Electrolytes, salting out

Conway, B.E. Local changes of solubility induced by electrolytes salting-out and ionic hydration. PureAppl. Chem. 1985, 57, 263-272. 

The constant K is termed the distribution or partition coefficient. As a very rough approximation the distribution coefficient may be assumed equal to the ratio of the solubilities in the two solvents. Organic compounds are usually relatively more soluble in organic solvents than in water, hence they may be extracted from aqueous solutions. If electrolytes, e.g., sodium chloride, are added to the aqueous solution, the solubility of the organic substance is lowered, i.e., it will be salted out this will assist the extraction of the organic compound. 

Comparatively large concentrations of electrolytes are required to cause precipitation ( salting out ). The change is, in general, reversible, and reversal is effected by the addition of a solvent (water). 

Therefore some indirect methods have been worked out to determine the value of ff=0.154,259 In particular (1) salting out of organic compounds from a surface-inactive electrolyte solution, (2) F"" for 1-pentanol or other organic compounds with a high attractive interaction constant a, and (3) dependence of the capacitance minimum on thiourea concentration. It should be noted that indirect estimates based on TU adsorption give... 

By definition, in a solution all ions belong to the same phase, even though counterions may cluster more or less diffusely around the macroions. When significant amounts of a simple 1 1 electrolyte (such as KCl) are added to a polyelectrolyte solution, dissociation of the polyelectrolyte macromolecule is repressed in an extreme case the polyelectrolyte may be salted out. An undissociated polyacid may be precipitated by generous addition of a simple acid such as HCl. 

Proteins that are polyampholytes, upon addition of electrolyte, first undergo some salting in up to a certain ionic strength, since electrostatic interactions between ions are shielded by the additional simple ions of both sign. Adding more salt will then cause salting out as the added ions compete for water that would otherwise solvate the protein. 

The effect of dissolved hydrophilic electrolytes on the interaction between organic solutes and water can be described by the salting-in and salting-out effects. Dissolved electrolytes usually increase the internal pressure in water, through a volume-reducing process that... 

When the dissolved salt increases the internal pressure of aqueous solution to a certain extent, the nonelectrolyte is squeezed out (salting out). On the other hand, when the dissolved salt reduces the internal pressure of the solution, more of the nonelectrolyte is able to dissolve (salting in). All the electrolytes except perchloric acid increase the internal pressure of water and cause a salting out of organic species. For example, saturated sodium chloride is used to separate organic compounds from water. 

Similarly, concepts of solvation must be employed in the measurement of equilibrium quantities to explain some anomalies, primarily the salting-out effect. Addition of an electrolyte to an aqueous solution of a non-electrolyte results in transfer of part of the water to the hydration sheath of the ion, decreasing the amount of free solvent, and the solubility of the nonelectrolyte decreases. This effect depends, however, on the electrolyte selected. In addition, the activity coefficient values (obtained, for example, by measuring the freezing point) can indicate the magnitude of hydration numbers. Exchange of the open structure of pure water for the more compact structure of the hydration sheath is the cause of lower compressibility of the electrolyte solution compared to pure water and of lower apparent volumes of the ions in solution in comparison with their effective volumes in the crystals. Again, this method yields the overall hydration number. 

Lang W, Zander R (1986) Salting-out of oxygen from aqueous electrolyte solutions prediction and measurement. Ind Eng Chem Fundam 25 775-782... 

For most organic chemicals the solubility is reported at a defined temperature in distilled water. For substances which dissociate (e.g., phenols, carboxylic acids and amines) it is essential to report the pH of the determination because the extent of dissociation affects the solubility. It is common to maintain the desired pH by buffering with an appropriate electrolyte mixture. This raises the complication that the presence of electrolytes modifies the water structure and changes the solubility. The effect is usually salting-out. For example, many hydrocarbons have solubilities in seawater about 75% of their solubilities in distilled water. Care must thus be taken to interpret and use reported data properly when electrolytes are present. 

The solubility of any solid can be either increased or decreased by the addition of an electrolyte to the solvent, a phenomenon known as the salt effect. Salting-out describes the situation in which the solubility of the solid is decreased by the salt effect, whereas salting-in is the term used when the solubility is increased. Salting-out takes place when the added electrolyte sufficiently modifies the water structure so that the amount of water available for solute dissolution is effectively reduced, and it is a procedure convenient for the isolation of highly soluble substances. 

Towards high concentrations of the electrolyte, the microemulsion changes to an emulsion containing normal monomer droplets. With a further increase in the electrolyte concentration, the emulsion becomes unstable and breaks down ( salting out ). 

A second type of ternary electrolyte systems is solvent -supercritical molecular solute - salt systems. The concentration of supercritical molecular solutes in these systems is generally very low. Therefore, the salting out effects are essentially effects of the presence of salts on the unsymmetric activity coefficient of molecular solutes at infinite dilution. The interaction parameters for NaCl-C02 binary pair and KCI-CO2 binary pair are shown in Table 8. Water-electrolyte binary parameters were obtained from Table 1. Water-carbon dioxide binary parameters were correlated assuming dissociation of carbon dioxide in water is negligible. It is interesting to note that the Setschenow equation fits only approximately these two systems (Yasunishi and Yoshida, (24)). 

The semi-empirical Pitzer equation for modeling equilibrium in aqueous electrolyte systems has been extended in a thermodynamically consistent manner to allow for molecular as well as ionic solutes. Under limiting conditions, the extended model reduces to the well-known Setschenow equation for the salting out effect of molecular solutes. To test the validity of the model, correlations of vapor-liquid equilibrium data were carried out for three systems the hydrochloric acid aqueous solution at 298.15°K and concentrations up to 18 molal the NH3-CO2 aqueous solution studied by van Krevelen, et al. 

Salting-out Coefficients at 25°C. The effect of an electrolyte on the solubility or activity of a gas dissolved in an aqueous solution is commonly expressed as a salting out coefficient ... 

Coefficient expressing the effect of concentration of gas on its activity coefficient, kg H20/mole Coefficient expressing the effect of change of partial molal volume of electrolyte (with temperature) on the salting-out coefficient, kg H20/cm3. Salt concentration, mol/2. 

An extended form of the Debye-Huckel equation is the hydration one of Robinson and Stokes (11). It contains two adjustable parameters, ap and h, where h is rilated to the hydration number. It can be fitted to y for several electrolytes for concentrations in excess of 1 m. Their equation has the valuable feature of describing not only the salting-in but also the salting-out part of the y+ versus m curve. It should be noted, however, that the... 

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