(a) Crystallisation

Crystallisation is one of the most commonly used techniques for the purification of solid

organic compounds.

Principle: It is based on the difference in the solubilites of the compound and the

impurities in a given solvent. The impure compound gets dissolved in the solvent in which

it is sparingly soluble at room temperature, but appreciably soluble at higher temperature.

The solution is concentrated to obtain a nearly saturated solution. On cooling the solution,

the pure compound crystallises out and is removed by filtration.

For example, pure aspirin is obtained by recrystallising crude aspirin. Approximately 2 – 4

g of crude aspirin is dissolved in about 20 mL of ethyl alcohol. The solution is heated (if

necessary) to ensure complete dissolution. The solution is then left undisturbed until some

crystals start to separate out. The crystals are then filtered and dried.

(b) Distillation

This method is used to separate volatile liquids from non-volatile impurities or a mixture

of those liquids that have a sufficient difference in their boiling points.

Principle: It is based on the fact that liquids having different boiling points vapourise at

different temperatures. The vapours are then cooled and the liquids so formed are

collected separately.

For example, a mixture of chloroform (b.p = 334 K) and aniline (b.p = 457 K) can be

separated by the method of distillation. The mixture is taken in a round bottom flask fitted

with a condenser. It is then heated. Chloroform, being more volatile, vaporizes first and

passes into the condenser. In the condenser, the vapours condense and chloroform trickles

down. In the round bottom flask, aniline is left behind.

(c) Chromatography

It is one of the most useful methods for the separation and purification of organic

compounds.

Principle: It is based on the difference in movement of individual components of a mixture

through the stationary phase under the influence of mobile phase.

For example, a mixture of red and blue ink can be separated by chromatography. A drop

of the mixture is placed on the chromatogram. The component of the ink, which is less

adsorbed on the chromatogram, moves with the mobile phase while the less adsorbed

component remains almost stationary.