Liquid-liquid extraction

Liquid-liquid extraction is a separation method based on the physical properties of different compounds. Here, a solute of interest is extracted from one liquid phase into another immiscible liquid phase, in which it is relatively more soluble. The two immiscible liquid phases are typically water (aqueous phase) and an organic solvent (organic phase) and the extraction is most commonly carried out using a separatory funnel.

The method of liquid-liquid extraction is based on the concept of relative solubility. The solute will distribute itself in a certain, predictable, ratio between the two immiscible solvents depending on how soluble it is in each. It is important to ensure that the solute is more soluble in the extraction solvent than the other solvent, so it will preferentially partition into this phase. The relative solubility of a solute between two immiscible solvents can be measured as the partition coefficient (P), the ratio of concentrations of a solute between two immiscible solvents at equilibrium.

The solubility of a solute of interest is based upon the principle of ‘like dissolves like’ in that, a highly polar solute will be more soluble in a highly polar solvent, and vice versa. For example, if an organic compound (such as caffeine) is dissolved in an aqueous solvent (water) and then mixed with an organic solvent (such as dichloromethane) the caffeine will preferentially partition into the organic solvent as it is more soluble in this phase.

It is important to note that the solute will still be present in both phases, and multiple rounds of extraction may be required to transfer a high yield of the solute into the extraction solvent.