Azeotropic Mixture

Azeotropic Mixture

Some liquids on mixed from azeotropes which are binary mixtures having the same composition liquid and vapor phase and boil at a constant temperature. In such cases, it is not possible to separate the components by fractional distillation. There are two types of azeotropes called as minimum boiling azeotrope and maximum boiling azeotrope, respectively. Solutions of ethanol and water show such a large deviation from Raoult’s law that there is a maximum in the vapour pressure curve and hence a minimum in the boiling point.

Ethanol-water mixtures (obtained by fermentation of sugars) are rich in water. Fractional distillation is able to concentrate the alcohol to at best, the azeotropic composition of approximately 95% by volume of ethanol. Once this composition has been achieved, the liquid and vapour have the composition, and no additional fractional occurs. Other methods of separation have to be used for preparing 100% C2H5OH.

There are also solutions that show a large negative deviation from ideality and, therefore, have a minimum in their vapor pressure curves. This leads to a maximum on the boiling point diagram. HNO3 and H2O form examples of this class of the azeotrope. This azeotrope has the approximate composition, 68% nitric acid and 32% water by mass, with a boiling point of 393K.