Extraction of Molecular Species

The distribution of simple molecular species, that do not undergo dissociation or polymerization reactions is straightforward. The corresponding partition isotherm is a straight line with a slope of +1. and represents the case where D_C remains constant.

The percentage of solute extracted into solvent b from solvent a is given by:

where V_a and V_b are the volumes of solvents a and b, and D is defined according to Eq. 10. It is obvious that %E can be increased by increasing V_b. More effective, however, are multiple extractions with smaller volumes V_a and V_b. If V_a = V_b, Eq. 10 simplifies to

Extraction is considered to be quantitative when 99.9% or more of the solute is extracted.

Extraction of Metal Complexes

Metal ions easily form complexes in aqueous solution, when an appropriate ligand is available. Because of their electrical charge, metal ions, even if hydrated, usually do not extract well into an organic phase. The situation may be quite different with neutral complexes, especially if these are covalent in nature. An example is given in Fig. 3 where the extraction of thallium(III) from aqueous chloride solutions with tributyl phosphate (TBP) in hexane is shown. The extracted species is T1C1₃. Therefore we can write:

From these relationships, the complex formation constants in aqueous solutions may be derived as:

Ion Exchange

The solubility of a solid in a solvent is a well-defined property of all crystalline bodies. In aqueous solutions, most solids dissociate and exist as the corresponding cations and anions. If the solution contains no other electrolyte, the anions and cations are present in the solution in the stoichiometric ratio of the solid and the solubility can be expressed in terms of K_sp. However, this general behavior is not always observed. Many minerals behave quite differently. It is often seen that the anion (e.g., silicates) is part of an insoluble rigid crystal structure (matrix) and that the cations are present only to compensate for the excess negative charge of the rigidly fixed anions. This is so when the cations are held in the crystal lattice by purely electrostatic forces. In the process of dissolution, the polar bonds (electrostatic) can easily be broken by the water dipoles (likes dissolves like), but the covalent bonds are quite resistant to interaction with the water molecules.

Since the cations are held in the crystal lattice by electrostatic forces, they occupy holes, interstices, or cavities of the lattice, and they can easily be replaced by other cations of similar charge and size. However, cations with a large charge and smaller ionic radii are retained much better on the solid surface than cations with small charge and large ionic radii. Clays belong to this class of minerals. In this case the stoichiometry of the compounds is not fixed and cannot be expressed by simple integral numbers. These materials exhibit low solubility, but they can exchange certain cations in their lattice with cations present in an aqueous solution (e.g., seawater, which contains about 0.7 mol L^-1 electrolytes). This replacement is called an ion exchange process and can be expressed by the equilibrium:

and the appropriate equilibrium constant K_eq:

where YR and XR are the fractions of sites occupied by Y⁺ and X⁺.

In modern laboratory applications, the natural ion exchangers (minerals, such as clays and zeolites) are replaced by synthetic organic products, the so-called ion exchange resins.