Desiccant Initial use Moisture can cause mold and spoilage in many items. It can also damage electronics by causing condensation and shorten the lifespan of chemicals, like those in vitamins. Silica gel packets help by absorbing moisture and extending the life of these items. 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 (see also
humidity buffering). 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, degrading the signal. It is common for a small compressed air system (similar to a small home aquarium pump) 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. Prior to widespread use of air-conditioning, salt shakers with caps containing silica gel beads to keep the salt dry enough to prevent clumping were marketed in the US, replacing the practice of including a few grains of rice in salt shakers to effect the same drying. Silica gel is sometimes used as a
preservation tool to control relative humidity in museum and library exhibitions and storage. Other applications include diagnostic test strips, inhalation devices,
syringes,
drug test kits, and hospital sanitation kits. Silica gel is commonly described as a way to remove moisture from phones and electronics accidentally exposed to water, but there are no known controlled studies that compare its actual efficacy compared to simple air or fan exposure. The average person is also unlikely to have a sufficient quantity of ready-to-use activated or
regenerated silica gel.
Regeneration Once saturated with water, the gel may be regenerated by heating it to about Regenerating silica gel is important for both economic and environmental reasons. It can be regenerated thermally or chemically. Chemical methods involve the use of desiccating agents or solvents to desorb water from silica gel, though these are generally less practical for routine regeneration. Thermal regeneration can be applied also to packed silica gel beds without opening them, and can be done by conventional heating in an oven at a temperature around . Another option for thermal regeneration is by using microwaves to apply heat more evenly and reduce regeneration time compared to conventional ovens, but this process should be tightly controlled to avoid overheating. Yet another option to apply heat is using a solar heater with a compound parabolic concentrator. There are also studies on using ultrasonic waves to remove water molecules from silica gel at which are lower temperatures than that of conventional heating.
Chemistry 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 μm. Different particle sizes are used for different kinds of column chromatography as the particle size is related to surface area. The differences in particle size dictate if the silica gel should be used for flash or gravity chromatography. 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 (such as C18 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 aluminium, glass, or plastic sheets for
thin layer chromatography. The hydroxy (OH) groups on the surface of silica can be functionalized to afford specialty silica gels that exhibit unique stationary phase parameters. These so-called functionalized silica gels are also used in organic synthesis and purification as insoluble reagents and
scavengers. Chelating groups have also been covalently bound to silica gel. These materials have the ability to remove metal ions selectively from aqueous solutions. 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. (See
SiGNa chemistry) Silica gel is not expected to biodegrade in either water or soil.
Cat litter Silica gel is also used as
cat litter, by itself or in combination with more traditional materials, such as clays including
bentonite. It is non-tracking and virtually odorless.
Food additive Silica gel, also referred to as silicon dioxide or synthetic amorphous silica (SAS), is listed by the FDA in the United States as
generally recognized as safe (GRAS), meaning it can be added to food products without needing approval. Silica is allowed to be added to food in the US at up to 2% as permitted under 21 CFR 172.480. In the EU, it can be in up to 5% concentrations. In 2018, a re-evaluation by the EFSA Panel on Food Additives and Nutrient Sources added to Food found no indications of toxicity even at the highest estimates of exposure level. Listed uses include:
anticaking agent, defoaming agent, stabilizer, adsorbent, carrier, conditioning agent, chill proofing agent, filter aid, emulsifying agent, viscosity control agent, and anti-settling agent. Silica can be found commonly in foods including baked goods, spices and herbs, dairy products, cocoa products, and more. The surface structure of silica gel allows the adsorption of some minerals that are dissolved in the water, or "ion exchange" as it is marketed. Due to the lack of regulations for domestic water filtration products, no studies validate the manufacturer claims regarding the effectiveness of the filtration system.
Humidity indicator (color-changing silica gel) Silica gel may be doped with a moisture indicator that gradually changes its color when it transitions from the
anhydrous (dry) state to the hydrated (wet) state. Common indicators are
cobalt(II) chloride and
methyl violet. Cobalt (II) chloride is deep blue when dry and pink when wet, but it is toxic and carcinogenic, and was reclassified by the
European Union in July 2000 as a toxic material.
Methyl violet may be formulated to change from orange to green, or orange to colorless. It also is toxic and potentially carcinogenic, but is safe enough to have medicinal uses.
Ferric and
ferrous salts, sometimes combined with small amounts of
sodium hydroxide, provide a better alternative. In particular,
ferric sulfate and
double salts like
ammonium iron(III) sulfate (iron alum),
ammonium iron(II) sulfate, and
potassium iron(III) sulfate all result in a color change from amber/yellow when dry to colorless/white when saturated. ==Hazards==