What is an azeotrope? – Definition and examples

Chemists John Wade and Richard William Merriman coined the word “azeotropic” in a 1911 paper describing the behaviour of ethanol-water mixtures. The term comes from the Greek works for “boiling” and “turning”, with the prefix a- (not), meaning “no change of boiling”. In contrast, a zeotrope is a mixture of liquids that have different boiling points and can be separated by distillation.

An azeotrope is a mixture that boils at a constant temperature and has the same composition in its liquid and vapour phases.

Types of azeotropes

Azeotropes are classified according to the number of components, whether they are homogeneous or heterogeneous, and whether their boiling point is higher or lower than that of the components.

• Binary and ternary azeotropes: A binary azeotrope is an azeotrope consisting of two components. A ternary azeotrope consists of three components. There are also azeotropes composed of more than three components.
• Homogeneous and heterogeneous azeotropes: A homogeneous azeotrope consists of miscible liquids. Ethanol and water form a homogeneous azeotrope. A heterogeneous azeotrope consists of immiscible liquids that separate into two phases. Chloroform and water form a heterogeneous azeotropic mixture. The upper layer is mainly water with a small amount of dissolved chloroform, while the lower layer is mainly chloroform with a small amount of dissolved water. When the two layers are boiled together, the resulting vapour consists of 97% chloroform and 3% water, regardless of the proportion of liquid.
• Positive and negative azeotropes: A positive azeotrope o minimum boiling point azeotrope has lower boiling points than its constituents. For example, a zeotopic mixture of ethanol and water (approximately 96% ethanol and 4% water) boils at 78.174°C, while pure ethanol boils at 78.3°C and pure water boils at 100°C. A negative azeotrope o maximum boiling point azeotrope has a higher boiling point than its components. The chloride of hydrogen chloride and water form a negative azeotrope. The azeotropic mixture boils at 110 ° C, while water boils at 100 ° C and HCl boils at -85 ° C.

Why you can’t distil 100% ethanol

For example, you cannot distil a mixture of ethanol (grain alcohol) and water to obtain pure alcohol because the two compounds form an azeotropic mixture. The best you can get is approximately 95.6% ethanol.

Let’s say you start with a mixture containing some alcohol in water. If you distil it, collect the vapour and condense it as a liquid, you will have an alcohol-enriched mixture. You can repeat the process until you get a mixture that is 95.6% ethanol and 0.4% water. Then, you hit a wall because the azeotrope vapour is identical to its liquid composition. Essentially, an azeotropic mixture boils as if it were a pure liquid.

Azeotrope uses

One use of azeotropes is to separate zeotropic mixtures more easily. For example, the acetic acid and water form a zeotopic mixture. But acetic acid has a boiling point (118.1 °C) too close to that of water for efficient distillation. The addition of ethyl acetate forms an azeotrope with water with an azeotropic boiling point of 70,4 ° C. The ethyl acetate acts as a carry-over agent, so that water and ethyl acetate evaporate, leaving almost pure acetic acid.

Azeotropes are also used as standards for testing gas detectors and gas chromatographs.

How to separate azeotropes

Although simple distillation cannot separate the components of an azeotropic mixture, there are other methods used to isolate the components.

• Pressure change distillation uses pressure changes to change the composition of a mixture, enriching the distillate with the desired component.
• A carry-over agent alters the volatility of one of the azeotropic components. Sometimes the carrier reacts with a component to form a non-volatile compound. Distillation using a carrier is called azeotropic distillation.
• Pervaporation separates the components by means of a membrane that is more permeable to one component than the other. Vapour permeation is a related technique, which uses a membrane that is more permeable to the vapour phase of one component than the other.