The removal of a solute from its solution by the addition of a solvent in which the solute is more soluble but which itself is not soluble in solution is called solvent extraction.
Many organic compounds are produced along with minor products in aqueous solution. Such compounds are shaken with ether taken in a separating funnel. Pure organic compounds are removed from their aqueous solution by ether in which organic compound is more soluble but which itself is not soluble in water. Ethereal layer containing organic compound is removed from the separating funnel.
Distribution Law (Partition Law):
Solvent extraction is an equilibrium process controlled by distribution law or partition law.
It states that a solute distributes itself between two immiscible liquids in a constant ratio of concentrations irrespective of the amount of solute added.
The law is based on experimental evidence.
Consider, for example, the distribution of iodine between the two immiscible solvents, water in the presence of KI and carbon tetrachloride. The following dynamic equilibrium is established:
At this point the rate at which iodine passes from CCl4 to water equals the rate at which it passes from water to CCl4.
So if we add CCl4 to an aqueous solution of I3 ions, the iodine will transfer from the aqueous layer into the organic layer. As a result the brown colour of the tri-iodide ions fades and the purple colour of free iodine molecules appear in organic phase. No matter how much iodine is used, the ratio of the final concentrations of equilibrium is constant.
The constant is called distribution coefficient, K. At 25°C the value of K is 85 which indicates that non-polar iodine is 85 times more soluble in non-polar solvent then in polar water.