Question

Add up the total grams of organic material extracted via 3 extractions and compare it to the grams extracted with only 1 extraction. What conclusions can you make about the use of multiple extractions?

Answer 1

Transferring a solute from one solvent into another is called Extraction, or more precisely, liquid-liquid extraction. The solute is extracted from one solvent into the other because the solute is more soluble in the second solvent than in the first. When a solution (solute A in solvent 1) is shaken with a second solvent (solvent 2) with which it is not miscible the solute distributes itself between the two liquid phases. When the two phases have separated again into two distinct solvent layers, and equilibrium will be achieved such that the ratio of the concentrations of the solute in each layer is defined by a constant. The constant, called the distribution coefficient (or partition coefficient) K, is defined by

K = C2/C1

where C1 and C2 are the concentrations at equilibrium, in grams per liter (or milligrams per milliliter) of solute A in solvent 1 and in solvent 2, respectively. This relationship is a ratio of two concentrations and is independent of the actual amounts of the two solvents mixed. The distribution coefficient has a constant value for each solute considered and depends on the nature of the solvent used in each case.

Every time an extraction is carried there will be mass transfer of solute from solvent 1 to solvent 2.
The solute will distribute itself in the two liquid phases and a new equlibrium will be estblished
to achieve K - the partition coefficient which is constant

Therefore multiple small extractions that generate multiple equilibrium stages of mass transfer of solute are more efficient than a single extraction which involves a single equilibrium stage of mass transfer of solute . Considering the fact that the partition coefficient of solute between the two solvents is independent of the quantity of solvent being used for extraction a large volume of solvent in a single extraction is just adding cost without any value addition.