Activity coefficients high salt concentration

This definition of x and y is more realistic at low and moderate salt concentrations and is in agreement with that of Sada and Morisue (17). Broul and Hala also assumed complete salt dissociation. The assumption of full dissociation of the salt may not be entirely valid at high salt concentrations, especially where the concentration of the nonaqueous solvent is also high. However, even in those instances where the assumption of full dissociation of the salt may be invalid, it appears to describe the system better than ignoring salt ionization completely. The terms x/ and y/ are referred to hereafter as ionic mole fraction and ionic activity coefficient, respectively. These should not be confused with the mean ionic terms used by Hala which are also based on complete salt dissociation, but are defined differently. No convergence problems were encountered when the ionic quantities were employed. 

Figure 10.10 Limiting current density and operating current density as a function of feed water salt concentration. The change in slope of the curves at about 3000 ppm salt reflects the change in the activity coefficient of the ions at high salt concentrations [23]...

Where the neutral species is present at sufficiently high concentration, its influence on the activity coefficients of dissolved salts and the osmotic coefficient of the solution must be considered. The equation for the mean activity coefficient of salt M +Xv. dissolved in an aqueous solution containing NH3 (and neglecting dissociation) is given by ... 

It must be remembered that the cell potential is proportional to the In(activity) of the ion rather than its concentration. The activity is a measure of the extent of thermodynamic nonideality in the solution. The activity coefficient is usually less than unity, so the activity of a solution is generally lower than the total concentration, but the values of activity and concentration approach each other with increasing dilution. If a compound is not completely ionized, the activity is further decreased. Decreased ionization can be brought about because of a weak ionization constant, chemical complexation, or a high salt concentration in the solution. Any of these factors will cause a change in the potential of the cell, even if the ion concentration is constant. In practice, it is better to determine the relationship between the cell potential and ion concentrations experimentally. 

Finally, as an example of a highly soluble salt, we may take sodium chloride at 25° the concentration of the saturated solution is 6.16 molal. The activity coefficient of NaCl, like that of NaBr plotted in Fig. 72, passes through a minimum at a concentration between 1.0 and 1.5 molal and it has been estimated2 that in the saturated solution the activity coefficient has risen to a value very near unity. Writing y = 1.0, we find that the work required to take a pair of ions from the surface of NaCl into pure water at 25° has the rather small value... 

Activity Coefficients at High Concentrations in Multicomponent Salt Systems... 

The integral extends from LH to Ld. Equation 5.47b demonstrates that the solvent solubility IE offers a convenient way to determine the IE on the standard state partial molar free energy for the salt provided the concentration dependence of its activity coefficient in one solvent, most likely H20, is available at high concentration. 

When the solvent is a good solvater, the determination of the solvation number b is difficult, unless the dependence of the extractant concentration on the solvent can be obtained. Solvation numbers can be obtained in mixtures of a solvating extractant and an inert diluent like hexane. Further, in these systems the extraction of the metal commonly requires high concentrations of salt or acid in the aqneons phase, so the activity coefficients of the solutes must be taken into acconnt. 

At low ionic strength, y I for neutral compounds. At high ionic strength, most neutral molecules can be salted out of aqueous solution. That is, when a high concentration (typically > 1 M) of a salt such as NaCl is added to an aqueous solution, neutral molecules usually become less soluble. Does the activity coefficient, -yclher, increase or decrease at high ionic strength ... 

Electrodes respond to the activity of uncomplexed analyte ion. Therefore, ligands must be absent or masked. Because we usually wish to know concentrations, not activities, an inert salt is often used to bring all standards and samples to a high, constant ionic strength. If activity coefficients are constant, the electrode potential gives concentrations directly. 

Other than specific effects that result from conventional chemical interactions (such as acid-base or complex formation), the main factors to be considered are hydration of ions, electrostatic effects, and change in dielectric constant of the solvent. For example, hydration of ions of added salt effectively removes some of the free solvent, so that less is available for solution of the nonelectrolyte. The Setschenow equation probably best represents the activity coefficient of dilute solutions (less than 0.1 M) of nonelectrolytes in aqueous solutions of salts up to relatively high concentrations (about 5 M) ... 

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