An azeotrope is a mixture of two or more liquids with a constant boiling point. The vapor composition and liquid composition are identical, so the components of the mixture cannot be separated by simple distillation. The mixture proportions forming an azeotropic are called the azeotropic composition. The temperature the liquid boils (at a given pressure) is the azeotropic temperature. The azeotropic temperature can be either higher or lower than the boiling points of the mixture components.
Chemists John Wade and Richard William Merriman coined the word “azeotrope” in a 1911 paper describing the behavior of ethanol and water mixtures. The term comes from the Greek works for “boil” and “turning,” with the prefix a- (no), meaning “no change from 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 vapor 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 made of more than three constituents.
- 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 top layer is mostly water with a small amount of dissolved chloroform, while the bottom layer is mostly chloroform with a small amount of dissolved water. When the two layers are boiled together, the resulting vapor consists of 97% chloroform and 3% water, regardless of the liquid ratio.
- Positive and Negative Azeotropes: A positive azeotrope or minimum boiling point azeotrope has a lower boiling points than its constituents. For example, a zeotropic mixture of ethanol and water (about 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 or maximum boiling point azeotrope has a higher boiling point than its components. 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 Distill 100% Ethanol
For example, you can’t distill a mixture of ethanol (grain alcohol) and water to get pure alcohol because the two compounds form an azeotropic mixture. The best you can get is about 95.6% ethanol.
Let’s say you start with a mixture that contains some alcohol in water. If you distill it, collect the vapor, and condense it as a liquid, you’ll have a mixture that is enriched in alcohol. You can repeat the process until you reach a mixture that is 95.6% ethanol and 0.4% water. Then, you hit a wall because the azeotrope’s vapor is identical to its liquid composition. Essentially, an azeotropic mixture boils as if it was a pure liquid.
Azeotrope Uses
One use of azeotropes is to separate zeotropic mixtures more easily. For example, acetic acid and water form a zeotropic mixture. But, acetic acid has a boiling point (118.1 °C) too close to that of water for effective distillation. Adding ethyl acetate forms an azeotrope with water with an azeotropic boiling point of 70.4 °C. Ethyl acetate acts as an entrainer, so the water and ethyl acetate boil away, leaving nearly pure acetic acid.
Azeotropes also find use as standards to test detectors and gas chromatographs.
How to Separate Azeotropes
Although simple distillation cannot separate components of an azeotropic mixture, there are other methods used to isolate consituents.
- Pressure swing distillation uses pressure changes to change the composition of a mixture, enriching the distillate with the desired component.
- An entrainer alters the volatility of one of the azeotrope components. Sometimes, the entrainer reacts with a component to form a nonvolatile compound. Distillation using an entrainer is called azeotropic distillation.
- Pervaporation separates components using a membrane that is more permeable to one constituent than the other. Vapor permeation is a related technique, using a membrane more permeable to the vapor phase of one component than another.
References
- Morrison, Robert Thornton; Boyd, Robert Neilson (1972). Organic Chemistry (2nd ed.). Allyn and Bacon.
- Petrucci, Harwood; Herring, Madura (2007). General Chemistry: Principles & Modern Applications (9th ed.). Upper Saddle River, NJ: Pearson Education, Inc.
- Rousseau, Ronald W.; James R. Fair (1987). Handbook of Separation Process Technology. Wiley-IEEE. ISBN 978-0-471-89558-9.
- Wade, John; Merriman, Richard William (1911). “CIV.—Influence of Water on the Boiling Point of Ethyl Alcohol at Pressures above and Below the Atmospheric Pressure.” Journal of the Chemical Society, Transactions 99: 997–1011. doi:10.1039/CT9119900997