Mixed electrolyte solutions

Henri CL, Dalton CN, Scruton L, Craig VSJ (2007) Ion-specific coalescence on bubbles in mixed electrolyte solutions. J Phys Chem C 2007 1015-1023... 

Also attracting growing attention is the phase coexistence curve characteristic of ionic systems it plays a role in some ionic solution phenomena, although examples in aqueous solutions are not known at this time. Other new features are the intense concentration dependence - at low concentration - of certain of the Hamed coefficients that characterize mixed electrolyte solutions and the evidence for a solvent-separated state of the hydrophobic bond, the attractive force between hydrophobic ions, even those of zero charge, in water. 

Siveral approaches are available in the case of mixed electrolyte solutions. The Guntelberg equation can be used at very high dilutions to avoid the ambiguity in the meaning of aD, the distance of closest approach, when several electrolytes are present. This equation is empirical and has fewer terms than the Debye-Huckel extended equation. I found it to yield poor agreement with experimental results even at m = 0.01 for NaCl at 25°C (y+ caic = 0.8985 and y+ exp = 0.9024). For the Davies equation for m = 0.20 one obtains y+ calc = 0.752 andy+exo = 0.735 also for NaCl at 25°C. 

Next, predictive equations for activity coefficients in mixed electrolyte solutions, based upon results in simpler ones, will be mentioned. The work of Brjlnsted (12) and of Guggenheim (13) led to the specific interaction equation... 

The pressure-volume-temperature (PVT) properties of aqueous electrolyte and mixed electrolyte solutions are frequently needed to make practical engineering calculations. For example precise PVT properties of natural waters like seawater are required to determine the vertical stability, the circulation, and the mixing of waters in the oceans. Besides the practical interest, the PVT properties of aqueous electrolyte solutions can also yield information on the structure of solutions and the ionic interactions that occur in solution. The derived partial molal volumes of electrolytes yield information on ion-water and ion-ion interactions (1,2 ). The effect of pressure on chemical equilibria can also be derived from partial molal volume data (3). 

The PVT properties of aqueous solutions can be determined by direct measurements or estimated using various models for the ionic interactions that occur in electrolyte solutions. In this paper a review will be made of the methods presently being used to determine the density and compressibility of electrolyte solutions. A brief review of high-pressure equations of state used to represent the experimental PVT properties will also be made. Simple additivity methods of estimating the density of mixed electrolyte solutions like seawater and geothermal brines will be presented. The predicted PVT properties for a number of mixed electrolyte solutions are found to be in good agreement with direct measurements. 

The use of equation (52) to estimate the density of a mixed electrolyte solution can be demonstrated by considering average seawater. The equivalent fraction of the cations and anions in seawater are shown in Figure 18. The solution mainly consists of the ions Na+, Mg2+, Cl-, and S042-. The calculation of v° and by for "sea salt" is given in Table III (151). 

The compressibilities of mixed electrolyte solutions (48,86, 88,148) can also be estimated. The adiabatic apparent molal compressibility of a mixed electrolyte is given by... 

Once the density and compressibilities of mixed electrolyte solutions are known at 1 atm, values at high pressures can be made by using the secant bulk modulus equation of state. The major difficulty, at present, with using additivity methods to estimate the PVT properties of mixed electrolytes is the lack of experimental data for binary solutions over a wide range of concentrations and temperatures. Hopefully, in the near future we will be able to provide some of these data by measurements in our laboratory in Miami. 

Equilibrium and Kinetic Problems in Mixed Electrolyte Solutions... 

The Important conclusion is that complex controlling processes can occur in solubility phenomena in mixed electrolyte solutions. This is especially true of surface coatings formed kinetically or by multistate thermodynamics and which prevent the aqueous solution from interaction with internal bulk phases. One should remember of course that, when the degree of supersaturation is large enough for bulk precipitation to occur, the kinetic and multiphase thermodynamic processes studied above will apply to the actual bulk phases. 

The stoichiometric measurements were made using a similar dialysis technique. Exactly 50 mg zeolite ( 0.1-0.2 mg) was weighed into a dialysis membrane to which 5 ml 0.01 NaCl (or NaNOs) was pipetted. The mixture was equilibrated with 40 ml of a mixed electrolyte solution of 0.01 total normality, containing sodium and the bivalent cation (Ni, Zn, Co) in various proportions. Each combination requires two identical experiments, involving either a 22Na or a label of the transition element. Equilibrations were made in an end-over-end shaker (5 or 25°C) for one to two weeks. The ion distributions were calculated from the amounts of radioactivity of the initial and the equilibrium solutions for assays of duplicate 5 ml samples. For precision, the total number of counts collected in the 22Na-labeled samples was always about 10 . 

In Scotland, Danesh, Todd, and coworkers measured the inhibition of multiphase systems with methanol (Avlonitis, 1994) and mixed electrolyte solutions (Tohidi et al., 1993,1994a, 1995b,c). They also performed the most comprehensive study of systems with heavy hydrocarbons such as might be produced/transported intheNorth Sea (Avlonitis et al., 1989 Tohidi etal., 1993,1994b, 1996) including systems with structure H hydrate formers. 

Dholabhai, P.D., Incipient Equilibrium Conditions for Methane Hydrate Formation in Aqueous Mixed Electrolyte Solutions, M.Sc. Thesis, University of Calgary, Calgary (1989). 

Pytkowicz M. and Cole M. R. (1980) Equilibrium and kinetic problems in mixed electrolyte solutions. In Thermodynamics of Aqueous Systems with Industrial Applications (ed. S.A. Newman), ACS Symp. Series, 133, Washington, D.C., 644-652. 

The influence of the membrane selectivity on the resistance could nicely be demonstrated in a series of experiments with mixed electrolyte solutions. The NaCl concentration was maintained at 0.1 M and the KC1 concentration was varied from 0.1 M to 10-6 M. In the concentration range 0.1-104 M KC1 there is virtually no change in the spectra but at 1 O 5 M KC1 there is a small change which becomes very prominent at 10 M KC1. Experiments starting from a low... 

At higher ionic strength values, an additional dependence on [i] is often required to fit the observed solubility data. Alternatively, the Pitzer model can be used with a high degree of accuracy to describe the short-range binary (neutral-neutral, neutral-cation, neutral-anion) and ternary (neutral-neutral-neutral, neutral-cation-anion) interactions between ions and neutral species in single and mixed electrolyte solutions. ... 

There are two alternatives available for calculating the surface species distribution in a sample or a mixed electrolyte solution. One approach is the solution equilibrium computer program MINEQL (32) as modified to include surface species by Davis al. (17). The surface species distribution is calculated by simultaneously solving the equations for charge, potential, total surface sites and individual surface species. 

Equation lA corresponds to equation (A-1) of R.eilly, Wood, and Robinson (15) for the total excess free energy for a general mixed electrolyte solution containing m moles of cation with charges Z, m moles of anions Xj with charges Zj, and one kilogram of solvent. 

Equation 17 relates the excess free energy of a mixed electrolyte solution to the osmotic coefficient of the solution and the activity coefficients of its component ions. 

The Osmotic Coefficient of a Mixed Electrolyte Solution. It may be seen that differentiation of equation 17 with respect to m will leave a term including the osmotic coefficient of the solution, cj). 

In order to evaluate (j), the total excess free energy expression given in equation 17 will be differentiated with respect to the solvent concentration. This involves no changes in the concentration ratios of any ionic specie m to the total concentration of all ionic species m. Differentiation of equation 17 with respect to m results in the working equation for the osmotic coefficient of a mixed electrolyte solution. Details of the differentiation are given elsewhere (15,16). 

Activity Coefficient for a Trace Component in a Mixed Electrolyte Solution. If the salt MpXq is present in the electrolyte mixture in only a very minute quantity, it may be seen that its contribution to I, E, and m are negligible. Applying equation 19, and treating the trace salt as any other component, it becomes apparent that any interaction terms with an Mp dependence will be vanishingly small. Also, when k = p, any terms multiplied by E may be neglected since they will approach zero. The result of such a treatment is that the trace salt activity coefficient will be largely determined by the activity coefficient of the pure salt in a solution at the same ionic strength as the mixture (15). 

Although the above equations are derived for pure solutions of MCI and MCI2, the following technique was used to adapt their use to a mixed electrolyte solution in which chloride is the dominant anion ... 

The model was shown to predict osmotic coefficients to within the experimental error for the determination of osmotic coefficients from saturated vapor pressure measurements. Activity coefficient calculations appeared to be consistent with the available data for mixed electrolyte solutions. 

Reilly, P. J., Wood, R. H., and Robinson, R. A. The prediction of osmotic and activity coefficients in mixed-electrolyte solutions. Phys. Chem. 75, 1305-1315 (1971). 

The ionic interactions in a mixed electrolyte solution like seawater can affect the physical properties (density, heat capacity, etc.). Since the composition of natural waters can be quite different, it is useful to have models that can be used to describe how the ionic components affect the physical properties. This requires knowledge of ionic interactions in the solutions of interest. Over the years, a great deal of progress has been made in interpreting and modeling the physical-chemical properties of mixed electrolyte solutions (Millero, 2001). This has led to the development of models that can be used to estimate the properties of namral... 

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