Azeotrope
The term was coined in 1911 by English chemist John Wade[8] and Richard William Merriman.
[9] Because their composition is unchanged by distillation, azeotropes are also called (especially in older texts) constant boiling point mixtures.
A solution that shows greater positive deviation from Raoult's law forms a minimum boiling azeotrope at a specific composition.
In general, a positive azeotrope boils at a lower temperature than any other ratio of its constituents.
A well-known example of a positive azeotrope is an ethanol–water mixture (obtained by fermentation of sugars) consisting of 95.63% ethanol and 4.37% water (by mass), which boils at 78.2 °C.
The boiling and recondensation of a mixture of two solvents are changes of chemical state; as such, they are best illustrated with a phase diagram.
The shape of the curves requires that the vapor at B be richer in constituent X than the liquid at point A.
[2] The vapor is physically separated from the VLE (vapor-liquid equilibrium) system and is cooled to point C, where it condenses.
A solution that shows large negative deviation from Raoult's law forms a maximum boiling azeotrope at a specific composition.
This azeotrope has an approximate composition of 68% nitric acid and 32% water by mass, with a boiling point of 393.5 K (120.4 °C).
In general, a negative azeotrope boils at a higher temperature than any other ratio of its constituents.
An example of a negative azeotrope is hydrochloric acid at a concentration of 20.2% and 79.8% water (by mass).
Other examples: The adjacent diagram shows a negative azeotrope of ideal constituents, X and Y.
Because this process has removed a greater fraction of Y from the liquid than it had originally, the residue must be poorer in Y and richer in X after distillation than before.
In other words: Raoult's law predicts the vapor pressures of ideal mixtures as a function of composition ratio.
In this case because the molecules in the mixture are sticking together more than in the pure constituents, they are more reluctant to escape the stuck-together liquid phase.
[2] When the deviation is great enough to cause a local maxima or minima in the vapor pressure versus mole fraction graph (i.e.
[16] The adjacent diagram illustrates total vapor pressure of three hypothetical mixtures of constituents, X, and Y.
The center trace is a straight line, which is what Raoult's law predicts for an ideal mixture.
The top trace deviates sufficiently that there is a point on the curve where its tangent is horizontal.
Further repeated distillations will produce mixtures that are progressively closer to the azeotropic ratio of 95.5/4.5%.
[20] Distillation is one of the primary tools that chemists and chemical engineers use to separate mixtures into their constituents.
[13] Indeed, this difficulty led some early investigators to believe that azeotropes were actually compounds of their constituents.
That azeotropic composition can be affected by pressure suggests a means by which such a mixture can be separated.
The composition of the azeotrope is substantially different between the high- and low-pressure plots: higher in X for the high-pressure system.
By contrast the composition of the water to ethanol azeotrope discussed earlier is not affected enough by pressure to be easily separated using pressure swings[13] and instead, an entrainer may be added that either modifies the azeotropic composition and exhibits immiscibility with one of the components, or extractive distillation may be used.
[22] Other methods of separation involve introducing an additional agent, called an entrainer, that will affect the volatility of one of the azeotrope constituents more than another.
When the mixture is then boiled, the azeotrope vaporizes leaving a residue composed almost entirely of the excess ethanol.
In this way, for example, it is possible to break the water/ethanol azeotrope by dissolving potassium acetate in it and distilling the result.
For example, the azeotrope of 20% acetone with 80% chloroform can be broken by adding water and distilling the result.
