Methods of purification of organic compounds:
The various methods involved in the purification are:
- Differential extraction
Sublimation: It is the process in which solid directly changes to gaseous form on heating and on cooling the gaseous state convert in to solid state.
The idea behind using this technique is to separate substance,that show sublimation from substance that do not sublime.
Crystallisation: This method is based on the difference in the solubility of compound and the impurities in the suitable solvent.
- The impure compound in dissolved in a solvent in which it is sparingly soluble at room temperature, but appreciably soluble at higher temperature.
- The solution is concentrated to get saturated solution.
- On cooling the solution pure compound crystallizes out and is removed by filtration.
- The filtrate contains impurities and small quantity of compound .
Distillation : This method is used to separate volatile liquids from non volatile impurities and also the liquids that have difference in their boiling points .
- The liquids with different boiling point vaporise at different temperatures .Then these vapours are cooled and the liquids formed are collected separately.
Like chloroform and aniline can be easily separated by this technique as both have different boiling points.
For this technique:
(i) Take the mixture in a distillation flask. Fit the flask with a thermometer.
(ii) Arrange the apparatus as shown in the figure.
(iii) Heat the mixture slowly keeping a close watch at the thermometer and observe what happens.The vapours of one component with lower boiling point can be seen rising up in the distillation flask with the increase in temperature.
(ii) These vapours get condensed in the condenser and can be collected (as pure liquid distillate) from the condenser outlet.
Fractional distillation: This method is used for those liquids which have nearly same boiling points that is the difference in their boiling point is not much .So, as they have same boiling points. Therefore ,liquids from both liquids condense and formed at same time.
- In this technique ,vapours of liquid mixture are passed through a fractionating column before condensation .This fractionating column is fitted over round bottom flask.
- Vapours of liquid with higher boiling point condense before the vapours of liquid with lower boiling point.
- On reaching the top, the vapours become pure in low boiling component and passed through the condenser. The pure liquid is collected in receiver .Each successive condensation and vaporisation unit in the fractionating column is called theoretical plate. Example: separation of different components of crude oil
Distillation under reduced pressure: This method is used to purify liquids having very high boiling points and those which decompose at or below their boiling points.
- Such liquids are made to boil at a temperature lower than their boiling points, by reducing the pressure on their surface. A liquid boils at a temperature, at which its vapour pressure is equal to external pressure .The pressure is reduced with the help of water pump. Example glycerol can be separated from spent-lye by this method
Steam distillation: This technique is applied to separate substances which are steam volatile and are immiscible in water.
- In steam distillation steam from a steam generator is passed through a heated flask containing the liquid to be distilled.
- The mixture of steam and the volatile organic compound is condensed and collected. Then compound is later separated from water using separating funnel.
- In it the liquid boils ,when sum of vapour pressures due to organic liquid and due to water becomes equal to the atmospheric pressure.
i.e. =P1 +p2
- Since p1 is lower than p, the organic liquid vaporises at lower temperature than its boiling point.
Example: Aniline and water can be separated by this method