Liquid-liquid extraction is a unit operation frequently employed in the pharmaceutical industry, as in many others, for recovery and purification of a desired ingredient from the solution in which it was prepared. Extraction may also be used to remove impurities from a feed stream. Extraction is the removal of a soluble constituent from one liquid into another. By convention, the first liquid is the feed (F) which contains the solute at an initial concentrationXf The second liquid is the solvent (S) which is at least partially immiscible with the feed. The solvent may also have some solute present at an initial concentration of but usually is essentially zero. The solvent does the extraction, so the solvent-rich liquid leaving the extractor is the extract (E). With the solute partially or completely removed from the feed, the feed has become rejned so the feed-rich liquid leaving the extractor is the raflnate (R). When the feed and solvent are brought together, the solute (A) will distribute itselfbetween the two liquid phases. At equilibrium, the ratio ofthis distribution is called the distribution coeficient (m):
