Azeotrope
2-propanol/water Equilibrium with an azeotrope
An azeotrope is a mixture of two or more liquids (chemicals) in such a ratio that its
composition cannot be changed by simple distillation. This occurs because, when an
azeotrope is boiled, the resulting vapor has the same ratio of constituents as the original
mixture. Because their composition is unchanged by distillation, azeotropes are also
called constant boiling mixtures. The word azeotrope is derived from the Greek word
meaning, “no change on boiling.”
Types
Positive and negative azeotropes
Each azeotrope has a characteristic boiling point. The boiling point temperature of an
azeotrope is either less than the boiling point temperatures of any of its constituents (a
positive azeotrope), or greater than the boiling point temperatures of any of its
constituents (a negative azeotrope).
A well known example of a positive azeotrope is 95.63% ethanol and 4.37% water (by
weight). Ethanol boils at 78.4°C, water boils at 100°C, but the azeotrope boils at 78.2°C,
which is lower than either of its constituents. Indeed 78.2°C is the minimum temperature
at which any ethanol/water solution can boil at atmospheric pressure. In general, a
positive azeotrope boils at a lower temperature than any other ratio of its constituents.
Positive azeotropes are also called minimum boiling mixtures or pressure maximum
azeotrope. An example of a negative azeotrope is hydrochloric acid at a concentration of
20.2% 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.
The maximum temperature at which any hydrochloric acid solution can boil is 110°C. In
general, a negative azeotrope boils at a higher temperature than any other ratio of its
constituents. Negative azeotropes are also called maximum boiling mixtures or
pressure minimum azeotrope.
Homogeneous and heterogeneous azeotropes
If the constituents of a mixture are not completely miscible an azeotrope can be found
inside the miscibility gap. This type of azeotrope is called heterogeneous azeotrope. If the
azeotropic composition is outside the miscibility gap or the constituents of the mixture
are completely miscible the type of azeotrope is called homogeneous azeotrope.
Distillation of mixtures
If two solvents can form a positive azeotrope, then distillation of any mixture of those
constituents will result in the distillate being closer in composition to the azeotrope than
the starting mixture. For example, if a 50/50 mixture of ethanol and water is distilled
once, the distillate will be 80% ethanol and 20% water, which is closer to the azeotropic
mixture than the original. Distilling the 80/20% mixture produces a distillate that is 87%
ethanol and 13% water. Further repeated distillations will produce mixtures that are
progressively closer to the azeotropic ratio of 95.5/4.5%.
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 is 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 is 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.
Examples of azeotropes
Proportions are by weight.
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nitric acid (68%) / water, boils at 120.5°C at 1 atm (negative azeotrope)
hydrofluoric acid (35.6%) / water, boils at 111.35°C (negative azeotrope)
ethanol (96%) / water, boils at 78.1°C
sulfuric acid (98.3%) / water, boils at 338°C
acetone / methanol / chloroform form an intermediate boiling (saddle) azeotrope
Diethyl ether (33%) / halothane (66%) a mixture once commonly used in
anesthesia.
Benzene / hexafluorobenzene form a double binary azeotrope.
A mixture is the blending of two or more dissimilar substances that do not chemically
combine to form compounds and that can typically be separated by non-chemical means.
Mixtures can be classified into three types: suspension mixture, colloidal mixture or
solution, according to how they combine and can be separated.
Suspension mixture
A suspension mixture is usually created by stirring together two or more ingredients,
where the particles are typically large enough to be seen by the unaided eye or a
magnifying glass. The ingredients of a suspension mixture are heterogeneous, meaning
that they are not evening distributed throughout. Most mixtures are suspension mixtures.
Solid-solid mix
Many suspension mixtures consist of solids mixed with solids. Cake mix is an example of
visible solid particles mixed together by a means of stirring. Dirt or soil is another
example of a solid-solid suspension mixture.
These mixtures can be separated by sifting. Sometimes shaking will cause the heavier
particles to settle to the bottom.
Solid-fluid mix
If solid particles are mixed in a liquid or gas to form a suspension mixture, the
ingredients will soon separate, with the heavier solid particles settling at the bottom. For
example, if you mixed sand and water, the sand would soon sink to the bottom.
If the solid particles are lighter than the liquid--as in the case of sawdust mixed in water-they will separate and float to the top.
A major part of air pollution consists of smoke and dust particles mixed within the
atmosphere. This is a suspension mixture. After a while, the solid particles will settle to
the ground.
Besides settling, filtration can also be used to separate the ingredients.
Fluid-fluid mix
If visible globules of a liquid are mixed in a liquid or gas solvent, the ingredients will
soon separate. If the globules are heavier, they will settle at the bottom. If the globules
are lighter, they will float to the top.
Colloidal mixture
A colloidal mixture is a homogeneous combination of solid or liquid particles mixed
within a liquid or gas solvent.