Silica gel
| General | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Surname | Silica gel | ||||||||||||
| other names |
Silica, colloidal |
||||||||||||
| Molecular formula | SiO 2 | ||||||||||||
| Brief description |
colorless, odorless, hygroscopic, non-flammable solid |
||||||||||||
| External identifiers / databases | |||||||||||||
|
|||||||||||||
| properties | |||||||||||||
| Molar mass | 60.08 g mol −1 | ||||||||||||
| Physical state |
firmly |
||||||||||||
| density |
2.2 g cm −3 |
||||||||||||
| Melting point |
1710 ° C |
||||||||||||
| boiling point |
2230 ° C |
||||||||||||
| solubility |
practically insoluble in water |
||||||||||||
| safety instructions | |||||||||||||
|
|||||||||||||
| As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions . | |||||||||||||
Silica gel , silica gel or silica gel ( English silica gel ) is a colorless, amorphous silicon dioxide with a gel-like, rubbery to solid consistency. It has a large internal surface (approx. 600 m² / g). It is highly hygroscopic (water-attracting) and is suitable as a gelling agent, filter, adsorption material and drying agent . It is counted among the xerogels .
Silica gel was already known around 1640. At the time of the First World War , it was patented for its property as an adsorbent for binding vapors and gases.
Manufacturing
If acid is added to water glass , the insoluble, clear gel is created. For the desiccant beads, water glass is dripped into an acid bath. The gel spheres are then washed and dried. The pore size of the silica gel can be varied by changing the pH value during washing.
use
Desiccant
Silica gel can be used as a drying agent because it absorbs water on its large internal surface. This process is reversible. Small paper bags or pillows filled with silica gel ( anti-condensation bags ) are included with many shipments that are sensitive to moisture (e.g. electronic devices, leather goods and nori seaweed for Japanese cuisine ) to keep their contents dry during shipping and to prevent condensation to prevent moving from cold to warm surroundings. Only uncolored (white) silica gel that is harmless to health should be used in conjunction with food to keep them dry.
Silica gel is often used as a drying agent in desiccators . Silica gel can also be used to dry seeds and keep them dry. It is also offered in stores as a highly efficient cat litter . The advantages are a longer service life and the low weight of the individual packs compared to other types.
The maximum water absorption of "normal" silica gel is around 20% to 33% of its own weight. There is also large-pored silica gel that can adsorb up to 66% water. There is always an equilibrium between adsorbed water and the relative humidity (RH). This means that the maximum water absorption applies to air that is saturated with water vapor. This capacity is not achieved at normal humidity (approx. 40–60% RH). When the air humidity is very low, the silica gel also releases the water again, i.e. even when it is heated, which is used for regeneration.
Colored silica gel
In order to be able to visually assess the loading of silica gel with water easily, colored silica gel is also available commercially. The dye changes its color depending on the water content of the gel and thus allows a simple assessment of the loading of the silica gel with water.
One dye used is cobalt dichloride , which is blue in the dry state ; the silica gel colored in this way is called blue gel . When the blue gel binds water, the red hexaqua complex [Co (H 2 O) 6 ] Cl 2 is formed , so that the gel changes color from blue to pale pink. Blue gel has been considered carcinogenic since 2000 (then Category 2) and is classified in Category 1B (i.e. a carcinogenic effect was demonstrated in animal experiments with high doses). It is therefore to be provided with the H-phrase 350i ("Can cause cancer if inhaled."), The pictogram GHS08 (health hazard) and the signal word danger . The dusts that z. B. occur when decanting.
A variant of methyl violet , which is orange when dry, is also used as a moisture indicator. The silica gel colored with it is called orange gel . Depending on the modification of methyl violet, the orange gel becomes either colorless or dark green when moist. Methyl violet is also considered to be potentially carcinogenic and is very toxic to aquatic life.
Because of the harmfulness of the dyes, colored silica gel should not be used in the field of medical products or food.
Regeneration
Silica gel remains free-flowing and dimensionally stable even when it is exhausted. Colorless silica gel can be regenerated at around 120 to 150 ° C, with colored silica gel the temperature should be below 120 ° C in order not to destroy the dye. During the regeneration, the absorbed water is expelled ( heating ). If the silica gel is in permeable plastic bags, for example made from Tyvek , drying may only take place at a maximum of approx. 80 ° C in order not to damage the casing. A microwave oven is unsuitable for drying because the high power density means that the water evaporates very quickly and cannot be removed quickly enough; as a result, the silica gel particles burst.
Rubber additive for vehicle tires
Silica gel in mixtures together with other substances in vehicle tires as a substitute of rubber as a bonding agent used, which significantly improves the wet grip of the tires. Here part of the significantly reduced coefficient of friction between rubber and asphalt when wet is compensated for by the additional adsorption (more precisely physisorption ) of the water molecules ( hydrogen bond ). Furthermore, the mixture becomes softer and therefore more adhesive on dry roads, while abrasion and thus wear are reduced.
Stationary phase in chromatography
Silica gel is the most frequently used stationary phase in thin layer chromatography (TLC) and column chromatography . For the TLC, the silica gel is applied to suitable carrier materials (e.g. glass, aluminum). Often it is also mixed with additives (e.g. fluorescent indicators).
Abbreviations for certain additions:
- Fluorescence: F 366 for long-wave and F 254 for short-wave ultraviolet
- Plaster of Paris: G
- without additives: H
- for preparative layer chromatography: P
- highly purified: R
The free OH groups on the surface of the silica gel make it polar and preferentially bind polar molecules by means of hydrogen bonds. Very strongly polar substances cannot be eluted again. The use of reversed-phase silica gel is recommended here. Reversed phase means that the surface z. B. has been modified with C2, C4, C8 or C18 chains and is therefore non-polar.
additional
- In biology , especially in phytology and microbiology , silica gel is used as a gelling agent for culture media in addition to agar gel or gelatin .
- The silica gel is impregnated with chromosulfuric acid in alcohol test tubes, the passing alcohol vapor ( breath alcohol determination ) reduces the dichromate to green chromium compounds. The degree of alcoholisation can be estimated from the strength and extent of the discoloration.
- When cooling by drying and evaporation , silica gel can be used as a regenerable drying agent.
- In the 1930s there were attempts with zeppelins to use silica gel to compensate for buoyancy . The moisture absorbed should compensate for the weight loss in the fuel consumed.
- Modified silica gel is used under various brand names ( Percoll , Sepracell ) for the purification of mononuclear cells of peripheral blood ( PBMC ) by density gradient centrifugation .
- Another application is the use as thermochemical heat storage . Silica gel has the property of emitting heat when it absorbs water ( adsorption ) and absorbs heat when it is dried ( desorption ). This process can be repeated as often as you like.
- The protein stabilization of beer with the help of silica gel was initiated by Karl Raible .
Hazard assessment
In 2012, silica gel was included in the EU's ongoing action plan ( CoRAP ) in accordance with Regulation (EC) No. 1907/2006 (REACH) as part of substance evaluation . The effects of the substance on human health and the environment are re-evaluated and, if necessary, follow-up measures are initiated. The inclusion of silica gel was caused by concerns about other hazard-related concerns. The re-evaluation has been running since 2012 and is carried out by the Netherlands . In order to be able to reach a final assessment, further information was requested.
literature
- Ralph K. Iler: The Chemistry of Silica. Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry. John Wiley & Sons, New York / Chicester / Brisbane / Toronto / Singapore 1979, ISBN 0-471-02404-X .
Web links
Individual evidence
- ↑ a b c d e f g h Entry for CAS no. 7631-86-9 in the GESTIS substance database of the IFA , accessed on May 22, 2016(JavaScript required) .
- ↑ Data sheet Silica gel, packet of 5 g desiccant from Sigma-Aldrich , accessed on May 22, 2016 ( PDF ).
- ^ Maryann Feldman, Pierre Desrochers: Research Universities and Local Economic Development. In: Industry and Innovation. Volume 10, number 1, pp. 5–24, March 2003 ( PDF, p. 18 ( Memento of the original dated November 12, 2005 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and Archive link according to instructions and then remove this note. ).
- ↑ Werner Kast: Adsorption from the gas phase , VCH Verlagsgesellschaft mbH, Weinheim 1988, ISBN 3-527-26719-0 , p. 13.
- ↑ Harmonized classification and labeling of cobalt dichloride in the Classification and Labeling Inventory of the European Chemicals Agency (ECHA), accessed on September 13, 2016.
- ↑ Safety of silica gels with color indicators. Retrieved August 28, 2017 .
- ↑ India rubber directory , PRECIPITATED SILICA, Samir Majumdar, accessed November 22, 2015.
- ↑ Brewing Industry 1/2019, p. 7
- ↑ Community rolling action plan ( CoRAP ) of the European Chemicals Agency (ECHA): Silicon dioxide , accessed on March 26, 2019.
