Silica gel

Silica, colloidal

White, odorless, hygroscopic, non -combustible solid

Fixed

Practically insoluble in water

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Silica gel, silica gel or silica gel (english silica gel ) is a gel-like amorphous silica of, rubbery to solid consistency and is colorless. It has a large internal surface area. It is highly hygroscopic (water- attracting) and is suitable as a gelling agent, filter, adsorbent and desiccant. One expects it to xerogels.

Silica gel was already known in 1640. During the First World War it was patented as an adsorbent for binding of vapors and gases due to the property.

Production

If water glass with acid, there arises the insoluble, clear gel. For the desiccant beads water glass is dropped into an acid bath. The gel beads are then washed and subjected to drying. By changing the pH during the wash, the pore size of the silica gel can be varied.

Use

Desiccant

Silica gel is used as a drying agent, it is often treated with a water indicator; to blue cobalt ( II ) chloride ( CoCl2 ) is mostly used. If the silica gel has bound water, the indicator changes color from blue to pale pink to whitish; there arises the Hexaaqua complex [ Co ( H2O ) 6] Cl2. Upon drying, this process is reversible.

Filled with silica gel small paper bag or pillow ( bag of desiccant ) are many moisture-sensitive goods consignments (eg electronic equipment, leather goods and nori seaweed for Japanese cuisine), to keep the contents dry during shipping and the formation of condensation moisture at to prevent the movement of warm in cold environments. Also in moisture- sensitive electronic equipment, they are often to be found; for example, be used with silica gel sachet in underwater photography to keep dry the inside of the camera.

Silica gel is often employed as the desiccant in desiccators. Silica gel can also be used for drying and keeping dry seed. Further, it is offered as a highly efficient cat litter on the market. The advantages are a longer period of use and the other varieties low in comparison weight of individual packages.

While silica gel itself is not harmful to health, blue gel is colored blue with cobalt ( II ) chloride and is classified as carcinogenic category 2 since 2000 (that is, in animal studies with high doses of a carcinogenic effect detected ). It is therefore with the R-phrase 49 (may cause cancer by inhalation ) and the symbol to be provided poisonous. Are dangerous dusts that occur, for example when refilling. As an alternative to blue gel orange gel is used (see photo); The silica gel is here colored with a cobalt-free indicator loading of orange turns at about 6% to colorless.

The maximum water absorption of the "normal " silica is about one third of its own weight. There are also large-pore silica gel ( photo), which can adsorb up to two thirds water. There is always an equilibrium of adsorbed water and humidity. That is, the maximum water absorption is true for water- vapor-saturated air. In normal relative humidity ( RH 40-60% ) is not reached this capacity. At very low humidity the silica gel are also the water again ( eg in the heat, see the following section " regeneration ").

Regeneration of the drying agent

Silica gel remains pourable and dimensionally stable even in the depleted state. It can be regenerated, if necessary, is applied on a metallic screen mesh or baking tin, for instance in an oven, at about 120 to 150 ° C again by the absorbed water is driven off again. Located at the silica gel in permeable plastic packaging, such as Tyvek, the drying at a maximum of about 80 ° C must be done. A microwave oven for drying fundamentally unsuitable, as the water evaporates very quickly due to the high power of the microwave and can not be dissipated quickly enough; as a result of bursting the silica particles.

Alcohol test

In alcohol test tube, the silica gel is impregnated with chromic acid, the passing alcohol vapor reduces the dichromate to the green chromium compounds. From the degree and extent of discoloration can be the level of blood alcohol estimate.

Heat storage

A further application is the use as thermochemical heat store. Silica gel has a property to make the heat in the adsorption, that is, the absorption of water and to include in the desorption, the drying heat. This process works as a chemical heat pump, and can be repeated as often as desired. The internal surface of silica is about 600 m² / g

Stationary phase in chromatography

Silica gel stationary phase is most widely used 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 (eg fluorescent indicators ) mixed with additives.

Abbreviations for certain additives:

• Fluorescence: F366 F254 for long-wave and short-wave UV light for
• Gypsum: G
• Without additives: H
• For preparative layer: P
• Highly purified: R

The free -OH groups on the surface of the silica gel make it polar and preferentially bind polar molecules by hydrogen bonds. Very strongly polar substances can not be eluted again. Here offers the use of reversed - phase silica gel. Reversed- phase means that the surface has been modified, for example, C2, C4, C8 or C18 chains, and thus is non-polar.

Density

The modified silica gel is used under different brand names ( Percoll, Sepracell ) for purification of mononuclear peripheral blood cells (PBMC ) by density gradient centrifugation.

Gel culture media

In biology, especially in the Phytologie and Microbiology, silica gel is used in addition to agar gel or gelatin as a gelling agent for culture media.

Counterbalance in airships

In the 1930s, there were attempts on the Zeppelins to use silica gel for buoyancy compensation. The absorbed moisture should compensate for the loss of weight of fuel consumed.