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a substance that promotes drying (e.g., calcium oxide absorbs water and is used to remove moisture) (同)drying_agent, drier, siccative

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(薬品など)乾燥力のある / 乾燥剤

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/08/26 15:10:22」(JST)

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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 in a sachet or porous packet

A desiccant is a hygroscopic substance that induces or sustains a state of dryness (desiccation) in its vicinity. Commonly encountered pre-packaged desiccants are solids that adsorb water. Desiccants for specialized purposes may be in forms other than solid, and may work through other principles, such as chemical bonding of water molecules. They are commonly encountered in foods to retain crispness. Industrially, desiccants are widely used to control the level of water in gas streams.

Types of desiccants

Desiccants are chemically stable or chemically inert, and a number of useful desiccants with this property have been identified and are in common use. The most common desiccant is silica, an otherwise inert, nontoxic, water-insoluble white solid. Tens of thousands of tons are produced annually for this purpose.^[1] Other desiccants include activated charcoal, calcium sulfate (Drierite),^[2] calcium chloride, and molecular sieves (typically, zeolites).

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 a 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.

Applications

One example of desiccant usage is in the manufacture of insulated windows where zeolite spheroids fill a rectangular spacer tube at the perimeter of the panes of glass. The desiccant helps to prevent the condensation of moisture between the panes. Another use of zeolites is in the dryer component of air conditioning systems to help maintain the efficacy of the refrigerant. Desiccants are also commonly used to protect goods in shipping containers against moisture damage. Hygroscopic cargo, such as cocoa, coffee, and various nuts and grains, are particularly susceptible to mold and rot when exposed to condensation and humidity. Because of this, shippers often take precautionary measures to protect against cargo loss.

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.

Air conditioning systems can be made based on desiccants.^[3]

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.

Desiccants are also used to remove water from solvents, typically required by chemical reactions that do not tolerate water, e.g., the Grignard reaction. The method generally, though not always, involves mixing the solvent with the solid desiccant. The dried solvent is then separated from the desiccant by filtration or distillation.^[4]^[5]

References

^ Otto W. Flörke, et al. "Silica" in Ullmann's Encyclopedia of Industrial Chemistry, 2008, Weinheim: Wiley-VCH, . doi:10.1002/14356007.a23_583.pub3.
^ http://www.drierite.com
^ Daou, K; Wang, Xia (2005). "Desiccant cooling air conditioning: a review". Renewable and Sustainable Energy Reviews 10 (2): 55–77. doi:10.1016/j.rser.2004.09.010.
^ Chai, Christina Li Lin; Armarego, W. L. F. (2003). Purification of laboratory chemicals. Oxford: Butterworth-Heinemann. ISBN 0-7506-7571-3.
^ Williams, D. B. G., Lawton, M., "Drying of Organic Solvents: Quantitative Evaluation of the Efficiency of Several Desiccants", The Journal of Organic Chemistry 2010, vol. 75, 8351. doi: 10.1021/jo101589h

External links

Drierite Desiccants
Desiccant Requirement Chart (printable)
A Desiccant Requirements Calculator
Information on Desiccant Dryers
Moisture In Electronics

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English Journal

Highly selective water adsorption in a lanthanum metal-organic framework.

Plessius R1, Kromhout R, Ramos AL, Ferbinteanu M, Mittelmeijer-Hazeleger MC, Krishna R, Rothenberg G, Tanase S.
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We present a new metal-organic framework (MOF) built from lanthanum and pyrazine-2,5-dicarboxylate (pyzdc) ions. This MOF, [La(pyzdc)1.5 (H2 O)2 ]⋅2 H2 O, is microporous, with 1D channels that easily accommodate water molecules. Its framework is highly robust to dehydration/hydration cycles. Unu
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Controlling the catalytic aerobic oxidation of phenols.

Esguerra KV1, Fall Y, Petitjean L, Lumb JP.
Journal of the American Chemical Society.J Am Chem Soc.2014 May 28;136(21):7662-8. doi: 10.1021/ja501789x. Epub 2014 May 16.
The oxidation of phenols is the subject of extensive investigation, but there are few catalytic aerobic examples that are chemo- and regioselective. Here we describe conditions for the ortho-oxygenation or oxidative coupling of phenols under copper (Cu)-catalyzed aerobic conditions that give rise to
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Synergistic interaction between the fungus Beauveria bassiana and desiccant dusts applied against poultry red mites (Dermanyssus gallinae).

Steenberg T1, Kilpinen O.
Experimental & applied acarology.Exp Appl Acarol.2014 Apr;62(4):511-24. doi: 10.1007/s10493-013-9757-8. Epub 2013 Nov 20.
The poultry red mite, Dermanyssus gallinae, is a major pest in egg production, feeding on laying hens. Widely used non-chemical control methods include desiccant dusts, although their persistence under field conditions is often short. Entomopathogenic fungi may also hold potential for mite control,
PMID 24253584

Microporous coordination polymers as efficient sorbents for air dehumidification.

Guo P1, Wong-Foy AG, Matzger AJ.
Langmuir : the ACS journal of surfaces and colloids.Langmuir.2014 Mar 4;30(8):1921-5. doi: 10.1021/la4043556. Epub 2014 Feb 17.
Air drying is a widespread and critical industrial process. Removal of water from air is commonly accomplished by passage through a desiccant such as alumina; modest water capacity and energy intensive regeneration are limitations of currently used sorbents. Microporous coordination polymers (MCPs)
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