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.
Desiccator
Desiccators are sealable enclosures containing desiccants used for preserving moisture-sensitive items such as cobalt chloride paper for another use. A common use for desiccators is to protect chemicals which are hygroscopic or which react with w ...
s can also be used to store reagents in dry conditions.
Common
desiccants include
phosphorus pentoxide
Phosphorus pentoxide is a chemical compound with molecular formula P4 O10 (with its common name derived from its empirical formula, P2O5). This white crystalline solid is the anhydride of phosphoric acid. It is a powerful desiccant and dehydra ...
and
silica gel
Silica gel is an amorphous and porous form of silicon dioxide (silica), consisting of an irregular tridimensional framework of alternating silicon and oxygen atoms with nanometer-scale voids and pores. The voids may contain water or some other ...
.
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
The Grignard reaction () is an organometallic chemical reaction in which alkyl, allyl, vinyl, or aryl-magnesium halides (Grignard reagent) is added to a carbonyl group in an aldehyde or ketone. This reaction is important for the formation of ...
and the
Wurtz reaction
In organic chemistry, the Wurtz reaction, named after Charles Adolphe Wurtz, is a coupling reaction whereby two alkyl halides are treated with sodium metal to form a higher alkane.
: 2 R−X + 2 Na → R−R + 2 NaX
The reaction is of little v ...
.
Solvents have typically been dried using distillation or by reaction with reactive metals or
metal hydride
In chemistry, a hydride is formally the anion of hydrogen( H−). The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms are called hydrides: water (H2O) is a hydride of oxygen, ammonia is a hydride of ...
s. These methods can be dangerous and are a common cause of lab fires. More modern techniques include the use of
molecular sieve
A molecular sieve is a material with pores (very small holes) of uniform size. These pore diameters are similar in size to small molecules, and thus large molecules cannot enter or be adsorbed, while smaller molecules can. As a mixture of molecu ...
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. 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.
Anhydrous solvents are commercially available from chemical suppliers, and are packaged in sealed containers to maintain dryness. 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
Magnesium sulfate or magnesium sulphate (in English-speaking countries other than the US) is a chemical compound, a salt with the formula , consisting of magnesium cations (20.19% by mass) and sulfate anions . It is a white crystalline solid, s ...
or a similar drying agent to remove most remaining water.
Anhydrous acetic acid is known as
glacial acetic acid
Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main componen ...
.
Gases
Several substances that exist as gases at
standard conditions of temperature and pressure
Standard temperature and pressure (STP) are standard sets of conditions for experimental measurements to be established to allow comparisons to be made between different sets of data. The most used standards are those of the International Union ...
are commonly used as concentrated
aqueous solution
An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be ...
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
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...
is generally referred to as ''anhydrous'' ammonia, to distinguish it from its solution in water, household
ammonia solution
Ammonia solution, also known as ammonia water, ammonium hydroxide, ammoniacal liquor, ammonia liquor, aqua ammonia, aqueous ammonia, or (inaccurately) ammonia, is a solution of ammonia in water. It can be denoted by the symbols NH3(aq). Although ...
, also known as ammonium hydroxide.
*Gaseous
hydrogen chloride
The compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colourless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hydrogen chloride g ...
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
upDean–Stark apparatus setup for an azeotrope with a solvent whose density is ''less than'' that of water:
The Dean–Stark apparatus, Dean–Stark receiver, distilling trap, or Dean–Stark Head is a piece of laboratory glassware used in syn ...
.
See also
*
Air-free technique Air-free techniques refer to a range of manipulations in the chemistry laboratory for the handling of compounds that are air-sensitive. These techniques prevent the compounds from reacting with components of air, usually water and oxygen; less comm ...
*
Acidic oxide
An acidic oxide is an oxide that either produces an acidic solution upon addition to water, or acts as an acceptor of hydroxide ions effectively functioning as a Lewis acid. Acidic oxides will typically have a low pKa and may be inorganic or org ...
, a.k.a. acid anhydride
*
Base anhydride
A base anhydride is an oxide of a chemical element from group 1 or 2 (the alkali metals and alkaline earth metals, respectively). They are obtained by removing water from the corresponding hydroxide base. If water is added to a base anhydride, a ...
*
Hydrate, a
hemicalsubstance that contains water or its constituent elements
References
{{Reflist
Chemical properties