Azeotropes - Example, Types, Applications, Minimum and Maximum boiling azeotrope

An azeotrope is the mixture of the vapor phase and liquid phase that exhibits the same concentration. In simple words, it is a liquid containing a mixture of two liquids that contain the same concentration of liquid phase and vapor phase. Azeotropes are known to have either a low boiling point or high boiling point for one of their components. This liquid mixture also has a constant boiling point, and the fraction of mixed liquids cannot be altered by simple distillation.

The explanation behind the formation of azeotropes is because whenever an azeotrope is boiled, the vapor formed is observed to have the exact composition of its initial constituents as that of the liquid phase. This helps to conclude the fact that simple distillation does not affect the constituents of an azeotrope.

In comparison to ideal solutions, azeotropes do not conform to Raoult's law. According to Raoult's law, when several substances are mixed with, it is observed that the solvent's partial vapor pressure is equivalent to or similar to the vapor pressure of the pure solvent multiplied by the mole fraction in the solution. However, azeotropes exist in a solution at only a specific boiling point for any specific components.

With the help of the figure, we can understand that all azeotropes exist only at one single temperature. This is because, at different temperatures, the composition of its constituent also changes. It has to be noted that azeotropes are considered to be binary solutions. Here, only one single solute and one single solvent are utilized in the mixture. For instance, azeotrope ethyl alcohol contains a mixture of ethyl alcohol and water.

Types of azeotropes

There are types of azeotrope such as:-

1. Minimum boiling azeotrope
If the azeotropes show positive deviation through Raoult's law, it is known as a minimum boiling azeotrope. It is also most commonly known as a positive boiling azeotrope. It is also known that this positive azeotrope or minimum boiling azeotropes contain a boiling point lower than the boiling point of all of its constituents. Minimum boiling azeotropes are formed when the mixture has a lower boiling point and higher vapor point.
An example of the minimum boiling azeotrope is a large concentration of ethanol and a lower concentration of water.

2. Maximum boiling azeotrope
Maximum boiling azeotropes are defined as the liquid mixtures that demonstrate a negative deviation through Raoult's law. Therefore, it is also called a negative azeotrope. Maximum boiling azeotrope is formed when the mixture has lower vapor pressure and the highest boiling point.

3. Homogeneous Azeotrope
If the constituents of an azeotropic mixture are completely miscible 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. One example of a homogeneous azeotropic mixture includes water mixed with ethanol.

4. Heterogeneous Azeotrope
If the azeotropes are present as a constituent in several mixtures and if these mixtures are not entirely miscible, they are considered heterogeneous azeotropes. These heterogeneous azeotropes are incompletely miscible and form two liquid phases. Examples of heterogeneous azeotropes include mixtures such as chloroform and water, benzene, and water.

5. Binary Azeotrope
Azeotropes that contain precisely two constituents as their components are called binary azeotropes. Examples of binary azeotrope include the mixture of diethyl ether (33%) and halothane (66%). This azeotropic mixture is most commonly used in medical fields as anesthesia.

6. Ternary Azeotrope
Azeotropes which contain three constituents as their components are called ternary azeotropes. Examples of ternary azeotrope include the azeotropic mixture of acetone, methanol, and chloroform.

Application of Azeotrope

Some applications of azeotropes include:-

1. It is mainly used as a standard in several fields, such as testing chromatographs, detectors, and columns.
2. It is also utilized 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. It is observed to boil at 70.4℃. Thus, 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, i.e., ethyl acetate and water, and leave approximately almost pure acetic acid in the distillation flask. The boiling point of acetic acid is 118.1℃, due to which it is left behind in the distillation flask.