What is Azeotrope?

An azeotrope is a mixture of two or more liquids which displays the same level of concentration in the liquid and vapour phase. Simple distillation cannot alter their proportions. These mixtures can either have a lower boiling point or a higher boiling point of the components.

An azeotrope is a mixture that, at a given pressure (the azeotropic pressure), boils at a constant temperature (the azeotropic temperature), and has the same composition (azeotropic composition) in the equilibrium vapour and liquid phases.


Azeotrope Mixture Distillation Curve

They are termed as constant boiling mixtures as their composition remains unchanged by distillation. That is the constitutions stays in the same proportion even when the azeotrope is boiled. When distillation cannot alter a fraction of a liquid, it results in azeotropes.

Azeotrope Mixture

An azeotropic compound is a mixture of two substances which distil at the same temperature. An azeotrope is a mixture of compounds with the same composition in the vapour as in the liquid. In other words, an azeotrope is a liquid mixture that has a constant boiling point and whose vapour has the same composition as the liquid. Several compounds, such as toluene, benzene, and cyclohexane, form suitable azeotropes with water.

Azeotrope Distillation

Azeotropic distillation as on early and important special distillation process is commonly used in laboratory and industry. Azeotropic distillation is accomplished by adding to the liquid phase a volatile third component which changes the volatility of one of the two components more than the other so that the components are separated by distillation.

  • An azeotropic distillation performed in a laboratory is normally done in batches in a conventional fractional distillation column. In such a case, an excess of the usually required amount of the azeotropic agent is added with the charge to the still.
  • The mixture is then distilled in the same manner as a conventional fractional distillation. If the mixture of binary compound and azeotrope is a minimum boiling point type then the distillate product should contain only one of the original components and the azeotropic agent.
  • The composition of the distillate product then changes as the distillation proceeds since the separation taking place is between the different azeotropes.
  • Then the distillation should proceed until all other components have been removed as a distillate product leaving the bottom product as the remaining component and the azeotropic agent.

Types of Azeotropes

1. Heterogeneous and Homogeneous Azeotropes

If azeotropes are found in constitutions of mixtures and if they are not completely miscible then they are termed as heterogeneous azeotropes. Homogeneous Azeotropes are azeotropes in which the constitutions of a mixture are miscible completely. Hetero Azeotropic distillation is comprised of two liquid phases.

2. Positive Azeotropes

Azeotropic Mixture which has a higher boiling point than its constituents is termed as positive azeotropes. For instance consider a positive azeotrope, ethanol consisting of a concentration of around ninety-five percent and four percent of water in terms of weight.

Water boils at hundred degree Celsius, and ethanol boils at around seventy-eight-degree Celsius, whereas azeotropes boil at around seventy-eight-degree celsius indicating that it has a lower boiling point than its constituents.

3. Negative Azeotropes

Azeotropic mixtures with less boiling point then its constitutions is termed as negative azeotropes.  For instance consider a negative azeotrope, hydrochloric acid consisting of a concentration of around twenty percent and seventy-nine percent of water in terms of weight.

Water boils at hundred degree Celsius and hydrochloric acid boil at around minus eight four degree Celsius, whereas azeotropes boil at around seventy-eight degree Celsius indicating that it has a higher boiling point than its constituents.

4. Binary Azeotropes

Binary Azeotropes are azeotropes consisting of two constitutions as mentioned above illustration. Ternary Azeotropes are azeotropes consisting of more than three constitutions.

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