Anhydrous
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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
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
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
Gases
Several substances that exist as gases at
- 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.
See also
- Air-free technique
- Acidic oxide, a.k.a. acid anhydride
- Base anhydride
- Hydrate, a [chemical] substance that contains water or its constituent elements
References
- PMID 20945830.
- ^ "Guidelines for solvent purification at UC Davis". Archived from the original on September 4, 2006.
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: 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.