Azeotropes

What are Azeotropes? 

Azeotropes are mixtures of at least two liquids which exhibit the same concentration at liquid phase and vapor 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 higher or lower boiling point than another component. In these mixtures, components exhibit the same mole fraction in liquid phase and vapor 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 25K. Another condition for fractional distillation is that components of the mixture must not have equal mole fractions at liquid phase and vapor phase. Fractional distillation is used to separate different gases from 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. 

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Thus, 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 liquid state in the azeotropic mixture. For example, in azeotropic mixture of ethyl alcohol and water, both are taken in 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 is mixed. 

  • Azeotropic mixture solution boils at constant temperature. For example, azeotropic mixture of ethyl alcohol (96%) and water (4%) boils at 78.1

  • Separation of constituents of azeotropic mixture by fractional distillation is not possible. For example, as mole fraction of ethyl alcohol is same at liquid phase and vapor phase and mole fraction of water is also same at liquid phase and vapor phase so the azeotropic solution of ethanol and water boils at 78.1 and cannot be separated by fractional distillation. 

Types of Azeotropes 

Azeotropes can be divided into 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 Roult’s Law at specific composition are called positive azeotropes. They show boiling points lower than boiling points of its constituents. That’s why they are also called minimum boiling azeotropes. These types of mixtures exhibit highest vapor pressure and lowest boiling point. For example, 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 –

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This azeotropic mixture boils at 78.2 while water boils at 100 and ethanol at 78.5. 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 Roult’s Law at specific composition are called negative azeotropes. They show boiling points higher than boiling points of its constituents. That’s why they are also called maximum boiling azeotropes. These types of mixtures exhibit lower vapor pressure and highest boiling point. For example, azeotropic mixture of hydrogen chloride and water is negative azeotrope. It shows a large negative deviation from Raoult’s law. It is represented by the graph below –

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This azeotropic mixture boils at 110 while water boils at 100 and HCl at -84. 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 large deviation from Raoult’s law. This leads to the formation of a minimum boiling heterogeneous azeotrope which has a vapor 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, 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 53.3 while boiling point of water is 100℃ and chloroform is 61.2. Graphical representation of heterogeneous azeotrope is given below –

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In the above graph, the vertical axis is temperature (T), 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 which 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 which 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. 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 70.4. So, by adding ethyl acetate in the mixture of acetic acid and water as an entrainer (solvent), it is possible to distill away the azeotrope (ethyl acetate and water) and leave nearly pure acetic acid in the distillation flask. As boiling point of acetic acid is 118.1

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