Desiccant

Canisters are commonly filled with silica gel and other molecular sieves used as desiccant in drug containers to keep contents dry; shown here next to a U.S. quarter for size comparison.

Silica gel

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. A humectant is also a hygroscopic substance, but it is used to help a product retain water. Both are frequently used in food and cosmetic products. Glycerine is often used as a humectant.

Commonly encountered pre-packaged desiccants are solids which 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 desiccants are most commonly used to remove excessive humidity that would normally degrade or even destroy certain products which are sensitive to moisture. Some commonly used desiccants are: silica gel, activated charcoal, calcium sulfate, calcium chloride, montmorillonite clay, and molecular sieves.

Applications

One manufacturing example of desiccant usage would be in the manufacturing of insulated windows where it is mixed with the edge spacer between the panes of glass. This helps to prevent the accumulation of moisture condensation between the panes.

The most common use of desiccants are in packaged foods.

Chemistry

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.

Desiccants induce dryness in any environment and reduce the amount of moisture present in air. Desiccants come in various forms and have found widespread use in the food, pharmaceuticals, packing, electronics and many manufacturing industries.

Performance efficiency

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.

Coloured saturation indicators

Indicating Silica Gel

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.

Drying of solvents

Toluene is refluxed with sodium and benzophenone to produce dry, oxygen-free toluene. The toluene is dry and oxygen free when the intense blue coloration from the benzophenone ketyl radical is observed.

See also: air-free technique

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.

Types of desiccants

Main article: List of desiccants

See also

• Desiccator
• Humidity buffering
• Humidity indicator card
• Hygroscopy
• Solar air conditioning
• Sorbent
• Zeolites

Further reading

• Chai, Christina Li Lin; Armarego, W. L. F. (2003). Purification of laboratory chemicals. Oxford: Butterworth-Heinemann. ISBN 0-7506-7571-3. 
• Lavan, Z.; Jean-Baptiste Monnier, Worek, W. M. (1982). "Second Law Analysis of Desiccant Cooling Systems". Journal of Solar Energy Engineering 104 (3): 229–236. doi:10.1115/1.3266307. 
• S. Sadik; J. W. White (1982). "True potato seed drying over rice". Potato Research 25 (3): 269. doi:10.1007/BF02357312. 

External links

• Desiccant Science and Tehnology Published Paper
• Desiccant Requirement Chart (printable)
• Latest US desiccants patents information.
• A Desiccant Requirements Calculator
• Combating Moisture and Humidity in Nutraceutical Packaging
• Education Center
• Liquid Desiccant Waterfall for attractive building dehumidification
• Information on Desiccant Dryers
• Moisture In Electronics