A substance, such as calcium oxide or silica gel, that has a high
affinity for water and is used as a drying agent.
A substance that promotes drying (e.g., calcium oxide absorbs water
and is used to remove moisture) .
A substance (adsorbant) used to withdraw moisture from other materials.
Although the removal of large quantities of water is done by evaporation,
aided by moving air currents and by elevated temperature, the last
traces of moisture are often held very tightly and do not evaporate
readily. Furthermore, evaporation ceases when the moisture content
of the material is reduced to that of the drying-air current. For
final drying, a desiccant is used. It may react with water chemically
or retain water through capillarity of adsorption. The drying agent
is placed directly into the gas or liquid to be dried; solid materials
are placed in a desiccator, a closed vessel in which moisture diffuses
to the desiccant through the dry desiccator atmosphere. A desiccant
loses potency as it takes on water; often it can be renewed by heating.
Desiccants which form hydrates can be selected to maintain certain
levels of low humidity in a closed vessel. See also Adsorption;
Among the more important types of solid desiccants are silica gel,
activated alumina, anhydrous calcium sulfate, magnesium perchlorate,
oxides (of barium and calcium), and activated carbon.