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. Hygroscopy is the ability of a substance to attract Water Molecules from the surrounding environment through either absorption or Adsorption Desiccation is the state of extreme dryness or the process of extreme drying

Commonly encountered pre-packaged desiccants are solids, and work through absorption or adsorption of water, or a combination of the two. Absorption, in Chemistry, is a physical or chemical Phenomenon or a process in which Atoms Molecules, or Ions enter some Adsorption is a process that occurs when a gas or liquid Solute accumulates on the surface of a solid or a liquid (adsorbent forming a film of molecules or atoms (the Water is a common Chemical substance that is essential for the survival of all known forms of Life. 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. Humidity is the amount of water vapor in the air In daily language the term "humidity" is normally taken to mean Relative humidity. Silica gel, calcium sulfate, calcium chloride, montmorillonite clay, and molecular sieves are commonly used as desiccants. Silica gel is a granular porous form of Silica made synthetically from Sodium silicate. Calcium sulfate is a common laboratory and industrial chemical Calcium chloride (CaCl2 is an ionic compound of Calcium and Chlorine. Montmorillonite is a very soft phyllosilicate mineral that typically forms in microscopic Crystals forming a clay. A molecular sieve is a material containing tiny pores of a precise and uniform size that is used as an adsorbent for Gases and Liquids Molecules

Rice is a common "low-tech" alternative, frequently used for example in salt-shakers to maintain granularity of table-salt for effective pouring or shaking. Rice is a Cereal foodstuff which forms an important part of the diet of many people worldwide and as such it is a staple food for many The term low-technology is a description of those Crafts and Tools whose inception (typically predates the Industrial Revolution. Salt and pepper shakers are Condiment holders used in Western culture that are designed to allow food eaters to distribute Edible salt Salt is a Dietary mineral composed primarily of Sodium chloride that is essential for Animal life but toxic to most land plants 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 foodstuffs. Drying is a method of Food preservation that works by removing Water from the food which prevents the growth of Microorganisms and Decay.

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/fungicidal/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. In modern usage an antibiotic is a Chemotherapeutic agent with activity against Microorganisms such as Bacteria, fungi or Protozoa Fungicides are Chemical compounds or biological organisms used to kill or inhibit fungi or fungal spores A pesticide is a substance or mixture of substances used to kill a pest.

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. In English to be inert is to be in a state of doing little or nothing

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). In Mathematics, a percentage is a way of expressing a number as a Fraction of 100 ( per cent meaning "per hundred" Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object In the Physical sciences weight is a Measurement of the gravitational Force acting on an object

Another measure is the residual relative humidity of the air or other fluid being dried. Relative humidity is a measurement of the amount of Water vapor that exists in a gaseous mixture of air and water

The performance of any desiccant varies with temperature and both relative humidity and absolute humidity. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature Relative humidity is a measurement of the amount of Water vapor that exists in a gaseous mixture of air and water Humidity is the amount of water vapor in the air In daily language the term "humidity" is normally taken to mean Relative 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. A humidity indicator is a moisture-sensitive chemical that it will change color when the indicated Relative humidity is exceeded One commonly used indicator is cobalt chloride (CoCl₂). Cobalt(II chloride is the Chemical compound with the formula Co[[chlorine Cl2]] although the term is used also to refer to the hexahydrate which Anhydrous cobalt chloride is blue. As a general term a substance is said to be anhydrous if it contains no Water. 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.

Desiccant regeneration

Most useful desiccants can be recycled by thermally-induced drying, for example, in a conventional kitchen oven, or with solar energy. Recycling involves processing used materials into new products in order to prevent the waste of potentially useful materials reduce the consumption of fresh raw materials reduce A thermal column (or thermal) is a column of rising Air in the lower altitudes of the Earth's atmosphere. An oven is an enclosed compartment for Heating, Baking or Drying. Solar energy is the Light and radiant heat from the Sun that powers Earth 's Climate and Weather and sustains Life A cost-effective, low-energy, continuous-cycle desiccant dehumidifier or desiccant regeneration system can be easily designed from off-the-shelf component parts. A dehumidifier is a Household appliance that reduces the level of Humidity in air usually for health reasons as humid air can cause Mold and In finance or when buying things off the shelf refers to products that have already been designed and made compared to " Made to measure," (or " One-off

Drying of solvents

Toluene is refluxed with sodium and benzophenone to produce dry, oxygen-free toluene. Sodium (ˈsoʊdiəm is an element which has the symbol Na( Latin natrium, from Arabic natrun) atomic number 11 atomic mass 22 Benzophenone is the Organic compound with the formula ( C 6 H 52C O, generally abbreviated Ph 2CO 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. Air-free techniques refer to a range of manipulations in the chemistry Laboratory for the handling of compounds that are Air-sensitive. A solvent is a liquid or gas that dissolves a solid liquid or gaseous Solute, resulting in a Solution. g. the Grignard reaction). The Grignard reaction, named for the French chemist François Auguste Victor Grignard, is an organometallic Chemical reaction in which Alkyl - 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 rig is used to remove moisture and other impurities simultaneously. Distillation is a method of separating Mixtures based on differences in their volatilities in a boiling liquid mixture

References

Lavan, Z. ; Jean-Baptiste Monnier, Worek, W. M. ; 1982, "Second Law Analysis of Desiccant Cooling Systems", Journal of Solar Energy Engineering, Vol. 104, pp. 229-236.

See also

• List of desiccants
• Hygroscopy
• Solar air conditioning

External links

• Desiccant Requirement Chart (printable)
• Desiccant info
• Latest US desiccants patents information.
• A Desiccant Requirements Calculator
• Combating Moisture and Humidity in Nutraceutical Packaging
• Desiccant Selection Guide
• Drying agents brochure from Merck
• True potato seed drying over rice
• Education Center
• Desiccant Cooling and Dehumidification
• Liquid Desiccant Waterfall for attractive building dehumidification