An azeotropic mixture or a constant boiling point mixture forms from a mixture of substances that have the same concentration of vapor and fluid phases.
Azeotropic Mixture Explained
An Azeotrope is basically a mixture whose proportion cannot be changed by distillation. This happens because when the mixture is boiled, it produces the vapor, and the proportion of the constituents in this vapor is the same as the unboiled mixture.
For this reason, these mixtures are also known as constant boiling point mixtures. It is not possible to separate the components of the mixture using fractional distillation in such a case.
Related Topic: Difference Between Mixture and Compound
Types of Azeotropic Mixture
Azeotropes are primarily of two types:
Minimum Boiling Azeotrope
These azeotropic mixtures have a boiling point lesser than their respective constituents. They are also known as positive azeotropes or pressure maximum azeotropes.
An example is 95.63% ethanol and 4.37% water (by mass), which boils at 78.2℃. Ethanol’s boiling point is 78.4℃, and water has a boiling point of 100℃ but the azeotrope boils at 78.2℃.
Maximum Boiling Azeotrope
These mixtures have a boiling point greater than their respective constituents. They are also known as negative azeotropes or pressure minimum azeotropes.
An example is hydrochloric acid at a concentration of 20.2% and 7.8% water (by mass), with a boiling point of 110℃. HCl has a boiling point of -84℃ and water has a boiling point of 100℃.
Homogeneous azeotropes are azeotropes whose constituents are completely miscible.
An example of a homogeneous azeotrope is any amount of ethanol and any amount of water.
Heterogeneous azeotropes are present in mixture constitutions and are not fully miscible.
An example of a heterogeneous azeotrope is equal volumes of chloroform and water.
Based on the Number of Constituents
Azeotropic mixtures that consist of two constituents are called binary azeotropes, whereas azeotropes consisting of three constituents are called ternary azeotropes.
An example of a binary azeotrope is a mixture of diethyl ether(33%) and halothane(66%).
An example of a ternary azeotrope is a mixture of acetone, methanol, and chloroform.
Separation Process of Azeotropes
There are different distillation methods that are used for the separation of azeotropes. They are discussed below.
Azeotropic Distillation: Entrainer is used, which is a material that changes the stability of one of the azeotrope components. The result is a heterogeneous azeotrope.
Pressure Swing Distillation: In this method, the pressure is changed in order to modify the mixture composition and enrich the distillate with the desired portion.
Pervaporation: This method separates the constituents with the help of a membrane that will be permeable to only one material compared to the other.
Distillation using a dissolved salt: Generally, a salt upon dissolving in a solvent raises the boiling point of that solvent. In this method, a salt that is readily soluble in one constituent but not in another is used.
Applications of Azeotropic Mixture
- Azeotropes are also used to separate the constituents of zeotropic mixtures.
- Azeotropic mixtures find applications as standards in testing detectors, gas chromatographs, and columns.
An azeotropic mixture is a mixture of substances that have the same concentration of vapor and fluid phases. These mixtures are generally not separable using simple or fractional distillation.
The different types of azeotropic mixtures are:
– Positive or negative azeotropes
– Homogeneous or heterogeneous azeotropes
– Binary or ternary azeotropes
Azeotropic mixture of HCl and water is an example of a negative azeotrope. The boiling point of water is 100℃ while for hydrochloric acid is -8℃ but their azeotropic mixture boils at 78℃ which indicates that the boiling point is greater than the boiling points of its constituents. An azeotropic mixture of HCl and water has 20.2% HCl.