Salt-fractionation effects

Our third approximation is a brutal one, but it is consistent with the prediction that the minimum in the pair potential always lies at 4/k and should give reasonable results in the limit of dilute electrolyte solutions. With this simplification, we can obtain a clean analytical prediction for the salt-fractionation effect. 

The values for g obtained from Equation 4.22 do not seem to be very different from those obtained from Equation 4.20, as shown in Table 4.1 and Figure 4.3a. It is easy to see that g must be equal to /2 as Os tends to zero by expanding the exponentials in the linear approximation (Debye limit). Naturally, Equation 4.15 gives us i = 1 in this limit, as an uncharged layer does not expel co-ions and salt is equally distributed between regions I and II. However, as shown in Table 4.2 and Figure 4.3b, the predicted salt-fractionation effect obtained by substituting Equation 4.22 into Equation 4.15 is markedly different from the Donnan equilibrium. 

Twelve Sets of Results Obtained for the Salt Fractionation Effect... 

FIGURE 5.5 The salt fractionation effect in n-butylammonium vermiculite swelling. The salt fractionation factor s = ceJc x is plotted as a function of cex, the salt concentration in the supernatant fluid. The solid line shows the coulombic attraction theory prediction, s = 2.8. 

In Table XVIII, there are several trends that can be noted in k if one proceeds through the R4NBr series. First of all, k tends to decrease as the size of the TAA cation increases and, in fact, tetra-n-butylammonium bromide shows a large salting-in effect. This trend is emphatically demonstrated by Figure 13, which shows the smoothed salt effects of the various salts studied in the ethanol-water system at x = 0.206. Secondly, it appears that there is a larger salting-out effect as the mole fraction of ethanol increases in the binary solvent mixture. 

Figure 16.5. Supersaturation behavior, (a) Schematic plot of the Gibbs energy of a solid solute and solvent mixture at a fixed temperature. The true equilibrium compositions are given by points b and e, the limits of metastability by the inflection points c and d. For a salt-water system, point d virtually coincides with the 100% salt point e, with water contents of the order of 10-6 mol fraction with common salts, (b) Effects of supersaturation and temperature on the linear growth rate of sucrose crystals [data of Smythe (1967) analyzed by Ohara and Reid, 1973],...

A commonly employed first separation step is ammonium sulfate precipitation. This technique exploits the fact that the solubility of most proteins is lowered at high salt concentrations. As the salt concentration is increased, a point is reached where the protein comes out of solution and precipitates. The concentration of salt required for this salting out effect varies from protein to protein, and thus this procedure can be used to fractionate a mixture of proteins. For example, 0.8 M ammonium sulfate precipitates out the clotting protein fibrinogen from blood serum, whereas 2.4 M ammonium sulfate is required to precipitate albumin. Salting out is also sometimes used at later stages in a purification procedure to concentrate a dilute solution of the protein since the protein precipitates and can then be redissolved in a smaller volume of buffer. 

The presence of D-fructose does not interfere with the isolation of aldo-benzimidazoles by the above method. It has no effect on the potassium salt fraction but it will precipitate with the barium salts, and later, by degradation, may give rise to small amounts of D-arabo-ben-zimidazole. Therefore, say Moore and Link,16 in the examination of natural products the isolation of D-arabo-benzimidazole from the barium salt fraction, rather than from the potassium salt fraction as expected, is to be regarded with suspicion, for it may have been formed from D-fructose instead of D-arabinose. They suggest in questionable cases that pentose and ketose tests then be tried on the original sample as an aid in solving the problem. 

Although the introduction of the highly dissociative salt group proved to be effective in increasing the number of carrier ions, the increase of the salt fraction significantly elevated the Tg. The characteristics of salt turned dominant by increasing the salt fraction, as mentioned above. Since the carrier ions can migrate faster in... 

The solubility of a dissolved non-electrolyte solute can be reduced by the addition of a salt. This phenomenon, known as the salting-out effect, is of practical importance for the isolation of organic compounds from their solutions. In the presence of a dissolved dissociated salt, a fraction of the solvent molecules becomes involved in solva-tional interaction with the ions of the electrolyte, whereby their activity is diminished, leading to salting-out of the dissolved non-electrolyte solute. In other words, the salting-out can be considered as the difference in solubility in two kinds of solvents, the ion-free and the ion-containing one [248]. 

A relationship between the derivative of the activity coefficient of the protein with respect to the mole fraction of water at infinite dilution of protein and the preferential binding parameter was used to connect the solubility of a protein in an aqueous mixed solvent to the preferential binding parameter. This relation was used to examine the salting-in and salting-out effect of various compounds on the protein solubility in water and to predict the protein solubility. 

The product is often purified further after the recovery phase, particularly when the desired product is a protein of pharmaceutical interest. Precipitation is a widely used method that can be induced by the addition of salts, organic solvents, or heats. It can both purify and concentrate a particular protein fraction, and is frequently accomplished by the addition of salts for a salting out effect. The addition of a salt precipitates proteins because increasing salt concentrations reduce the solubility of a protein in a solution. Even though precipitation is an effective and relatively inexpensive method, it is also a fairly crude step and is often followed by chromatographic separations. ... 

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