Question -
Answer -
(a) Crystallisation
Crystallisation is oneof the most commonly used techniques for the purification of solid organiccompounds.
Principle:┬аIt is based on thedifference in the solubilites of the compound and the impurities in a givensolvent. The impure compound gets dissolved in the solvent in which it issparingly soluble at room temperature, but appreciably soluble at highertemperature. The solution is concentrated to obtain a nearly saturatedsolution. On cooling the solution, the pure compound crystallises out and isremoved by filtration.
For example, pureaspirin is obtained by recrystallising crude aspirin. Approximately 2 тАУ 4 g ofcrude aspirin is dissolved in about 20 mL of ethyl alcohol. The solution isheated (if necessary) to ensure complete dissolution. The solution is then leftundisturbed until some crystals start to separate out. The crystals are thenfiltered and dried.
(b) Distillation
This method is used toseparate volatile liquids from non-volatile impurities or a mixture of thoseliquids that have a sufficient difference in their boiling points.
Principle:┬аIt is based on thefact that liquids having different boiling points vapourise at differenttemperatures. The vapours are then cooled and the liquids so formed arecollected separately.
For example, a mixtureof chloroform (b.p = 334 K) and aniline (b.p = 457 K) can be separated by themethod of distillation. The mixture is taken in a round bottom flask fittedwith a condenser. It is then heated. Chloroform, being more volatile, vaporizesfirst and passes into the condenser. In the condenser, the vapours condense andchloroform trickles down. In the round bottom flask, aniline is left behind.
(c) Chromatography
It is one of the mostuseful methods for the separation and purification of organic compounds.
Principle:┬аIt is based on thedifference in movement of individual components of a mixture through thestationary phase under the influence of mobile phase.
For example, a mixtureof red and blue ink can be separated by chromatography. A drop of the mixtureis placed on the chromatogram. The component of the ink, which is less adsorbedon the chromatogram, moves with the mobile phase while the less adsorbedcomponent remains almost stationary.