is quite satisfactory and better than that of Bromley s equation (J). 

Two activity coefficient models have been developed for vapor-liquid equilibrium of electrolyte systems. The first model is an extension of the Pitzer equation and is applicable to aqueous electrolyte systems containing any number of molecular and ionic solutes. The validity of the model has been shown by data correlation studies on three aqueous electrolyte systems of industrial interest. The second model is based on the local composition concept and is designed to be applicable to all kinds of electrolyte systems. Preliminary data correlation results on many binary and ternary electrolyte systems suggest the validity of the local composition model. 

ZJ at 293.15°K and the K2CO3-CO2 aqueous solution of tTTe Hot Carbonate Process with temperatures from 343.15°K to 413.15°K and concentrations up to 40 weight percent equivalent potassium carbonate. The success of the correlations suggests the validity of the model for aqueous electrolyte systems of industrial interest. 

Chen, C., H. I. Britt, J. F. Boston, and L. B. Evans, "Extension and Application of the Pitzer equation for Vapor-Liquid Equilibrium of Aqueous Electrolyte Systems with Molecular Solutes," AIChE J., 1979, 25, 820. 

It is shown that the properties of fully ionized aqueous electrolyte systems can be represented by relatively simple equations over wide ranges of composition. There are only a few systems for which data are available over the full range to fused salt. A simple equation commonly used for nonelectrolytes fits the measured vapor pressure of water reasonably well and further refinements are clearly possible. Over the somewhat more limited composition range up to saturation of typical salts such as NaCl, the equations representing thermodynamic properties with a Debye-Hiickel term plus second and third virial coefficients are very successful and these coefficients are known for nearly 300 electrolytes at room temperature. These same equations effectively predict the properties of mixed electrolytes. A stringent test is offered by the calculation of the solubility relationships of the system Na-K-Mg-Ca-Cl-SO - O and the calculated results of Harvie and Weare show excellent agreement with experiment. 

Franck, E. U. "Equilibrium in Aqueous Electrolyte Systems at High Temperatures and Pressures" in "Phase Equilibria and Fluid Properties in the Chemical Industry" Storvick, F. S. Sandler, S. I., Eds., ACS Symposium Series 60, American Chemical Society,... 

Phase behavior 1n concentrated aqueous electrolyte systems is of interest for a variety of applications such as separation processes for complex salts, hydrometal 1urgical extraction of metals, interpretation of geological data and development of high energy density batteries. Our interest in developing simple thermodynamic correlations for concentrated salt systems was motivated by the need to interpret the complex solid-liquid equilibria which occur in the extraction of sodium nitrate from complex salt mixtures which occur in Northern Chile (Chilean saltpeter). However, we believe the thermodynamic approach can also be applied to other areas of technological interest. 

Although there is a large number of experimental data (1, 2.,.3) for ternary aqueous electrolyte systems, few equations are available to correlate the activity coefficients of these systems 1n the concentrated region. The most successful present techniques are those discussed by Meissner and co-workers (4 ) and Bromley ( )... 

We have presented a thermodynamic technique which is useful for the correlation of thermodynamic data of aqueous electrolyte systems in the concentrated region. The approach was illustrated using the ternary system of HC1-NaCl-H20. The correlation gives a good description of solid-liquid and vapor-1iquid equilibria the two ternary parameters required to calculate the activity coefficients of the electrolytes are simple functions of the temperature and the total molality. 

The hot H3PO4 electrolyte rejects water, the reaction product. The high temperature favors H2O2 decomposition, and peroxide buildup is less pronounced than for the aqueous electrolyte systems. 

Conventional and Advanced Aqueous Electrolyte Systems Improvements are expected in all the factors that count energy density (ED), cost/kWh, safety, reliability, as well as turnaround efficiency and cycle life at deeper depths of discharge (dod). 

CZE is the most widely used mode due to its simplicity of operation and its versatility. Selectivity can be most readily altered through changes in running buffer pH or by use of buffer additives such as surfactants or chiral selectors. The major drawback with CZE is that it deals with aqueous electrolytic systems, whereas components can only be separated if they are charged and soluble in water. CZE separation of various antibacterials including penicillins, tetracyclines, and macrolides has been reported (86). Determination of cefixime, an oral cephalosporin antibiotic, and its metabolites in human urine has been also successfully carried out with CZE (87). 

For most potentiometric measurements, either the saturated calomel reference electrode or the silver/silver chloride reference electrode are used. These electrodes can be made compact, are easily produced, and provide reference potentials that do not vary more than a few mV. The silver/silver chloride electrode also finds application in non-aqueous solutions, although some solvents cause the silver chloride film to become soluble. Some experiments have utilised reference electrodes in non-aqueous solvents that are based on zinc or silver couples. From our own experience, aqueous reference electrodes are as convenient for non-aqueous systems as are any of the prototypes that have been developed to date. When there is a need to exclude water rigorously, double-salt bridges (aqueous/non-aqueous) are a convenient solution. This is true even though they involve a liquid junction between the aqueous electrolyte system and the non-aqueous solvent system of the sample solution. The use of conventional reference electrodes does cause some difficulties if the electrolyte of the reference electrode is insoluble in the sample solution. Hence, the use of a calomel electrode saturated with potassium chloride in conjunction with a sample solution that contains perchlorate ion can cause dramatic measurements due to the precipitation of potassium perchlorate at the junction. Such difficulties normally can be eliminated by using a double junction that inserts another inert electrolyte solution between the reference electrode and the sample solution (e.g., a sodium chloride solution). 

Chen CC, Evans LB. A local composition model for the excess Gibbs energy of aqueous electrolyte systems. AIChE J 1986 32 444-459. 

Using these methods to describe an aqueous electrolyte system with its associated chemical equilibria involves a unique set of highly nonlinear algebraic equations for each set of interest, even if not incorporated within the framework of a complex fractionation program. To overcome this difficulty, Zemaitis and Rafal (8) developed an automatic system, ECES, for finding accurate solutions to the equilibria of electrolyte systems which combines a unified and thermodynamically consistent treatment of electrolyte solution data and theory with computer software capable of automatic program generation from simple user input. 

Anderko and Lencka find. Eng. Chem. Res. 37, 2878 (1998)] These authors present an analysis of self-diffusion in multicomponent aqueous electrolyte systems. Their model includes contributions of long-range (Coulombic) and short-range (hard-sphere) interactions. Their mixing rule was based on equations of nonequilibrium thermodynamics. The model accurately predicts self-diffusivities of ions and gases in aqueous solutions from dilute to about 30 mol/kg water. It makes it possible to take single-solute data and extend them to multicomponent mixtures. 

Highly water-insoluble compounds can present a difficulty in CE and, therefore, completely non-aqueous electrolyte systems have been developed for both acidic and basic insoluble compounds. Standard CE methods have been developed and validated for determination of either metal ions, small carboxylic acids and inorganic anions. These compounds have limited or no UV ab.sorbance and, therefore, indirect UV detection is employed. 

In the present work, the discharge behavior of the Li/C2F battery in a non-aqueous electrolyte system was investigated. 

The phenomenon of EPOC or NEMCA effect was first reported in solid electrolyte systems [23, 195-205], but several NEMCA studies already exist using aqueous electrolyte systems [23, 30, 31,145] or Nafion membranes [23]. The EPOC phenomenon leads to apparent Faradaic efficiencies, A, well in excess of 100% (values up to 105 have been measured in solid-state electrochemistry and up to 102 in aqueous electrochemistry). This is due to the fact that, as shown by a variety of surface science and electrochemical techniques [23, 40, 195-198, 206-209], the NEMCA effect is due to electrocatalytic (Faradaic) introduction of promoting species onto catalyst-electrode surfaces [23, 196], each of these promoting species being able to catalyze numerous (A) catalytic turnovers. 

Lincomycin hydrochloride and aminoglycoside hydrochloride were determined in pharmaceuticals by Isotachophoresis. In this method, a mixture of 0.02 M potassium acetate and 0.3 % hydroxypropylmethyl cellulose (15,000) was used as an aqueous electrolyte system, whereas a mixture of 20um 1,4-aminobutyric acid in acetic acid (pH 4.72) or 0.02 M glycylglycine or B -alanine was used as the terminator. The analysis was performed at a constant current of 200 jjA at 5°C. it was found that the minimum amount that could be quantitated by this method was 1.6 nmol and the relative standard deviation was 2%. 

In summary, the results which are presented in this section suggest that the charge transport of ions within paper and paperlike structures is essentially the same as that of the transport properties associated with aqueous electrolyte systems. Furthermore, the transient current behaviour which has been observed in these fibrous cellulosic systems show characteristics similar to the ionic transient current conduction exhibited in both dielectric fluids and aqueous ionic systems. 

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