Question 12.18:
Give a brief description of the principles of the following techniques taking an example in
each case.
(a) Crystallisation
(b) Distillation
(c) Chromatography
(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.