Sparingly Soluble Species—Dilute Solutions

As we have seen in the previous section, the solubility of materials varies according to their chemical composition and with temperature. Solubility is also affected by the presence of additional species in the solution, by the pH, and by the use of different solvents (or solvent mixtures). When discussing inorganic species, the solvent is usually water, while with organics, the solvent can be water or a number of organic solvents, or solvent mixtures. 

If we start with a sparingly soluble inorganic species such as silver chloride and add silver chloride to water in excess of the saturation concentration, we will eventually have equilibrium 

This equation represents the solubility product of silver chloride. Solubility products are generally used to describe the solubility and equilibria of sparingly soluble salts in aqueous solutions. Solubility products of a number of substances are given in Table 1.3. It is important to remember that use of solubility product relations based on concentrations assumes that the solution is saturated, in equilibrium, and ideal (the activity coefficient is equal to one), and is therefore an approximation, except with very dilute solutions of one solute. 

4) can be used for electrolytes in which there is a 1 1 molar ratio of the anion and cation. For an electrolyte that consists of univalent and bivalent ions, such as silver sulfate, which dissociates into 2 mol of silver ion for each mole of sulfate ion, the solubility product equation would be written as 

In the dissociation equation the concentration of the ions of each species are raised to the power of their stoichiometric number. 

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In aqueous solutions the micellar assembly structure allows sparingly soluble or water-insoluble chemical species to be solubilized, because they can associate and bind to the micelles. The interaction between surfactant and analyte can be electrostatic, hydrophobic, or a combination of both [76]. The solubilization site varies with the nature of the solubilized species and surfactant [77]. Micelles of nonionic surfactants demonstrate the greatest ability for solubilization of a wide group of various compounds for example, it is possible to solubilize hydrocarbons or metal complexes in aqueous solutions or polar compounds in nonpolar organic solutions. As the temperature of an aqueous nonionic surfactant solution is increased, the solution turns cloudy and phase separation occurs to give a surfactant-rich phase (SRP) of small volume containing the analyte trapped in micelle structures and a bulk diluted aqueous phase. The temperature at which phase separation occurs is known as the cloud point. Both CMC and cloud point depend on the structure of the surfactant and the presence of additives. Table 6.10 gives the values of CMC and cloud point for the surfactants most frequently applied in the CPE process. 

The activity of a pure solid is I, and, for dilute solutions (sparingly soluble salts), the activity of a solute species can be replaced by its molarity. 

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