4 Solvent Extraction Equilibria

The ability of a solute (inorganic or organic) to distribute itself between an aqueous solution and an immiscible organic solvent has long been applied to separation and purification of solutes either by extraction into the organic phase, leaving undesirable substances in the aqueous phase; or by extraction of the undesirable substances into the organic phase, leaving the desirable solute in the aqueous phase. The properties of the organic solvent, described in Chapter 2, require that the dissolved species be electrically neutral. Species that prefer the organic phase (e.g., most organic compounds) are said to be lipophilic (“liking fat”) or hydrophobic (“disliking water”), while the species that prefer water (e.g., electrolytes) are said to be hydrophilic (“liking water”), or lipophobic (“disliking fat”). Because of this, a hydrophilic inorganic solute must be rendered hydrophobic and lipophilic in order to enter the organic phase. Optimization of separation processes to produce the purest possible product at the highest yield and lowest possible cost, and under the most favorable environmental conditions, requires detailed knowledge about the solute reactions in the aqueous and the organic phases. In Chapter 2 we described physical factors that govern the solubility of a solute in a solvent phase; and in Chapter 3, we presented the interactions in water between metal cations and anions by