question_answer 43)

Give a brief description of the principles of the following techniques taking an example in each case: (a) crystallisation (b) distillation (c) chromatography  

Answer:

(a) Crystallisation: It involves the preparation of saturated solution of the impure organic compound in a suitable solvent at a higher temperature, i.e., near its boiling point. The solution so formed is filtered while it is hot to remove insoluble impurities. The clear solution is cooled when the solid organic compound separates out in the form of fine crystals. The solid is filtered and dried. If a compound is highly soluble in one solvent and sparingly soluble in other solvent then crystallization can be carried out in the mixture of these solvents. Fractional crystallisation is used for the separation of mixture of two compounds which are soluble in the same solvent but their solubilities are different. Note: Coloured impurities present with the solid can be removed by adsorption on the surface of activated charcoal. (b) Distillation: It is most important method for purifying the organic liquids Distillation is a process which involves two steps: (i) Vaporisation (ii) Condensation Several methods of distillation such as simple distillation, fractional distillation, vacuum distillation, steam distillation etc., simple distillation is used for those liquids which boil without decomposition. Similarly, steam distillation is used for the purification of the liquids which are steam volatile. The type of distillation depends mainly on the nature of organic liquid and the nature of impurities present in it. (c) Chromatography: It is most efficient technique given by Tswett, a botanist in 1906. Chromatography is based on selective distribution of the various constituents of a mixture between two phases, a stationary phase and a moving or mobile phase. Different constituents migrate, at a different rates, through the stationary phase. On the basis of nature of stationary phase, the chromatography techniques are divided as follows : (a) When stationary phase is solid, the chromatography is known as adsorption or column chromatography. Depending upon the nature of mobile phase, partition chromatography is further classified as follows: (b) When stationary phase is liquid, the chromatography is known as partition chromatography. Depending upon the nature of mobile phase, partition chromatography is further classified as follows: The most common ones are: (i) Column chromatography or adsorption chromatography, (ii) Thin layer chromatography (TLC), (iii) Paper chromatography, (iv) Gas chromatography, (v) Ion-exchange chromatography.