Question

Explain why 2 extractions with 2 smaller volumes of solvent is more effective than 1 larger extraction.

Answer 1

It has to do with the partition coefficient of a solute. Partition coefficient is the ratio of concentration of a solute in two immiscible solvents.  

Let us say that some organic solute is dissolved in water with other water soluble inorganic impurities. We want to extract that organic solute using some organic solvent.

If we add a large amount of organic solvent (let us say 50 mL) for one time extraction then most of the organic compound from aqueous medium will extract into the organic layer. But there still would be some organic compound remain dissolved in the aqueous layer due to the partition coefficient of that solute between these two immiscible solvents.

Now if we do two small extraction (using 2 x 25 mL of organic solvent). Then in the first time, most of the organic compound would be extracted from the aqueous layer. After the first extraction, there would be organic compound in the aqueous layer which can be extracted again using organic solvent. This second extraction will pull out organic solute from aqueous layer more effectively as the equilibrium establishes due to the partition coefficient of the solute between two layers.

Hence, 2 extractions with 2 smaller volumes of solvent is more effective than 1 larger extraction.