Azeotrope: Definition, Types, Azeotrope Distillation & Applications

Azeotrope is a mixture which at a given pressure (azeotropic pressure) boils at the same temperature (azeotropic temperature) and has the same composition in equilibrium vapour and liquid phases. An azeotrope is a mixture of at least two liquids that displays an equal concentration in the liquid as well as the vapour phase. The process of simple distillation cannot alter their proportions. Such mixtures can either have a lower boiling point or a higher boiling point of the components.

Key Terms: Azeotrope, Azeotrope Mixture, Azeotrope Distillation, Liquids, Vapour Phase, Concentration, Simple Distillation

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Definition of Azeotrope

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An azeotrope can be defined as a mixture of two or more liquids that displays the same level of concentration in the liquid and vapor phase. Its proportion cannot be changed by simple distillation because when the azeotrope is boiled it produces vapor and the proportion of constituents in the vapour and the liquid which is not boiled is the same, even after distillation the composition remains unchanged.

• Azeotropes are also called a ‘constant boiling mixture’. 
• Azeotropes do not follow Raoult’s Law which means these mixtures are not ideal solutions and show deviation from Raoult’s Law. 
• In this type of mixture, one component has either a higher or lower boiling point than the other component. 
• In these mixtures, the component exhibits the same mole fraction in the liquid phase and the vapor phase which is why they cannot be separated by the distillation method.

Azeotrope

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Types of Azeotropes

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There are different types of Azeotropes with different and unique properties from each other. They are the following:

• Minimum Boiling Azeotrope

If an azeotrope shows positive deviation from Raoult’s law then it is called a minimum boiling azeotrope. It is also known as the positive boiling azeotrope. These azeotropes that are positive contain a boiling point lower than the boiling point of all of their constituents. Minimum boiling azeotropes are formed when the mixture has a lower boiling point and higher vapour point.

• Maximum Boiling Azeotropes

Maximum boiling azeotropes are defined as the liquid mixtures which show negative deviation from Raoult's Law. Therefore, it is also referred to as a negative azeotrope. Maximum boiling azeotrope is formed when the mixture has lower vapour pressure and the highest boiling point.

• Homogeneous Azeotropes

If the constituents of an azeotropic mixture are completely mixable in all proportions with each other, then this type of azeotrope is considered to be a homogeneous azeotrope. For example, any amount of ethanol can be mixed with water to form a homogeneous azeotropic mixture. An example of a homogeneous azeotropic mixture includes water mixed with ethanol.

• Heterogeneous Mixture

If the azeotropes are present as a constituent in several mixtures and if these mixtures are not entirely mixable they are called heterogeneous azeotropes. These heterogeneous azeotropes are incompletely mixable and form two different liquid phases. For instance, mixtures such as chloroform and water, benzene, and water. This type of azeotrope is also called heteroazeotrope. In the condition of heteroazeotrope phase splitting may occur for a minimum boiling azeotrope with a large deviation from Raoult’s law.

• Binary Azeotrope

Azeotrope which has two constituents in a mixture is called the binary azeotrope. Examples of binary azeotrope include the mixture of diethyl ether (33%) and halothane (66%).

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Azeotropic Distillation

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Azeotropic distillation is a special type of distillation used in industries and laboratories. Azeotropic distillation is done by adding to the liquid phase a volatile third component which changes the volatility of one of the two components more than the other so that the components can be separated by distillation.

• Azeotropic distillation is performed in batches in the traditional fractional distillation method only but the difference is that a third volatile compound is also added so that such mixture can be separated. 
• If the mixture of binary solution and azeotrope is a minimum boiling point type then the distillate product should contain only one of the original components and the azeotropic agent. 
• The composition of the distillate product then changes as the distillation proceeds as the separation is taking place between the different azeotropes.

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Application of Azeotropes

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The applications of Azeotropes are as follows:

• Azeotropes are used as a standard in testing gas, chromatographs, detectors, and columns.
• The azeotropes are also used to separate the components of zeotropic mixtures. One example is that it is pretty difficult to separate the components of acetic acid and water. It is known that ethyl acetate forms azeotropes with water by adding small amounts of ethyl acetate in this mixture containing acetic acid and water, it would be possible to distill away the azeotropes formed.

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Things to Remember

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• An azeotrope can be defined as a mixture that boils at a constant temperature (the azeotropic temperature) at a given pressure (the azeotropic pressure) and has the same composition in the equilibrium vapour and liquid phases.
• Azeotrope is also referred to as constant boiling mixtures as their composition remains unchanged by distillation. 
• There are various types of Azeotropes such as Heterogeneous and Homogeneous Azeotropes, Maximum and Minimum Boiling Azeotropes and Binary Azeotropes.
• Azeotropic distillation is done by adding a volatile third component to the liquid phase which changes the volatility of one of the two components more than the other so that the components are separated by distillation.

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