Azeotropic Mixture

Last Updated : 17 Dec, 2023

Azeotropic Mixture is a mixture of two or more liquids with a similar boiling point and a similar composition in their vapor phase. Unlike most mixtures, an azeotropic mixture has a constant boiling point and composition during the vaporization process. In this article, we will study the different concepts related to Azeotropic mixture in detail.

Table of Content

What is Azeotropic Mixture?
Examples of Azeotropic Mixture
Types of Azeotropic Mixture
Ideal Solution Vs Azeotropes
Differentiate Between Positive and Negative Azeotropes
Separation Techniques for Azeotropic Mixture

What is Azeotropic Mixture?

Azeotropic Mixture is a mixture of two or more liquids with a similar boiling point and a similar composition in their vapor phase. Azeotropic mixture either has a higher or lower boiling point than its components.

Each component in a typical mixture may have a different boiling point, and when the mixture heats up, the components will vaporize at various rates.
On the other hand, at a given temperature and pressure, the composition of the liquid phase and vapor phase in an azeotropic mixture is identical.
The mixture consequently has a constant boiling point. Therefore, Azeotropic mixtures are also known as Constant Boiling Point Mixtures.

Examples of Azeotropic Mixture

Some examples of Azeotropic mixtures are,

Water-Ethanol (95.6% ethanol, 4.4% water)
Acetone-Chloroform (29.3% acetone, 70.7% chloroform)
Sulfuric Acid-Water (98.3% sulfuric acid, 1.7% water)
Hydrochloric Acid-Water (20.2% hydrochloric acid, 79.8% water)

Types of Azeotropic Mixture

Azeotropic mixtures are made up of one solute and one solvent. Both the solvent and the solute are in the liquid phase. Both the solute and the solvent have a fixed composition. The boiling point of azeotropic mixtures is fixed. There are two main types of Azeotropic Mixture that are,

Positive Azeotropes
Negative Azeotropes

Positive Azeotropes

Azeotropic mixtures with a lower boiling point than their constituents are known as Positive Azeotropes. They are also known as Maximum Boiling Azeotropes. Some examples of Positive Azeotropes are:

Acetone-Methanol (8.4% acetone, 91.6% methanol)
Chloroform-Ethanol (9.6% chloroform, 90.4% ethanol)
Toluene-Cyclohexane (67.2% toluene, 32.8% cyclohexane)

Negative Azeotropes

Azeotropic mixtures with a higher boiling point than their constituents are known as Negative Azeotropes. They are also known as Minimum Boiling Azeotropes. Some examples of Negative Azeotropes include:

Water-Ethanol (95.6% ethanol, 4.4% water)
Acetic Acid-Water (64.4% acetic acid, 35.6% water)
Sulfuric Acid-Water (98.3% sulfuric acid, 1.7% water)

Homogeneous and Heterogeneous Azeotropes

Azeotropic mixture, where its constituents are completely miscible (capable of being mixed), is known as homogeneous azeotropic mixtures. Examples include:

Toluene-Methanol (8.6% toluene, 91.4% methanol)
Water-Ethanol (95.6% ethanol, 4.4% water)

While Azeotropic mixture, where its constituents are not completely miscible (capable of being mixed), is known as heterogeneous azeotropic mixtures. Examples include:

Formic Acid-Water (approx. 90.4% formic acid in liquid, 74.4% formic acid in vapor)
Acetic Acid-Water (approx. 99.4% acetic acid in liquid, 80.4% acetic acid in vapor)

Ideal Solution Vs Azeotropes

The differences between Ideal Solution and Azeotropes are explained in the table added below,

Difference Between Ideal Solution and Azeotropes
Characteristics	Ideal Solution	Azeotropes
Definition	A mixture that obeys Raoult’s law for all components over the entire range of composition and temperature.	A mixture of two or more substances that has a constant boiling point and composition.
Raoult’s Law	Obeys Raoult’s Law	At Least one Component violate Raoult’s Law.
Separation	Components can be separated by simple distillation process.	Special process like Azeotropic distillation is used.
Boiling Point	The boiling point of each component in the ideal solution is affected by its own vapor pressure.	Azeotropes have a constant boiling point, different from the boiling points of individual components.
Example	Toluene and Benzene	Ethanol-water azeotrope

Differentiate Between Positive and Negative Azeotropes

Some differences between positive and negative azeotropes are added in the table below as,

Positive Azeotropes Vs Negative Azeotropes
Characteristics	Positive Azeotrope	Negative Azeotrope
Boiling Point	Higher than the components’ boiling points.	Lower than the components’ boiling points.
Raoult’s Law Deviation	Positive Deviation	Negative Deviation
Phase Diagram Appearance	Distillation curve shows a maximum boiling point	Distillation curve shows a minimum boiling point
Temperature Changes During Distillation	Temperature rises during distillation	Temperature decreases during distillation
Example	Acetone-Methanol (8.4% acetone, 91.6% methanol)	Water-Ethanol (95.6% ethanol, 4.4% water)

Separation Techniques for Azeotropic Mixture

Distillation cannot separate an azeotropic mixture into its constituents as constituents in the vapour phase have a similar proportion to that of the normal liquid phase azeotrope. Constituents of an azeotropic mixture boil at the same temperature, making it difficult to separate the mixture into its constituents.

Special distillation techniques like azeotropic distillation can separate the azeotropic mixtures which is discussed in the later part of article.

Azeotropic Distillation

Azeotropic distillation is a separation process used to break or overcome the azeotropic behavior of mixtures during distillation. Azeotropic distillation involves the addition of an entrainer or an extractive agent to alter the vapor-liquid equilibrium and facilitate the separation of components. Process of Azeotropic Distillation is discussed below:

The primary goal of azeotropic distillation is to separate the components of an azeotropic mixture, where traditional distillation is ineffective.
An entrainer or extractive agent is a third component added to the azeotropic mixture to disrupt the azeotropic behavior.
The entrainer forms a ternary mixture with the original components. This changes the vapor-liquid equilibrium, breaking the azeotropic behavior.
A fractional distillation column is often used in azeotropic distillation.
The column provides multiple theoretical plates for separation, allowing for better control over the vapor and liquid compositions at various stages of the distillation process.
Factors such as temperature, pressure, and the choice of entrainer impact the success of the separation.

Applications of Azeotropes

Some applications of Azeotropes are added below as,

Azeotropes are used as standards in testing gas chromatographs, detectors, and columns.
Azeotropes are used for the separation of constituents of zeotropic mixtures.
Azeotropes find extensive use in distillation and fractionation processes.
Certain azeotropic mixtures have suitable properties for use as refrigerants or working fluids in air conditioning and refrigeration systems.
Azeotropic mixtures can be utilized in polymerization reactions and polymer processing.

Sample Questions on Azeotropic Mixture

Here are some sample question on Azeotropic mixtures that include,

Q1: What are Azeotropic Mixtures?

Azeotropic mixture is a mixture of two or more liquids with a similar boiling point and a similar composition in their vapor phase. Azeotropic mixture either has a higher or lower boiling point than its components.

Examples of Azeotropic Mixture:

Water-Ethanol (95.6% ethanol, 4.4% water)

Acetone-Chloroform (29.3% acetone, 70.7% chloroform)

Sulfuric Acid-Water (98.3% sulfuric acid, 1.7% water)

Hydrochloric Acid-Water (20.2% hydrochloric acid, 79.8% water)

Q2: What are Type of Azeotropic Mixture?

There are following main types of Azeotropic Mixture:

Positive Azeotropes

Negative Azeotropes

Homogeneous and Heterogeneous Azeotropes

Q3: Explain Concept of Constant Boiling Azeotropes.

Because their composition is unchanged by distillation, azeotropes are also called constant boiling point mixtures.

Q4: Why is Distillation Challenging for Azeotropic Mixtures?

In the case of azeotropic mixtures, distillation becomes challenging due to the unique behavior of these mixtures, specifically their constant boiling points and vapor compositions at the azeotropic point. Here are the primary reasons why distillation is challenging for azeotropic mixtures:

Constant Boiling Point

Failure of Raoult’s Law

Vapor-Liquid Composition Equality

Read More,