Liquid-Liquid Extraction: Are n Extractions with V/n mL of Solvent Really More Effective Than One Extraction with V mL?

Liquid-liquid extraction is a widespread separation technique with applications in the analytical and the organic chemistry laboratory (13). A problem often tackled in textbooks is the efficiency of several extractions using small portions of solvent in relation to a single extraction with a volume of solvent equal to the sum of the small portions. I t is shown in most textbooks, with the aid of one or two examples, that several extractions with small volumes is a more efficient process than a single extraction using the combined volumes of solvent (13). Although this is always true, the effect of the partition coefficient on the relative effectiveness of both processes has been largely overlooked. In this paper I show that if the partition coeff~cient for a chemical between two solvents, K, is very large (K > 100) or very small (K < 0.01), then dividing up the total amount of solvent into small portions and carrying out several consecutive extractions does not translate into a significant increase in the eff~ciency of the process.