Everything about Azeotrope totally explained
An
azeotrope is a mixture of two or more pure compounds (chemicals) in such a ratio that its composition can't be changed by simple
distillation. This is because when an azeotrope is boiled, the resulting vapor has the same ratio of constituents as the original mixture of liquids. As the composition is unchanged by boiling, azeotropes are also known as
constant boiling mixtures (especially in older texts). The word azeotrope is derived from the Greek words "ζειν"=boil and "τρόπος"=change, combining with prefix "α-"=no to give the overall meaning "no change on boiling".
Types of azeotropes
Each azeotrope has a characteristic
boiling point. The boiling point of an azeotrope is either less than the boiling points of any of its constituents (a positive azeotrope), or greater than the boiling point of any of its constituents (a negative azeotrope).
A well known example of a positive azeotrope is 95.6%
ethanol and 4.4% water (by weight). Ethanol boils at 78.4°C, water boils at 100°C, but the azeotrope boils at 78.1°C, which is lower than either of its constituents. Indeed 78.1°C is the minimum temperature at which any ethanol/water solution can boil. It is generally true that a positive azeotrope boils at a lower temperature than any other ratio of its constituents. Positive azeotropes are also called
minimum boiling mixtures.
An example of a negative azeotrope is 20.2%
hydrogen chloride and 79.8% water (by weight). Hydrogen chloride boils at –84°C and water at 100°C, but the azeotrope boils at 110°C, which is higher than either of its constituents. Indeed 110°C is the maximum temperature at which any hydrochloric acid solution can boil. It is generally true that a negative azeotrope boils at a higher temperature than any other ratio of its constituents. Negative azeotropes are also called
maximum boiling mixtures.
Azeotropes consisting of two constituents, such as the two examples above, are called
binary azeotropes. Those consisting of three constituents are called
ternary azeotropes. Azeotropes of more than three constituents are also known.
More than 18,000 azeotropic mixtures have been documented.
If two solvents can form a negative azeotrope, then distillation of any mixture of those constituents will result in the
residue being closer in composition to the azeotrope than the original mixture. For example, if a
hydrochloric acid solution contains less than 20.2%
hydrogen chloride, boiling the mixture will leave behind a solution that's richer in hydrogen chloride than the original. If the solution initially contains more than 20.2% hydrogen chloride, then boiling will leave behind a solution that's poorer in hydrogen chloride than the original. Boiling of any hydrochloric acid solution long enough will cause the solution left behind to approach the azeotropic ratio.
Phase diagrams
The boiling and recondensation of a mixture of two solvents are changes of
state. As such, they're best illustrated with a
phase diagram. If pressure is held constant, the two parameters that can vary are the temperature and the composition.
The diagram on the right shows a positive azeotrope of hypothetical constituents, X and Y. The bottom trace illustrates the boiling temperature at various compositions. Below the bottom trace, the mixture must be entirely liquid phase. The top trace illustrates the condensation temperature of various compositions. Above the top trace the mixture must be entirely vapor phase. Between the two traces, liquid and vapor phases exist simultaneously. The azeotrope is the point on the diagram where the two curves touch. The horizontal and vertical steps show the path of repeated distillations. Point A is the boiling point of a nonazeotropic mixture. The vapor is separated at the same temperature at point B. Since B is vapor only, it must occur at the top trace. The shape of the curves requires that the vapor at B be richer in constituent X than the liquid at point A. Such a system of solvents is known as a
heteroazeotrope. The diagram illustrates how the various phases of a heteroazeotrope are related.
Heteroazeotropes are always minimum boiling mixtures.
Raoult's law predicts the
vapor pressures of
ideal mixtures as a function of composition ratio. In general only mixtures of chemically similar solvents, such as
n-hexane with
n-heptane, form nearly ideal mixtures that come close to obeying Raoult's law. Solvent combinations that can form azeotropes are always nonideal, and as such they deviate from Raoult's law.
The diagram on the right illustrates total vapor pressure of three hypothetical mixtures of constituents, X, and Y. The temperature throughout the plot is assumed to be constant.
The center trace is a straight line, which is what Raoult's law predicts for an ideal mixture. The top trace illustrates a nonideal mixture that has a positive deviation from Raoult's law, where the total combined vapor pressure of constituents, X and Y, is greater than what is predicted by Raoult's law. The top trace deviates sufficiently that there's a point on the curve where its
tangent is horizontal. Whenever a mixture has a positive deviation and has a point at which the tangent is horizontal, the composition at that point is a positive azeotrope. At that point the total vapor pressure is at a maximum. Likewise the bottom trace illustrates a nonideal mixture that has a negative deviation from Raoult's law, and at the composition where tangent to the trace is horizontal there's a negative azeotrope. This is also the point where total vapor pressure is minimum.
A more compelling reason for believing that azeotropes are not compounds is, as discussed in the last section, that the composition of an azeotrope can be affected by pressure. Contrast that with a true compound, carbon dioxide for example, which is two moles of oxygen for each mole of carbon no matter what pressure the gas is observed at. That azeotropic composition can be affected by pressure suggests a means by which such a mixture can be separated.
Pressure swing distillation
A hypothetical azeotrope of constituents X and Y is shown in the diagram to the right. Two plots are shown, one at low pressure and one at high pressure. The composition of the azeotrope is substantially different between the high and low pressure plots. The goal is to separate Y in as high a concentration as possible starting from point, A. At the low pressure, it's possible by progressive distillation to reach a distillate at the point, B, which is on the same side of the azeotrope as A. If that distillate is exposed to the high pressure, it boils at point, C. From C, by progressive distillation it's possible to reach a distillate at the point, D, which is on the same side of the high pressure azeotrope as C. If that distillate is then exposed again to the low pressure, it boils at point, E, which is on the
opposite side of the low pressure azeotrope as A. So by means of the pressure swings it was possible to cross over the low pressure azeotrope.
When the solution is boiled at point, E, the distillate is poorer in Y than point E. This means that the residue is made richer in Y than point E. Indeed progressive distillations can result in a residue that's as rich in Y as you like.
A mixture of 5% water with 95%
tetrahydrofuran is an example of an azeotrope that can be economically separated using a pressure swing — a swing in this case between 1
atm and 8
atm. By contrast the composition of the water/ethanol azeotrope discussed earlier isn't affected enough by pressure to be easily separated using pressure swings. Just enough cyclohexane is added to the water/ethanol azeotrope to engage all of the water into the ternary azeotrope. When the mixture is then boiled, the azeotrope vaporizes leaving a residue composed almost entirely of the excess ethanol.
Anhydrous calcium chloride is used as a desiccant for drying a wide variety of solvents since it's inexpensive and doesn't react with most
nonaqueous solvents.
Chloroform is an example of a solvent that can be effectively dried using calcium chloride.
Examples of azeotropes
Proportions are by weight.
See
azeotrope (data) for tables of many more azeotropes.
Further Information
Get more info on 'Azeotrope'.
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