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.
History
Silica gel was patented by chemistry professor Walter A. Patrick at Johns Hopkins University, Baltimore, Maryland in 1919. Prior to that it was already used in World War I for the adsorption of vapors and gases in gas mask canisters and before that the substance was in existence as early as the 1640s as a scientific curiosity (Feldman 2003). In World War II silica gel was indispensable in the war effort in keeping penicillin dry, in 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 alcohol, an integral part of the synthetic rubber program.Properties
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 carcinogenic chemical that changes color when moist. Cobalt (II) chloride is deep blue when dry and pink when moist. Another common indicator in use, phenolphthalein, becomes yellow when moist. While non-doped silica gel is non-toxic, the doping chemicals can be toxic and are the reason most silica gel is 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.
In 1998 the European Community issued a directive (adapting to technical progress for the 25 time Council Directive 67/548/EEC on the approximation of the laws, regulations and administrative provisions relating to the classification, packaging and labelling of dangerous substances) which classifies cobalt (II) chloride compounds containing cobalt chloride percentages (w/w) between 0,01 and 1 as an T (toxic) and R49 (may cause cancer if inhaled). These products shall be labeled in compliance with the new normative.
In layman's terms: In leather products and foods like pepperoni, the lack of moisture can limit the growth of mold and reduce spoilage. In electronics it prevents condensation, which might damage the electronics. If a bottle of vitamins contained any moisture vapor and were cooled rapidly, the condensing moisture would ruin the pills. You will find little silica gel packets in anything that would be adversely affected by excess moisture or condensation.
The problem has been solved by some companies which developed non cobalt indicators for silica gel, using organic indicators, such as phenolphthalein in 100 ppm concentration. Other indicators may be based on iron salts[link], or copper(II) chloride. Envirogel is a silica gel with an indicator that is green when saturated and yellow/orange when dry. [link] Brown Silica Gel changes from amber colour to light blue colour when saturated.
Other uses
Silica gel is used in chromatography as a stationary phase. In column chromatography the stationary phase is most often formed by using silica gel particles of 40-63μm; this powder is hazardous if inhaled. Silica gel combines with alkali metals to form a M-SG reducing agent. Silica gel properties are tuned in reverse phase silica gel by chemical modification.Silica gel may also used to keep the relative humidity inside a Waveguide (electromagnetism) as low as possible. Excessive moisture buildup within a waveguide can cause decreased TX/RX gain by increasing Voltage Standing Wave Ratio (VSWR). The beads of water that form and condense inside the waveguide change the charecteristic impedance and frequency.
Silica gel is also used as cat litter, by itself or in combination with more traditional materials, such as clay or bentonite. It is trackless and virtually odorless, albeit pricey.
References
External links
- [Silica gel: do not eat!] Describes silica gel in great detail.
- [FDA Food Grade silica gel - U.S. National Institue 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
- [Grace Davidson - a Biography]
- [Fuji Silysia Chemical Company History]
- [INEOS Company History]
- [Levosil Company]
- For a full list of external links to MSDSs, spectroscopic data, commercial chemicals suppliers etc. for this compound, see [Chemical sources].
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