Azeotropic Distillation - Purification of organic compounds

Azeotropic mixture is a mixture comprising constant boiling point. The most well known instance is a mixture of ethanol and water in the ratio of 95.87 : 4.13 (a ratio available in rectified spirit). It boils at temperature of 78.13^oC. The constituents of an azeotropic mixture cannot be separated by fractional distillation. Therefore a special type of distillation (azeotropic distillation) is employed for separating the constituents of an azeotropic mixture.

In this technique a third compound is used in distillation. The procedure is based on the fact that dehydrating agents such as C₆H₆, CCl₄, diethyl ether etc. depress the partial pressure of one of the original components. Since the result, the boiling point of that component is raised adequately and so the other component will distil over.

Dehydrating agents comprising low boiling point (e.g. C₆H₆, CCl₄, ether) depress the partial pressure of alcohol more than that of water; conversely, dehydrating agents having high boiling point (glycerol, glycol) depress the partial pressure of water much more than that of alcohol.