Azeotrope

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An azeotrope is a special homogenous mixture of 2 or more compounds (molecules). The ratio of the compounds, say in an azeotrope consisting of two compounds X and Y, is exactly the same in both the vapour form of the mixture, X:Y as in the liquid phase.

Due to the uniformity of liquid and vapor, chemical composition of the azeotrope cannot be changed by simple boiling (distillation). This constant composition will (almost always) occur at a minimum or maximum temperature over the composition space. The exception for binary mixtures is the rare double azeotrope. The exception for three or more components is the intermediate boiling azeotrope (saddle azeotrope). For example, if there are 3 parts of liquid x to every 4 parts of liquid y, then you could not separate the two by boiling them because of the rates at which they boil.

In order to enrich a mixture past the azeotrope (or "break" the azeotrope), one may use several techniques:

1. Use azeotropic distillation, by adding in a third component which will generate a lower boiling heterogeneous azeotrope that can be broken by phase separating the two immiscible liquids and decanting.
2. Use extractive distillation where a third compound is added near the top of a distillation column that reduces the volatility of one compound over another. For example, if you wanted to separate water from a type of alcohol by boiling, but they were at their azeotropic mixture, if you added salt to the mixture that would raise the boiling point of the water so you could boil more of the alcohol out of it.
3. Use liquid-liquid extraction to separate the compounds by segregating them into two different liquid phases.
4. Use a membrane to separate them, as in pervaporation.
5. For azeotropes that change composition when pressure is changed, you can use pressure swing distillation by operating two distillation columns at different pressures, and recycle material between these two distillation columns. One column will remove a pure component (two-component systems) in the bottom of one distillation column, the other will remove a pure component in the bottom of a second distillation column, and the azeotrope (of two different compositions) will be fed to the other column.

The word azeotrope comes from the Greek "zein tropos", or "constant boiling". An azeotrope is said to be positive if the constant boiling point is at a temperature maximum, and negative when the boiling point is at a temperature minimum. The vast majority of azeotropes are minimum boiling. All liquid mixtures which are immiscible and which form azeotropes are minimum boiling .

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In other words, solutions showing positive deviation (from Raoult's law) have an intermediate composition for which they have maximum vapor pressure and minimum boiling point. At this composition, which is called the minimum boiling azeotrope, the solution boils at constant temperature without any change in its composition.

Similarly, solutions showing negative deviation have an intermediate composition for which solutions have minimum vapor pressure and maximum boiling point. At this composition, the maximum boiling azeotrope, the solution also boils at constant temperature without any change in its composition.

[edit] Examples of azeotropes

• nitric acid (68%) / water, boils at 120.5°C at 1 atm
• perchloric acid (28.4%) / water, boils at 203°C (negative azeotrope)
• hydrofluoric acid (35.6%) / water, boils at 111.35°C (negative azeotrope)
• ethanol (96%) / water, boils at 78.2°C
• sulfuric acid (98.3%) / water, boils at 100°C
• acetone / methanol / chloroform form an intermediate boiling azeotrope
• diethyl ether (33%) / halothane (66%) a mixture once commonly used in anaesthesia.
• benzene / hexafluorobenzene forms a double binary azeotrope.

[edit] See also

• Azeotropic distillation
• Azeotrope (data)
• Eutectic
• Batch Distillation
• Heteroazeotrope
• Ebulliometer

[edit] External links

• Ponton, Jack (September 2001). Azeotrope Databank (Queriable database). The Edinburgh Collection of Open Software for Simulation and Education, Edinburgh University. Retrieved on 9 April 2007.
• Dr G. A. Busato; Professor G. Bashein (2004). "Update in Anaesthesia: The halothane/ether azeotrope - A reconsideration". Pharmacology (18) Retrieved on 9 April 2007. 
• Azeotrope defined with a limerick.
• Publications about the separation of the azeotropes. BUTE Department of Process Engineering, Hungary. Retrieved on 9 April 2007.