Silica gel
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Silica gel is a granular, porous form of silica made synthetically from sodium silicate. Despite the name, silica gel is a solid. It is usually distributed in the form of beads, which are packaged in a semi-permeable packet. It was created at Johns Hopkins University, Baltimore, Maryland in the 1920s. Its high porosity, around 800 m²/g, allows it to adsorb water readily, making it useful as a desiccant (drying agent). It can reduce the relative humidity in a closed space to around 40%. Once saturated with water, the gel can be regenerated (dried) by heating it to 150°C (300°F) for 1.5 hours per liter (about 1 dry quart measure or about 30oz weight) in a thick-walled Pyrex dish. Silica gel is non-toxic, non-flammable and chemically unreactive.
However, some of the beads may be doped with a moisture indicator, such as cobalt (II) chloride, a chemical that is known to cause cancer and changes color when moist. Cobalt (II) chloride is deep blue when dry and pink when moist. Another common indicator in use becomes yellow when moist. While non-doped silica gel is non-toxic, the chemicals used to dope it can be toxic and are the reason most silica gel is labeled as dangerous or poisonous when eaten. Silica dust can cause Silicosis.
External links
- Silica gel: do not eat! (http://users.adelphia.net/~jthm/) site describing silica gel in great detail.
- Silica Gel with Moisture indicator: Instructions for use (http://www.southernexposure.com/library/drying-seed-silica-gel.html) detailed use and regeneration (re-drying) infoja:シリカゲル