A desiccant is a hygroscopic substance that induces or sustains a state of dryness (desiccation) in its local vicinity in a moderately well-sealed container.
Commonly encountered pre-packaged desiccants are solids, and work through absorption or adsorption of water, or a combination of the two. Desiccants for specialized purposes may be in forms other than solid, and may work through other principles, such as chemical bonding of water molecules.
Pre-packaged desiccant is most commonly used to remove excessive humidity that would normally degrade or even destroy products sensitive to moisture. Some commonly used desiccants are: silica gel, activated charcoal, calcium sulfate, calcium chloride, montmorillonite clay, and molecular sieves. Silica gel is often used in musical instrument cases.
A specific usage of a desiccant would be in insulated windows where it is placed inside the spacer between the air space and window edge. This prevents moisture condensation between the panes.
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A desiccant may be chosen for a particular task based not only on its effectiveness at drying, but also perhaps for any natural antibiotic, fungicidal, pesticidal, or virucidal effect, or for a lack of harmful effect on humans. For instance, table salt has a natural antibiotic and anti-fungal effect and is not harmful to humans when consumed in moderate quantities, and is thus commonly used as a drying agent in the preparation of dried foods.
Because of their common use in preservation, it is usually desirable for a desiccant to be substantially chemically stable or chemically inert, and a number of useful desiccants with this property have been identified and are in common use, such as silica gel, chalk and clay-based desiccants.
One measure of desiccant efficiency is the ratio (or percentage) of water storable in the desiccant, relative to the mass of desiccant.
Another measure is the residual relative humidity of the air or other fluid being dried.
The performance of any desiccant varies with temperature and both relative humidity and absolute humidity. To some extent, desiccant performance can be precisely described, but most commonly, the final choice of which desiccant best suits a given situation, how much of it to use, and in what form, is made based on testing and practical experience.
Often some sort of humidity indicator is included in the desiccant to show, by color changes, the degree of water-saturation of the desiccant. One commonly used indicator is cobalt chloride (CoCl2). Anhydrous cobalt chloride is blue. When it bonds with two water molecules, (CoCl2•2H2O), it turns purple. Further hydration results in the pink hexaaquacobalt(II) chloride complex [Co(H2O)6]Cl2.
Desiccants are also used to remove water from solvents, typically used for moisture free reactions (e.g. the Grignard reaction). The method generally, though not always, involves stirring the solvent with the desiccant.
Often, the desiccant will react with moisture to form an insoluble solid that can be removed by filtration. When a very pure solvent is required, a distillation still is used to remove moisture and other impurities simultaneously.