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Silica gel
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Silica gel is a granular, vitreous, highly porous form of silica made synthetically from sodium silicate. Despite its name, silica gel is a solid.
Silica gel is most commonly encountered in everyday life as beads packed in a semi-permeable plastic. In this form, it is used as a desiccant to control local humidity in order to avoid spoilage of some goods.
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Encyclopedia
Silica gel is a granular, vitreous, highly porous form of silica made synthetically from sodium silicate. Despite its name, silica gel is a solid.
Silica gel is most commonly encountered in everyday life as beads packed in a semi-permeable plastic. In this form, it is used as a desiccant to control local humidity in order to avoid spoilage of some goods. Because of poisonous dopants (see below) and their very high adsorption of moisture, silica gel packets usually bear warnings for the user not to eat the contents. If consumed, the pure silica gel is unlikely to cause acute or chronic illness, but would be problematic nonetheless. However, some packaged desiccants may include fungicide and/or pesticide poisons. Food-grade desiccant should not include any poisons which would cause long-term harm to humans if consumed in the quantities normally included with the items of food. A chemically similar substance with far greater porosity is aerogel.
History
The synthetic route for silica gel was patented by chemistry professor Walter A. Patrick at Johns Hopkins University, Baltimore, Maryland in 1919. It was used in World War I for the absorption of vapors and gases in gas mask canisters, as part of his patent. The substance was in existence as early as the 1640s as a scientific curiosity.
In World War II, silica gel was indispensable in the war effort for keeping penicillin dry, protecting military equipment from moisture damage, as a fluid cracking catalyst for the production of high octane gasoline, and as a catalyst support for the manufacture of butadiene from ethanol, feedstock for the synthetic rubber program.
Properties
Silica gel's high surface area (around 800 m²/g) allows it to adsorb water readily, making it useful as a desiccant (drying agent). Once saturated with water, the gel can be regenerated by heating to 120 °C (250 °F) for two hours. Some types of silica gel will "pop" when exposed to enough water.
Preparation
A solution of sodium silicate is acidified to produce a gelatinous precipitate that is washed, then dehydrated to produce colorless silica gel. When a visible indication of the moisture content of the silica gel is required, ammonium tetrachlorocobaltate(II) (NH4)2CoCl4 or cobalt chloride CoCl2 is added. This will cause the gel to be blue when dry and pink when hydrated.
Applications
Desiccant
In many items from leather to pepperoni, moisture encourages the growth of mold and spoilage. Condensation may also damage other items like electronics and may speed the decomposition of chemicals, such as those in vitamin pills. By adding packets of silica gel, these items can be preserved longer.
Silica gel may also be used to keep the relative humidity inside a high frequency radio or satellite transmission system waveguide as low as possible. Excessive moisture buildup within a waveguide can cause arcing inside the waveguide itself, damaging the power amplifier feeding it. Also, the beads of water that form and condense inside the waveguide change the characteristic impedance and frequency, impeding the signal. It is common for a small compressed air system (similar to a small home aquarium pump) to be employed to circulate the air inside the waveguide over a jar of silica gel.
Silica gel is also used to dry the air in industrial compressed air systems. Air from the compressor discharge flows through a bed of silica gel beads. The silica gel adsorbs moisture from the air, preventing damage to the compressed air users due to condensation or moisture. The same system is used to dry the compressed air on railway locomotives, where condensation and ice in the brake air pipes can lead to brake failure.
Silica gel is sometimes used as a preservation tool to control relative humidity in museum and library exhibitions and storage.
Chemistry
In chemistry, silica gel is used in chromatography as a stationary phase. In column chromatography the stationary phase is most often composed of silica gel particles of 40-63 µm. In this application, due to silica gel's polarity, non-polar components tend to elute before more polar ones, hence the name normal phase chromatography. However, when hydrophobic groups (such as C18 groups) are attached to the silica gel then polar components elute first and the method is referred to as reverse phase chromatography. Silica gel is also applied to aluminum, glass, or plastic sheets for thin layer chromatography.
Chelating groups have also been covalently bound to silica gel. These materials have the ability to remove metal ions selectively from aqueous media. Chelating groups can be covalently bound to polyamines that have been grafted onto a silica gel surface producing a material of greater mechanical integrity. Silica gel is also combined with alkali metals to form a M-SG reducing agent.
Silica gel is not thought to biodegrade in either water or soil .
Cat litter
Silica gel is also used as cat litter, by itself or in combination with more traditional materials, such as clays including bentonite. It is trackless and virtually odorless, albeit expensive.
Silica in this form can be a cost effective way for private people to easily purchase silica gel for application in such things as keeping tools rust free in damp environments, long term storage, and preservation of dried food for long term storage.
Hazards
Silica gel is non-toxic, non-flammable, and non-reactive and stable with ordinary usage. It will react "with hydrogen fluoride, fluorine, oxygen difluoride, chlorine trifluoride, strong acids, strong bases, and oxidizers". Silica gel "is irritating to the respiratory tract", "May cause irritation of the digestive tract", and dust from the beads may cause irritation to the skin and eyes, so precautions should be taken .. Some of the beads may be doped with a moisture indicator, such as cobalt(II) chloride, which is toxic and may be carcinogenic. Cobalt (II) chloride is deep blue when dry (anhydrous) and pink when moist (hydrated).
Crystalline silica dust can cause silicosis but synthetic amorphous silica gel is non-friable, and so does not cause silicosis.
External links
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- Chemical properties
- Detailed use and re-drying info
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- Helpful Information on Rechargeable Silica Packs for Home Usage
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