Azeotropes

A mixture of two or more liquids whose proportion cannot be changed by the simple distillation is called Azeotrope. This phenomenon occurs because when the azeotrope is boiled, it produces the vapour, and the proportion of constituents in this vapour and the mixture which is not boiled is the same. And it happens because even after distillation the composition remains unchanged. Though the word Azeotrope is now used commonly for describing this phenomenon, in the older scientific texts the term “Constant boiling point mixture” is more common.

When the pairs of compounds of some azeotropic mixtures are known, and two, three, or even more compounds of many azeotropes are also known, then in such cases, separating the components by fractional distillation is not possible. Minimum boiling azeotropes and the maximum boiling azeotropes are the two types of Azeotropes.

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What are Azeotropes? 

Azeotropes are mixtures of at least two liquids that exhibit the same concentration at the liquid phase and vapour phase. Azeotropes do not follow Raoult’s Law. It means these mixtures are not ideal solutions and show deviation from Raoult’s Law. In azeotropic mixtures, one component has either a higher or lower boiling point than another component. In these mixtures, components exhibit the same mole fraction in the liquid phase and vapour phase. So, they cannot be separated by fractional distillation. 

Fractional distillation is a separation method to separate a mixture of two or more miscible liquids for which the difference in boiling points is less than 25 K. Another condition for fractional distillation is that components of the mixture must not have equal mole fractions at the liquid phase and vapour phase. Fractional distillation is used to separate different gases from the air, different fractions from petroleum products etc. The apparatus for fractional distillation has almost the same apparatus as simple distillation but with a fractionating column installed in between the distillation flask and the condenser. 

Thus, the main points about azeotropic mixtures can be listed as follows –

• These are binary solutions with one solute and one solvent. For example, in azeotrope ethyl alcohol, ethyl alcohol is mixed with water. 

• Solute and solvent both are in a liquid state in the azeotropic mixture. For example, in an azeotropic mixture of ethyl alcohol and water, both are taken in the liquid state. 

• Solute and solvent are taken in fixed compositions to form azeotropes. For example, in an azeotropic mixture of ethyl alcohol and water, 96% of ethyl alcohol and 4% of water are mixed. 

• Azeotropic mixture solution boils at a constant temperature. For example, an azeotropic mixture of ethyl alcohol (96%) and water (4%) boils at 78.10C. 

• Separation of constituents of an azeotropic mixture by fractional distillation is not possible. For example, as the mole fraction of ethyl alcohol is the same at the liquid phase and vapour phase and mole fraction of water is also the same at the liquid phase and vapour phase so the azeotropic solution of ethanol and water boils at 78.10C and cannot be separated by fractional distillation.

Types of Azeotropes 

Azeotropes can be divided into the following types based on their composition and deviation from Raoult’s Law –

• Positive azeotrope or minimum boiling azeotrope

• Negative azeotrope or maximum boiling azeotrope 

• Heterogeneous azeotrope 

• Homogeneous azeotrope

• Binary azeotrope 

• Ternary azeotrope

Positive Azeotrope or Minimum Boiling Azeotrope – Those azeotropic mixtures which show large positive deviation from Raoult's Law at specific composition are called positive azeotropes. They show boiling points lower than the boiling points of their constituents. That’s why they are also called minimum boiling azeotropes. These types of mixtures exhibit the highest vapour pressure and lowest boiling point. For example, an azeotropic mixture of 96% (or 95.6%) ethanol and 4% water is a positive azeotrope. It shows a large positive deviation from Raoult’s law. It is represented by the graph below, where A = water and B = ethanol –

This azeotropic mixture boils at 78.20C while the water boils at 1000C and ethanol at 78.50C. It is also represented by the graph below –

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Negative Azeotrope or Maximum Boiling Azeotrope – Those azeotropic mixtures which show large negative deviation from Raoult's Law at specific composition are called negative azeotropes. They show boiling points higher than the boiling points of their constituents. That’s why they are also called maximum boiling azeotropes. These types of mixtures exhibit lower vapour pressure and the highest boiling point. For example, an azeotropic mixture of hydrogen chloride and water is a negative azeotrope. It shows a large negative deviation from Raoult’s law. It is represented by the graph below –

This azeotropic mixture boils at 1100C while the water boils at 1000C and HCl at -840C. It is also represented by the graph below –

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Heterogeneous Azeotrope – If constituents of azeotropic mixtures are not completely miscible then an azeotrope can be found inside the miscibility gap. So, Phase splitting may occur for a minimum boiling azeotrope with a large deviation from Raoult’s law. This leads to the formation of a minimum boiling heterogeneous azeotrope which has a vapour phase in equilibrium with two liquid phases. Constituents of these azeotropes are not completely miscible. These types of azeotrope are also called heteroazeotrope. For example, an azeotropic mixture of chloroform and water. When they are shaken together and left to stand then they form two separate layers. This mixture boils at 53.30C while the boiling point of water is 1000C and chloroform is 61.20C. Graphical representation of heterogeneous azeotrope is given below –

In the above graph, the vertical axis is the temperature (T), the horizontal axis is composition, as represented. The dotted vertical line indicates the composition of the combined layers of the distillate whenever both layers are present in the original mixture.

Homogeneous Azeotrope – If constituents of an azeotropic mixture are completely miscible in all proportions with each other, then this type of azeotrope is called homogeneous azeotrope. For example, any amount of ethanol can be mixed with any amount of water to form a homogeneous azeotropic mixture. 

Binary Azeotrope – Azeotropes that have two constituents are called binary azeotropes. For example, a mixture of diethyl ether (33%) and halothane (66%). This azeotropic mixture is commonly used in anesthesia. 

Ternary Azeotrope – Azeotropes that have three constituents are called ternary azeotropes. For example, an azeotropic mixture of acetone, methanol, and chloroform. 

Those mixtures whose constituents possess different boiling points are called non – azeotropic mixtures or zeotropic mixtures. Azeotropic mixtures are useful in separating constituents of non – azeotropic mixtures.

Applications of Azeotropes 

Azeotropes have various applications. A few of them are listed below –

• Azeotropes are used as standards in testing gas chromatographs, detectors, and columns. 

• Azeotropes are used for the separation of constituents of zeotropic mixtures. For example, it is very difficult to separate pure acetic acid from a solution of acetic acid and water. As we know, ethyl acetate forms an azeotrope with water and boils at 70.40C. So, by adding ethyl acetate in the mixture of acetic acid and water as an entertainer (solvent), it is possible to distil away the azeotrope (ethyl acetate and water) and leave nearly pure acetic acid in the distillation flask. As the boiling point of acetic acid is 118.10C.