Silica gel
Identifiers | |
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ChemSpider |
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ECHA InfoCard
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100.065.880 |
UNII | |
CompTox Dashboard (EPA)
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Properties | |
SiO2 | |
Molar mass | 60.08 g/mol |
Appearance | Transparent beads |
Odor | Odorless |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Silica gel is an
Silica xerogel with an average pore size of 2.4 nanometers has a strong affinity for water molecules and is widely used as a
Silica xerogel is usually commercialized as coarse granules or beads, a few
'Wet' silica gel, as may be freshly prepared from alkali silicate solutions, may vary in consistency from a soft transparent gel, similar to gelatin or agar, to a hard solid, namely a water-logged xerogel. It is sometimes used in laboratory processes, for example to suppress convection in liquids or prevent settling of suspended particles.[4]
History
Silica gel was in existence as early as the 1640s as a scientific curiosity.
In
Types
- Type A – clear pellets, approximate pore diameter: 2.5 nm, drying and moistureproof properties, can be used as catalyst carriers, adsorbents, separators and variable-pressure adsorbent.
- Type B – translucent white pellets, pore diameter: 4.5–7.0 nm, liquid adsorbents, drier and perfume carriers, also may be used as catalyst carriers, cat litter.
- Type C – translucent, micro-pored structure, raw material for preparation of silica gel cat litter. Additionally dried and screened, it forms macro-pored silica gel which is used as drier, adsorbent and catalyst carrier.
Silica alumina gel - light yellow, chemically stable, flame-resistant, insoluble except in alkali or hydrofluoric acid. Superficial polarity, thermal stability, performance greater than fine-pored silica gel.
Stabilizing silica gel - non-crystalline micro-porous solid powder, nontoxic, flame-resisting, used in brewery of grains for beer to improve taste, clearness, color, and foam and for removal of non-micro-organism impurities.
Properties
Silica gel's high
Silica gel is able to adsorb up to 37% of its own weight in moisture in high-humidity environments.[7] This moisture can be released upon heating at 120 °C for extended periods of time. This makes it reusable multiple times with very little, if any, loss of efficiency.
Regeneration
Once saturated with water, the gel may be regenerated by heating it to 120 °C (248 °F) for 1–2 hours.[7] Some types of silica gel will "pop" when exposed to enough water. This is caused by breakage of the silica spheres when contacting the water.[8]
Preparation
An aqueous solution of sodium silicate is acidified to produce a gelatinous precipitate that is washed, then dehydrated to produce colorless silica gel.[6] When a visible indication of the moisture content of the silica gel is required, ammonium tetrachlorocobaltate(II) (NH4)2[CoCl4] or cobalt(II) chloride CoCl2 is added.[6] This will cause the gel to be blue when dry and pink when hydrated.[6] Due to a link between the cobalt chloride and cancer, it has been forbidden in Europe for use in silica gel.[9] An alternative indicator is methyl violet which is orange when dry and green when hydrated.
Uses
Desiccant
Moisture can cause mold and spoilage in many items.[10][11] It can also damage electronics by causing condensation and shorten the lifespan of chemicals, like those in vitamins.[citation needed] Silica gel packets help by absorbing moisture and extending the life of these items.[12][13][14] They can even be useful for drying out electronics that have gotten wet accidentally.[15][16][17]
Silica gel may also be used to keep the
Silica gel can adsorb about 40 percent of its weight in moisture. Once saturated, you can propel the moisture off and reuse silica gel by heating it above 300 degrees F (150 C).
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 at the point of use of the compressed air 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.
Prior to widespread use of air-conditioning, salt shakers with caps containing silica gel beads to keep the salt dry enough to prevent clumping were marketed in the USA, replacing the practice of including a few grains of rice in salt shakers to effect the same drying.
Silica gel is sometimes used as a
Other applications include diagnostic test strips, inhalation devices, syringes, drug test kits, and hospital sanitation kits.
Chemistry
In chemistry, silica gel is used in
The hydroxy (OH) groups on the surface of silica can be functionalized to afford specialty silica gels that exhibit unique stationary phase parameters. These so-called functionalized silica gels are also used in organic synthesis and purification as insoluble reagents and scavengers.
Chelating groups have also been covalently bound to silica gel. These materials have the ability to remove metal ions selectively from aqueous solutions. 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. (See SiGNa chemistry)
Silica gel is not expected to biodegrade in either water or soil.[19]
Cat litter
Silica gel is also used as
Food additive
Silica gel, also referred to as silicon dioxide or synthetic amorphous silica (SAS), is listed by the FDA in the United States as generally recognized as safe (GRAS), meaning it can be added to food products without needing approval. Silica is allowed to be added to food in the USA at up to 2% as permitted under 21 CFR 172.480. In the EU, it can be in up to 5% concentrations.[21] In 2018, a re-evaluation by the EFSA Panel on Food Additives and Nutrient Sources added to Food found no indications of toxicity even at the highest estimates of exposure level.[22]
Listed uses include: anticaking agent, defoaming agent, stabilizer, adsorbent, carrier, conditioning agent, chillproofing agent, filter aid, emulsifying agent, viscosity control agent, and anti-settling agent.[23] Silica can be found commonly in foods including baked goods, spices and herbs, dairy products, cocoa products, and more.[22]
Water filtration
Given the water adsorption properties of silica gel, it is used in domestic water filters.[24] The surface structure of silica gel allows the adsorption of some minerals that are dissolved in the water,[25] or "Ion-exchange" as it is marketed. Due to the lack of regulations for domestic water filtration products, no studies validate the manufacturer claims regarding the effectiveness of the filtration system.
Humidity indicator (color-changing silica gel)
Silica gel may be doped with a moisture indicator that gradually changes its color when it transitions from the anhydrous (dry) state to the hydrated (wet) state. Common indicators are cobalt(II) chloride and methyl violet. Cobalt (II) chloride is deep blue when dry and pink when wet, but it is toxic and carcinogenic, and was reclassified by the European Union in July 2000 as a toxic material.[26] Methyl violet may be formulated to change from orange to green, or orange to colorless. It also is toxic and potentially carcinogenic,[27] but is safe enough to have medicinal uses.
Hazards
Silica gel is non-toxic, non-flammable, and non-reactive and stable with ordinary usage. It will react with
References
- ^ Silica gel, site jtbaker.com
- ^ Silica gel, site chemcas.org
- ^ Silicon dioxide, site echa.europa.eu
- ISBN 0521345030.
- S2CID 154423229. Archived from the original(PDF) on 2005-11-12.
- ^ ISBN 978-0-08-037941-8.
- ^ a b Chandradhas, Susheel (27 August 2016). "How To Recharge Silica Gel Crystals". Beyond Photo Tips. Retrieved 2 November 2022.
- ^ Spence Konde, "Preparation of High-Silica Zeolite Beads From Silica Gel" Archived 2016-03-04 at the Wayback Machine, retrieved 2011-09-26
- ^ "Blue Silicagel & Conclusions: Safety information on blue silicagel". Archived from the original on 2016-01-05.
- ^ "Prevent bacteria from feasting on your fresh produce, causing spoilage". MSU Extension. 2013-01-30. Retrieved 2024-03-08.
- ^ Command, United States Naval Education and Training (1978). Mess Management Specialist 3 & 2. Department of Defense, Navy Department, Office of the Chief of Naval Operations], Naval Education and Training Command. p. 91.
- ^ Machinery and Production Engineering. Machinery Publishing Company. 1951.
- ^ Anderson, Gary A. (1988). Floral Design and Marketing. Ohio Agricultural Education Curriculum Materials Service, the Ohio State University.
- ISBN 978-2-912743-00-8.
- ^ Rudderham, T. A. iPhone 5s Guide. iOS Guides.
- ^ Birstein, S. J.; Lyon, A. M. (1949). Adsorption of Nitrogen Tetroxide on Silica Gel. United States Atomic Energy Commission, Technical Information Service.
- ^ Smits, Benjamin Levi (1926). Silica Gel. Michigan State College of Agriculture and Applied Science.
- ^ Sabry, Fouad (2022-01-16). Aerogel: Want to Colonize Mars? Aerogel could help us farm and survive on Mars "in our lifetimes". One Billion Knowledgeable.
- ^ a b Environmental Health and Safety (2007-09-10). "Silica Gel". Retrieved 2008-01-12.
- ^ Andrew Kantor (2004-12-10). "Non-Tech High Tech Litters the Landscape". USA Today. Retrieved 2008-03-02.
- ^ "Notification of the GRAS Determination of Silicon Dioxide When Added Directly or Indirectly to Human Food" (PDF). Archived from the original (PDF) on April 18, 2013.
- ^ S2CID 79503431.
- ^ "GRAS Notice (GRN) No. 298" (PDF). Archived from the original (PDF) on April 9, 2011.
- ^ ZeroWater
- .
- ^ "Classifications - CL Inventory".
- ^ "Methyl Violet Safety Data Sheet" (PDF). labchem.
- ^ WO patent WO2000065339A1, Stephen Moreton & Graham James Earl, "Humidity indicators", published 2000-11-02
- ^ "SORBSIL CHAMELEON Safety Data Sheet" (PDF). OkerChemie.
- ^ Fisher Scientific (1997-02-09). "Material Safety Data Sheet: Silica Gel Dessiccant". Retrieved 2008-01-12.
External links
- Silica gel in the Consumer Product Information Database
- UK Material Safety Datasheet
- Drying Seed with Color Indicating Silica Gel, Southern Exposure Seed Exchange
- Silica Gel Dehumidifier FAQs Helpful information on rechargeable silica packs for home use
- Silica-bound reagents Information on development and tables illustrating reactivity
- Silica-bound scavengers General notes and Tables that summarize scavenging capacity