Desiccant 293065 226048353 2008-07-16T16:27:01Z Rifleman 82 1255637 /* Drying of solvents */ distillation still 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 (chemistry)|absorption]] or [[adsorption]] of [[water]], or a combination of the two. Desiccants for specialised 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. [[Silica gel]], [[calcium sulfate]], [[calcium chloride]], [[montmorillonite clay]], and [[molecular sieve]]s are commonly used as desiccants. [[Rice]] is a common "[[low-tech]]" alternative, frequently used for example in [[Salt shaker|salt-shaker]]s to maintain [[w:granular|granular]]ity of [[Table salt|table-salt]] for effective pouring or shaking. Rice, however, is not a good general purpose desiccant since, unless immersed in an organism-hostile environment like pure salt, over time may be eaten by creatures that might in turn pollute the product that is being preserved. Salt itself is another effective desiccant, used for millennia in preparation of [[dried food]]stuffs. == Toxicity == A desiccant may be chosen for a particular task based not only on its effectiveness at drying, but also perhaps for any natural [[antibiotic]]/[[fungicide|fungicidal]]/[[pesticide|pesticidal]] effect, or for a lack of harmful effect on humans -- for instance salt has a natural antibiotic and anti-fungal effect and is not harmful to humans when consumed in moderate quantities, hence its popular use as drying agent in preparation of dried foods. == Chemical reactivity == Because of their common use in preservation, usually it is 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. == Performance efficiency == One measure of desiccant efficiency is the ratio (or [[percentage]]) of water storeable in the desiccant, relative to the [[mass]] of desiccant (ie [[weight]]). 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 == 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]] (CoCl<sub>2</sub>). [[Anhydrous]] cobalt chloride is blue. When it bonds with two water molecules, (CoCl<sub>2</sub>•2H<sub>2</sub>O), it turns purple. Further hydration results in the pink hexaaquacobalt(II) chloride complex [Co(H<sub>2</sub>O)<sub>6</sub>]Cl<sub>2</sub>. == Desiccant regeneration == Most useful desiccants can be [[recycled]] by [[thermal]]ly-induced drying, for example, in a conventional kitchen [[oven]], or with [[solar energy]]. A cost-effective, low-energy, continuous-cycle desiccant [[dehumidifier]] or desiccant [[regeneration]] system can be easily designed from [[off-the-shelf]] component parts. ==Drying of solvents== [[Image:Toluene with sodium-benzophenone.jpg|right|thumb|[[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 dry [[solvents]], typically used for moisture free reactions (e.g. the [[Grignard reaction]]). While various desiccants are used in a variety of ways, the method generally involves stirring the desired solvent with the drying agent. Often, the drying agent will react with moisture to form an insoluble solid in the solvent being dehydrated, thus the precipitate can be removed by filtration. For more demanding applications, requiring very pure, anhydrous solvents a [[distillation]] still is used to remove moisture and other impurities simultaneously. ==See also== * [[List of desiccants]] * [[Hygroscopy]] *[[Solar air conditioning]] ==References== {{refimprove}} ==Further reading== * {{cite book |author=Chai, Christina Li Lin; Armarego, W. L. F. |title=Purification of laboratory chemicals |publisher=Butterworth-Heinemann |location=Oxford |year=2003 |pages= |isbn=0-7506-7571-3 |oclc= |doi= |accessdate=}} * {{cite journal | author = Lavan, Z.; Jean-Baptiste Monnier, Worek, W. M. | year = 1982 | title = Second Law Analysis of Desiccant Cooling Systems | journal = [[Journal of Solar Energy Engineering]] | volume = 104 | pages = 229–236}} * {{cite journal | doi = 10.1007/BF02357312 | title = True potato seed drying over rice | year = 1982 | author = S. Sadik; J. W. White| journal = [[Potato Research]] | volume = 25 | pages = 269}} ==External links== *[http://foodsave.net/controlled_atmosphere_requiremen.htm Desiccant Requirement Chart (printable)] *[http://www.qiaoyun.net/desiccant/desiccant.htm Latest US desiccants patents information.] *[http://www.agmcontainer.com/desiccantcity/desiccant_calculator.php A Desiccant Requirements Calculator] *[http://www.naturalproductsinsider.com/articles/473/473_181pacfoc2.html Combating Moisture and Humidity in Nutraceutical Packaging] *[http://www.sud-chemie.com/scmcms/web/content.jsp?nodeId=3944&lang=en Education Center] *[http://www.advancedbuildings.org/main_t_heat_desiccant_cooling.htm Desiccant Cooling and Dehumidification] *[http://solarteam.org/page.php?id=641 Liquid Desiccant Waterfall for attractive building dehumidification] * {{cite web | publisher = [[J. T. Baker]] | title = Desiccant Selection Guide | url = http://www.mallbaker.com/techlib/documents/americas/3045.html | date = 2000}} [[Category:Desiccants|*]] [[lt:Desikantas]] [[ml:ഡെസിക്കന്റ്]] [[pt:Dessecante]] [[tr:Desikant]]