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A substance is anhydrous if it contains no water. Many processes in chemistry can be impeded by the presence of water; therefore, it is important that water-free reagents and techniques are used. In practice, however, it is very difficult to achieve perfect dryness; anhydrous compounds gradually absorb water from the atmosphere so they must be stored carefully.

Solids

Many salts and solids can be dried using heat, or under vacuum. Desiccators can also be used to store reagents in dry conditions.

Common desiccants include phosphorus pentoxide and silica gel. Chemists may also require dry glassware for sensitive reactions. This can be achieved by drying glassware in an oven, by flame, or under vacuum.

Dry solids can be produced by freeze-drying, which is also known as lyophilization.

Liquids or solvents

In many cases, the presence of water can prevent a reaction from happening, or cause undesirable products to form. To prevent this, anhydrous solvents must be used when performing certain reactions. Examples of reactions requiring the use of anhydrous solvents are the Grignard reaction and the Wurtz reaction.

Solvents have typically been dried using distillation or by reaction with reactive metals or metal hydrides. These methods can be dangerous and are a common cause of lab fires. More modern techniques include the use of molecular sieves or a column purification system. Molecular sieves are far more effective than most common methods for drying solvents and are safer and require no special equipment for handling.^[1] Column solvent purification devices (generally referred to as Grubb's columns) recently became available, reducing the hazards (water reactive substances, heat) from the classical dehydrating methods.^[2]^[3]

Anhydrous solvents are commercially available from chemical suppliers, and are packaged in sealed containers to maintain dryness.^[4] Typically anhydrous solvents will contain approximately 10 ppm of water and will increase in wetness if they are not properly stored.

Organic solutions can be dried using a range of drying agents. Typically following a workup the organic extract is dried using magnesium sulfate or a similar drying agent to remove most remaining water.^[5]

Anhydrous acetic acid is known as glacial acetic acid.

Gases

Several substances that exist as gases at standard conditions of temperature and pressure are commonly used as concentrated aqueous solutions. To clarify that it is the gaseous form that is being referred to, the term anhydrous is prefixed to the name of the substance:

Gaseous ammonia is generally referred to as anhydrous ammonia, to distinguish it from its solution in water, household ammonia solution, also known as ammonium hydroxide.
Gaseous hydrogen chloride is generally referred to as anhydrous, to distinguish it from its solution in water, hydrochloric acid.

Reactions which produce water can be kept dry using a Dean–Stark apparatus.

References

^ Williams, D. Bradley G.; Lawton, Michelle (2010-12-17). "Drying of Organic Solvents: Quantitative Evaluation of the Efficiency of Several Desiccants". The Journal of Organic Chemistry. 75 (24): 8351–8354. doi:10.1021/jo101589h. ISSN 0022-3263. PMID 20945830.
^ "Guidelines for solvent purification at UC Davis". Archived from the original on September 4, 2006.{{cite web}}: CS1 maint: unfit URL (link)
^ "Drying Solvents".
^ "New Aldrich Sure/Seal® packaging for Anhydrous Solvents and Air-Sensitive Reagents". Sigma-Aldrich. Retrieved 2018-03-26.
^ "Drying agents". www.chem.ucla.edu. Retrieved 2018-03-26.

[1] Williams, D. Bradley G.; Lawton, Michelle (2010-12-17). "Drying of Organic Solvents: Quantitative Evaluation of the Efficiency of Several Desiccants". The Journal of Organic Chemistry. 75 (24): 8351–8354. doi:10.1021/jo101589h. ISSN 0022-3263. PMID 20945830.

[grubb-2] "Guidelines for solvent purification at UC Davis". Archived from the original on September 4, 2006.{{cite web}}: CS1 maint: unfit URL (link)

[3] "Drying Solvents".

[4] "New Aldrich Sure/Seal® packaging for Anhydrous Solvents and Air-Sensitive Reagents". Sigma-Aldrich. Retrieved 2018-03-26.

[5] "Drying agents". www.chem.ucla.edu. Retrieved 2018-03-26.

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[2]

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@@ Line 1: / Line 1: @@
+{{short description|Waterless substance}}
-{{Refimprove|date=June 2010}}
+{{More citations needed|date=June 2010}}
+A substance is '''anhydrous''' if it contains no [[water]]. Many processes in chemistry can be impeded by the presence of water; therefore, it is important that water-free reagents and techniques are used. In practice, however, it is very difficult to achieve perfect dryness; anhydrous compounds gradually absorb water from the atmosphere so they must be stored carefully.
+==Solids==
-A substance is '''anhydrous''' if it contains no [[water]], for example, salts lacking their water of crystallisation.<ref>{{cite book|last=Daintith|first=edited by John|title=A dictionary of chemistry|date=2008|publisher=Oxford University Press|location=Oxford|isbn=9780199204632|page=37|edition=6th ed.}}</ref>
+Many salts and solids can be dried using heat, or under vacuum. [[Desiccator]]s can also be used to store reagents in dry conditions.
+Common [[desiccant]]s include [[phosphorus pentoxide]] and [[silica gel]]. Chemists may also require dry glassware for sensitive reactions. This can be achieved by drying glassware in an oven, by flame, or under vacuum.
+Dry solids can be produced by [[freeze-drying]], which is also known as lyophilization.
-The way of achieving the anhydrous form differs from one substance to another. That is to say, anhydrous matter is prepared or used as a form with no water, and specifically the form of a chemical without [[water of crystallization]].
+==Liquids or solvents==
-==Solvents==
+In many cases, the presence of water can prevent a reaction from happening, or cause undesirable products to form. To prevent this, anhydrous solvents must be used when performing certain reactions. Examples of reactions requiring the use of anhydrous solvents are the [[Grignard reaction]] and the [[Wurtz reaction]].
-{{See also|Air-free technique#Associated preparations}}
-In many cases, the presence of water can prevent a reaction from happening, or form undesirable products. To prevent this, anhydrous solvents must be used when performing certain reactions. Examples of reactions requiring the use of anhydrous solvents are the [[Grignard reaction]] and the [[Wurtz reaction]].
-Solvents are commonly rendered anhydrous by boiling them in the presence of a [[hygroscopic]] substance; metallic [[sodium]] is one of the most common metals used. Other methods include the addition of [[molecular sieve]]s or alkali bases such as [[potassium hydroxide]] or [[barium oxide]]. Column solvent purification devices (generally referred to as Grubb's columns) recently became available, reducing the hazards (water reactive substances, heat) from the classical dehydrating methods.<ref name="grubb">[http://fire.ucdavis.edu/main/ucdfdinfobull22.htm Guidelines for solvent purification at UC Davis]{{Dead link|date=June 2010}}</ref> <ref>{{cite web|title=Drying Solvents|url=http://chemwiki.ucdavis.edu/Reference/Lab_Techniques/Distillation/Drying_Solvents|work=UCDavis Chemwiki|accessdate=8 April 2014}}</ref>
+Solvents have typically been dried using distillation or by reaction with reactive metals or [[metal hydride]]s. These methods can be dangerous and are a common cause of lab fires. More modern techniques include the use of [[molecular sieve]]s or a column purification system. Molecular sieves are far more effective than most common methods for drying solvents and are safer and require no special equipment for handling.<ref>{{cite journal|last=Williams|first=D. Bradley G.|last2=Lawton|first2=Michelle|date=2010-12-17|title=Drying of Organic Solvents: Quantitative Evaluation of the Efficiency of Several Desiccants|journal=The Journal of Organic Chemistry|volume=75|issue=24|pages=8351–8354|doi=10.1021/jo101589h|pmid=20945830|issn=0022-3263}}</ref> Column solvent purification devices (generally referred to as Grubb's columns) recently became available, reducing the hazards (water reactive substances, heat) from the classical dehydrating methods.<ref name="grubb">{{cite web |title=Guidelines for solvent purification at UC Davis |url=http://fire.ucdavis.edu/main/ucdfdinfobull22.htm |url-status=unfit |archive-url=https://web.archive.org/web/20060904103225/http://fire.ucdavis.edu/main/ucdfdinfobull22.htm |archive-date=September 4, 2006}}</ref><ref>{{cite web|title=Drying Solvents|url=https://chem.libretexts.org/Ancillary_Materials/Demos%2C_Techniques%2C_and_Experiments/General_Lab_Techniques/Drying_Solvents}}</ref>
+Anhydrous solvents are commercially available from chemical suppliers, and are packaged in sealed containers to maintain dryness.<ref>{{cite news|url=https://www.sigmaaldrich.com/chemistry/solvents/sureseal.html|title=New Aldrich Sure/Seal® packaging for Anhydrous Solvents and Air-Sensitive Reagents|work=Sigma-Aldrich|access-date=2018-03-26|language=en}}</ref> Typically anhydrous solvents will contain approximately 10 ppm of water and will increase in wetness if they are not properly stored.
+Organic solutions can be dried using a range of [[desiccant|drying agents]]. Typically following a workup the organic extract is dried using [[magnesium sulfate]] or a similar drying agent to remove most remaining water.<ref>{{cite web|url=http://www.chem.ucla.edu/~bacher/Specialtopics/Drying%20Agents.html|title=Drying agents|website=www.chem.ucla.edu|access-date=2018-03-26}}</ref>
+Anhydrous acetic acid is known as [[glacial acetic acid]].
 ==Gases==
-Several substances that exist as gases at [[standard conditions of temperature and pressure]] are commonly used as concentrated aqueous solutions. To clarify that it is the gaseous form that is being referred to, the term ''anhydrous'' is prefixed to the name of the substance:
+Several substances that exist as gases at [[standard conditions of temperature and pressure]] are commonly used as concentrated [[aqueous solution]]s. To clarify that it is the gaseous form that is being referred to, the term ''anhydrous'' is prefixed to the name of the substance:
+*Gaseous [[ammonia]] is generally referred to as ''anhydrous'' ammonia, to distinguish it from its solution in water, household [[ammonia solution]], also known as ammonium hydroxide.
+*Gaseous [[hydrogen chloride]] is generally referred to as ''anhydrous'', to distinguish it from its solution in water, [[hydrochloric acid]].
+Reactions which produce water can be kept dry using a [[Dean–Stark apparatus]].
-* gaseous [[ammonia]] is generally referred to as ''anhydrous'' ammonia to distinguish it from household ammonia, which is an [[ammonium hydroxide]] [[aqueous solution]]
-* gaseous [[hydrogen chloride]] is generally referred to as ''anhydrous'' to distinguish it from the more commonly used 37% w/w solution in water
 ==See also==
-* [[Air-free technique]]
+*[[Air-free technique]]
+*[[Acidic oxide]], a.k.a. acid anhydride
+*[[Base anhydride]]
+*[[Hydrate]], a [chemical] substance that contains water or its constituent elements
 ==References==
 {{Reflist}}