Salting out constant

Fig. 6.4 Calculated, estimated, or apparent salting out constants for various chemicals (a) selected aromatic compounds, (b) selected aliphatic compounds, (c) natural or surrogate hgands, (d) anthropogenic ligands, (e) sediment organic matter (SOM), (f) transition metal complexes, (g) trace metal complexes in the Mersey Estuary. Reprinted with permission from Turner A, Martino M, Le Roux SM (2002) Trace metal distribution coefficients in the Mersey Estuary UK Evidence for salting out of metal complexes. Environ Sci Technol 36 4578-4584. Copyright 2002 American Chemical Society...

The basis for characterizing fractionated precipitation of proteins is the Cohn-Edsall equation (Cohn, 1943) [Eq. (8.60), where S is the solubility of the protein, is the solubility in salt-free solution, Ks is the salting-out constant, and I represents the ionic strength]. 

Setschenow (1889) gave the first quantitative description of protein solubility as a function of salt concentration, as in Eq. (8.61), where [E] is the solubility of the enzyme, [S] the salt concentration, and Ks the salting-out constant... 

It is not certain that the theoretical arguments, which led to the introduction of the term C t, are completely satisfactory, but it seems to be established that the experimental data require a term of this type. The aggregation of solvent molecules in the vicinity of an ion is the factor responsible for the so-called salting-out effect, namely, the decrease in solubility of neutral substances frequently observed in the presence of salts the constant C is consequently called the salting-out constant. The activity coefficient of a non-electrolyte, as measured by its solubility in the presence of electrolytes, is often given by an expression of the form log / = CV this is the result to which equation (62) would reduce for the activity of a non-electrolyte, i.e., when z+ and z arc zero, in a salt solution of ionic strength... 

It is shown 30) that the analysis of a relationship between the salting-out constants of a given protein and the molal surface tension increments makes it possible to estimate the relative surface hydrophobicity which is calculated as the ratio of the nonpolar surface area to the molecular weight of the protein. According to the concept developed by Melander et al 30), the hydrophobic character of a protein is believed to be constant at both high and physiological concentrations of different inorganic salts, which seems to be untrue in most cases (see below). 

This equation describes the relationship between the aqueous solubility of sparingly soluble salts (So) and the empirical Setschenow salting-out constant k = 0.217/Sq. This relationship and the Setschenow equation are valid only at low concentrations of added salt. As the concentration of added salt increases, the apparent k value is not constant, but is dependent on the solubility and the rate of change of solubility with added salt concentration. It was concluded that the Setschenow treatment is generally inappropriate for description and analysis of common ion equilibria. 

Conversely, some large ions (e.g. basic drugs) may rise the gelation temperature, and presumably the cloud point [89,90], The increase in hydration is considered to result from the adsorption of these ions carrying water onto the macromolecule. In this regard, Mitchell et al. [90] provide new data for two HPMCs 2208 (Table 18). The abilities of additives to depress the cloud point, or salting-out constants KCP, were calculated from the relationship ... 

Table 18. Salting-out constants Ka> of additives derived from cloud points of 2% solutions of HPMCs 2208 (15000 and 100000mPa s viscosity grades) [90]...

S.2 Salinity When salts dissolve, the ions produced associate strongly with the water molecules to form hydration shells. This bound water is not available to dissolve organic compounds reducing solubility and producing what is termed a salting out effect defined by the Setschenow or salting out constant (K ) with units, M ... 

Salting out constants, K, for a series of compounds are compiled in Table 2.9. It is apparent that this effect is more pronounced in compounds of lower aqueous solubility with K inversely related to log 5 (Fig. 2.6). It was observed that solubilities determined in sea water corresponded with those determined in sodium chloride solution (35 g kg H2O I = 0.60) whose concentration approximated the total salt concentration of sea water. It was concluded that the salting-out constants derived from observations in sodium chloride solutions could be used to predict solubilities in sea water. Using the regression equation relating K and log 5 ... 

It is obvious from this previous equation that the solubility values of some salts could be represented by a straight line relationship. According to the model of Van Krevelen and Hoftijzer (50), the ionic strength I is introduced as a better measure of electrolyte activity and the salting-out constant is considered to be the result of contributions from the various gas species (h ), and ions (h, h ) present ... 

They found the salting out constant increasing with e/k for all salts other than NaNO), which agreed with behavior noted by other authors for nitrate salts. 

Values of the Salting-Out Constants, K t, for Amino Acids and Proteins... 

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