Difference between revisions of "Silica gel" - New World Encyclopedia
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| − | [[Image:SilicaGel.jpg|thumb|300px|right|Beads of silica gel]] | + | {{Claimed}} |
| + | [[Image:SilicaGel.jpg|thumb|300px|right|Beads of silica gel.]] | ||
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'''Silica gel''' is a granular, [[porosity|porous]] form of [[silica]] made synthetically from [[sodium silicate]]. Despite the name, silica gel is a solid. | '''Silica gel''' is a granular, [[porosity|porous]] form of [[silica]] made synthetically from [[sodium silicate]]. Despite the name, silica gel is a solid. | ||
| − | Silica gel is most commonly encountered in everyday life as beads packed in a [[semipermeable membrane|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 [[dopant]]s (see below) and their very high absorption of moisture, silica gel packets usually bear warnings for the user not to eat the contents, but to throw them away instead. 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]]. It is not known whether these would be | + | Silica gel is most commonly encountered in everyday life as beads packed in a [[semipermeable membrane|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 [[dopant]]s (see below) and their very high absorption of moisture, silica gel packets usually bear warnings for the user not to eat the contents, but to throw them away instead. 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]]. It is not known whether these would be labeled specifically. 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. |
== History== | == History== | ||
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==Applications== | ==Applications== | ||
===Desiccant=== | ===Desiccant=== | ||
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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 sachets of silica gel, these items can be preserved longer. | 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 sachets of silica gel, these items can be preserved longer. | ||
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===Chemistry=== | ===Chemistry=== | ||
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| − | 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 metal]]s to form a [[ | + | 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 groups with long, C<sub>18</sub> chains) 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 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 metal]]s to form a [[reducing agent]]. | ||
===Cat litter=== | ===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 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. | ||
==Hazards== | ==Hazards== | ||
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| − | Crystalline silica dust can cause [[silicosis]] but synthetic amorphous silica, which is what silica gel is, does not cause silicosis . | + | Alone, silica gel is nontoxic, nonflammable, and chemically unreactive. However, 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]]). This is one reason most silica gel packets are labeled as dangerous or poisonous when eaten. |
| + | |||
| + | Crystalline silica dust can cause [[silicosis]], but synthetic amorphous silica, which is what silica gel is, does not cause silicosis . A chemically similar substance with far greater porosity is [[aerogel]]. | ||
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| + | == Notes == | ||
| + | <references /> | ||
==References== | ==References== | ||
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==External links== | ==External links== | ||
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* [http://foodsave.net/Bulkdesiccants.htm Silica gel mesh-metric conversions] | * [http://foodsave.net/Bulkdesiccants.htm Silica gel mesh-metric conversions] | ||
* [http://householdproducts.nlm.nih.gov/cgi-bin/household/brands?tbl=chem&id=1683 FDA Food Grade silica gel - U.S. National Institute of Health] Uses in toothpaste and other ingested consumable products | * [http://householdproducts.nlm.nih.gov/cgi-bin/household/brands?tbl=chem&id=1683 FDA Food Grade silica gel - U.S. National Institute of Health] Uses in toothpaste and other ingested consumable products | ||
Revision as of 03:06, 23 August 2007
Silica gel is a granular, porous form of silica made synthetically from sodium silicate. Despite the 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 absorption of moisture, silica gel packets usually bear warnings for the user not to eat the contents, but to throw them away instead. 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. It is not known whether these would be labeled specifically. 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.
History
Silica gel was patented by chemistry professor Walter A. Patrick at Johns Hopkins University, Baltimore, Maryland in 1919. Prior to that, it was used in World War I for the absorption of vapors and gases in gas mask canisters. The substance was in existence as early as the 1640s as a scientific curiosity.[1]
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 150 °C (300 °F) for 1.5 hours per litre of gel. Some types of silica gel will "pop" when exposed to enough water.
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 sachets 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 groups with long, C18 chains) 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 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 reducing agent.
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.
Hazards
Alone, silica gel is nontoxic, nonflammable, and chemically unreactive. However, 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). This is one reason most silica gel packets are labeled as dangerous or poisonous when eaten.
Crystalline silica dust can cause silicosis, but synthetic amorphous silica, which is what silica gel is, does not cause silicosis . A chemically similar substance with far greater porosity is aerogel.
Notes
- ↑ Maryann Feldman and Pierre Desrochers, March 2003. "Research Universities and Local Economic Development: Lessons from the History of the Johns Hopkins University," Industry and Innovation 10(1): 5-24. Retrieved August 23, 2007.
ReferencesISBN links support NWE through referral fees
External links
- Silica gel mesh-metric conversions
- FDA Food Grade silica gel - U.S. National Institute of Health Uses in toothpaste and other ingested consumable products
- UK Material Safety Datasheet
- Silica Gel: DC Chemical Co., Ltd. Chemical properties
- Silica Gel with Moisture indicator: Instructions for Use Detailed use and re-drying info
- Silica Gel (Sodium Silicate): Chemical Data & Physical Data
- Silica Gel Dehumidifier FAQs Helpful Information on Rechargeable Silica Packs for Home Usage
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