Silica gel
Silica gel is a granular, vitreous, highly porous form of silica made synthetically from sodium silicate and despite the name is actually solid.
Silica gel is most commonly encountered in everyday life as beads packed in a paper sachet. In this form, it is used as a desiccant to control local humidity in order to avoid spoilage or degradation of some goods. Because of poisonous dopants and their very high adsorption of moisture, silica gel packets usually bear warnings for the user not to eat the contents. If consumed, the pure silica gel is unlikely to cause acute or chronic illness, but would be problematic nonetheless. However, some packaged desiccants may include fungicide and pesticide poisons. Food-grade desiccant should not include any poisons which would cause long-term harm to humans if consumed in the quantities normally included with the items of food.
In many items from leather to pepperoni, moisture encourages the growth of mold and so silica gel sachets are used. Condensation may also damage other items like electronics and may speed up the decomposition of chemicals, such as those in vitamin pills. By adding packets of silica gel, these items can be preserved longer. Silica gel may also be used to keep the relative humidity inside a high frequency radio or satellite transmission system waveguide as low as possible. Excessive moisture buildup within a waveguide can cause arcing inside the waveguide itself, damaging the power amplifier feeding it. Also, the beads of water that form and condense inside the waveguide change the characteristic impedance and frequency, impeding the signal. It is common for a small compressed air system to be employed to circulate the air inside the waveguide over a jar of silica gel. 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. Silica gel is sometimes used as a preservation tool to control relative humidity in museum and library exhibitions and storage.
In chemistry, silica gel is used in chromatography as a stationary phase. In column chromatography the stationary phase is most often composed of silica gel particles of 40-63 ?m. Different particle sizes are used for achieving a desired separation of certain molecular sizes. In this application, due to silica gel's polarity, non-polar components tend to elute before more polar ones, hence the name normal phase chromatography. However, when hydrophobic groups are attached to the silica gel then polar components elute first and the method is referred to as reverse phase chromatography. Silica gel is also applied to aluminum, glass, or plastic sheets for thin layer chromatography. Chelating groups have also been covalently bound to silica gel. These materials have the ability to remove metal ions selectively from aqueous media. 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.
Kevin Thomas works for Davpack, a uk packaging supplier. Their friendly staff are waiting to help you choose the right packaging for your business.Text and content © Copyright of Davenport Paper Co. Ltd 2009
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