Editing Azeotrope (section)

== Types ==

=== Positive azeotropes ===
[[File:Positive Azeotrope.png|thumb|left|Positive azeotrope – mixture of [[chloroform]] and [[methanol]]]]
[[file:positiveAzeotropePhaseDiagram.png|thumb|Phase diagram of a positive azeotrope. Vertical axis is temperature, horizontal axis is composition]]

A [[solution (chemistry)|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. Positive azeotropes are also called ''minimum boiling mixtures'' or ''pressure maximum azeotropes''. A well-known example of a positive azeotrope is an [[ethanol]]&ndash;water mixture (obtained by [[fermentation]] of sugars) consisting of 95.63% [[ethanol]] and 4.37% water (by mass), which boils at 78.2&nbsp;°C.<ref>{{Cite web |last=National Institute of Standards and Technology |title=Standard Reference Material 1828: Ethanol-Water Solutions |url=http://ts.nist.gov/MeasurementServices/ReferenceMaterials/upload/1828.pdf |url-status=dead |archive-url=https://web.archive.org/web/20110608142735/http://ts.nist.gov/MeasurementServices/ReferenceMaterials/upload/1828.pdf |archive-date=8 June 2011}}</ref> Ethanol boils at 78.4&nbsp;°C, water boils at 100&nbsp;°C, but the azeotrope boils at 78.2&nbsp;°C, which is lower than either of its constituents.<ref>{{Cite book |last=Rousseau |first=Ronald W. |title=Handbook of separation process technology |last2=James R. Fair |publisher=Wiley-IEEE |year=1987 |isbn=978-0-471-89558-9 |pages=261–262}}</ref> Indeed, 78.2&nbsp;°C is the minimum temperature at which any ethanol/water solution can boil at atmospheric pressure. Once this composition has been achieved, the liquid and vapour have the same composition, and no further separation occurs.

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]]. If the pressure is held constant, the two variable parameters are the temperature and the composition.

The adjacent diagram shows a positive azeotrope of hypothetical constituents, X and Y. The bottom trace illustrates the boiling temperature of various compositions. Below the bottom trace, only the liquid phase is in equilibrium. The top trace illustrates the vapor composition above the liquid at a given temperature. Above the top trace, only the vapor is in equilibrium. Between the two traces, liquid and vapor phases exist simultaneously in equilibrium: for example, heating a 25% X : 75% Y mixture to temperature AB would generate vapor of composition B over liquid of composition A. 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 that separates at that temperature has composition B. The shape of the curves requires that the vapor at B be richer in constituent X than the liquid at point A.<ref name="moore" /> The vapor is physically separated from the VLE (vapor-liquid equilibrium) system and is cooled to point C, where it condenses. The resulting liquid (point C) is now richer in X than it was at point A. If the collected liquid is boiled again, it progresses to point D, and so on. The stepwise progression shows how repeated distillation can never produce a distillate that is richer in constituent X than the azeotrope. Note that starting to the right of the azeotrope point results in the same stepwise process closing in on the azeotrope point from the other direction.

=== Negative azeotropes ===
[[File:Negative Azeotrope.png|thumb|left|Negative azeotrope – mixture of [[formic acid]] and water]]
[[File:negativeAzeotropePhaseDiagram.png|thumb|Phase diagram of a negative azeotrope. Vertical axis is temperature, horizontal axis is composition]]

A solution that shows large negative deviation from [[Raoult's law]] forms a maximum boiling azeotrope at a specific composition. [[Nitric acid]] and water is an example of this class of azeotrope. This azeotrope has an approximate composition of 68% [[nitric acid]] and 32% water [[mass percent|by mass]], with a [[boiling point]] of {{convert|393.5|K|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 azeotropes''. An example of a negative azeotrope is [[hydrochloric acid]] at a concentration of 20.2% and 79.8% water (by mass). Hydrogen chloride boils at −85&nbsp;°C and water at 100&nbsp;°C, but the azeotrope boils at 110&nbsp;°C, which is higher than either of its constituents. The maximum boiling point of any hydrochloric acid solution is 110&nbsp;°C. Other examples:

* [[hydrofluoric acid]] (35.6%) / water, boils at 111.35&nbsp;°C
* [[nitric acid]] (68%) / [[water (molecule)|water]], boils at 120.2&nbsp;°C at 1 atm 
* [[perchloric acid]] (71.6%) / water, boils at 203&nbsp;°C 
* [[sulfuric acid]] (98.3%) / water, boils at 338&nbsp;°C

The adjacent diagram shows a negative azeotrope of ideal constituents, X and Y. Again the bottom trace illustrates the boiling temperature at various compositions, and again, below the bottom trace the mixture must be entirely liquid phase. The top trace again illustrates the condensation temperature of various compositions, and again, above the top trace the mixture must be entirely vapor phase. The point, A, shown here is a boiling point with a composition chosen very near to the azeotrope. The vapor is collected at the same temperature at point B. That vapor is cooled, condensed, and collected at point C. Because this example is a negative azeotrope rather than a positive one, the distillate is farther from the azeotrope than the original liquid mixture at point A was. So the distillate is poorer in constituent X and richer in constituent Y than the original mixture. 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.

If the point, A had been chosen to the right of the azeotrope rather than to the left, the distillate at point C would be farther to the right than A, which is to say that the distillate would be richer in X and poorer in Y than the original mixture. So in this case too, the distillate moves away from the azeotrope and the residue moves toward it. This is characteristic of negative azeotropes. No amount of distillation, however, can make either the distillate or the residue arrive on the opposite side of the azeotrope from the original mixture. This is characteristic of all azeotropes.

=== Double azeotropes ===
[[file:Benzene+Hexafluorobenzene (Double Azeotrope).png|thumb|Double azeotrope of benzene and hexafluorobenzene. Proportions are by weight]]

Also more complex azeotropes exist, which comprise both a minimum-boiling and a maximum-boiling point. Such a system is called a double azeotrope, and will have two azeotropic compositions and boiling points. An example is water and ''N''-methylethylenediamine<ref>{{Cite web |last=Olson|first=John D. |title=Thermodynamics of Hydrogen Bonding Mixtures 4. |url=http://www.symp14.nist.gov/bin/AQU01OLS.PDF |url-status=dead |archive-url=https://web.archive.org/web/20080916084546/http://www.symp14.nist.gov/bin/AQU01OLS.PDF |archive-date=16 September 2008 |access-date=27 January 2008}}</ref> as well as [[benzene]] and [[hexafluorobenzene]].

=== Complex systems ===
[[file:Saddle Azeotrope.png|thumb|Saddle azeotropic system Methanol/Acetone/Chloroform calculated with mod. [[UNIFAC]]]]

Some azeotropes fit into neither the positive nor negative categories. The best known of these is the ternary azeotrope formed by 30% [[acetone]], 47% [[chloroform]], and 23% [[methanol]], which boils at 57.5&nbsp;°C. Each pair of these constituents forms a binary azeotrope, but chloroform/methanol and acetone/methanol both form positive azeotropes while chloroform/acetone forms a negative azeotrope. The resulting ternary azeotrope is neither positive nor negative. Its boiling point falls between the boiling points of acetone and chloroform, so it is neither a maximum nor a minimum boiling point. This type of system is called a [[saddle surface|saddle]] azeotrope.<ref name="hilmen" /> Only systems of three or more constituents can form saddle azeotropes.