Silica gel is a drying agent that often comes in the form of small, clear beads or clear rock crystals placed in small packets made of paper or cloth. These packets are often packaged with commercial products to prevent damage caused by moisture. Silica gel packets can be found in a wide range of products, such as food, clothing, and electronics.‌

Silica gel is usually non-toxic, but it is a choking hazard, especially for small children. Silica gel packets are frequently labeled with “Do not eat” because of the risk of choking.

What is Desiccant Silica Gel?

Silica gel is a “desiccant,” which means that it's used to keep things dry. Silica gel is made from silicon dioxide — a compound naturally found in sand — and has small particles that can absorb large amounts of water. Therefore, desiccant silica gel packets are put in store-bought products to prevent damage due to humidity. Silica gel packets can often be found in the following products:

• Clothes (e.g., coats, shoes, and hats)

• Electronics (e.g., cellphone and camera boxes)

• Medication or vitamin bottles

• Foods (e.g., packets of dry fruit and beef jerky)

  
  

Manufacturers frequently label packets “Do not eat” because poison control centers have reported increases in incidences of people accidentally eating silica gel packets. Most cases involve young children.

What is silica gel used for?

Silica gel pouches are used to absorb moisture and keep things dry. Brands add them to new products, particularly shoes and handbags, to protect products from dampness.

From Wikipedia, the free encyclopedia

"Do not eat" redirects here. 

Colloidal silica gel with light opalescence

Silica gel is an amorphous and porous form of silicon dioxide (silica), consisting of an irregular tridimensional framework of alternating silicon and oxygen atoms with nanometer-scale voids and pores. The voids may contain water or some other liquids, or may be filled by gas or vacuum. In the last case, the material is properly called silica xerogel.

Silica xerogel with an average pore size of 2.4 nanometers has a strong affinity for water molecules and is widely used as a desiccant. It is hard and translucent, but considerably softer than massive silica glass or quartz; and remains hard when saturated with water.

Silica xerogel is usually commercialized as coarse granules or beads, a few millimeters in diameter. Some grains may contain small amounts of indicator substance that changes color when they have absorbed some water. Small paper envelopes containing silica xerogel pellets, usually with a "do not eat" warning, are often included in dry food packages to absorb any humidity that might cause spoilage of the food.

'Wet' silica gel, as may be freshly prepared from alkali silicate solutions, may vary in consistency from a soft transparent gel, similar to gelatin or agar, to a hard solid, namely a water-logged xerogel. It is sometimes used in laboratory processes, for example to suppress convection in liquids or prevent settling of suspended particles.

Properties

Silica gel's high specific surface area (around 750–800 m²/g (230,000–240,000 sq ft/oz)) allows it to adsorb water readily, making it useful as a desiccant (drying agent). Silica gel is often described as "absorbing" moisture, which may be appropriate when the gel's microscopic structure is ignored, as in silica gel packs or other products. However, material silica gel removes moisture by adsorption onto the surface of its numerous pores rather than by absorption into the bulk of the gel.

Silica gel is able to adsorb up to 37% of its own weight in moisture in high-humidity environments.This moisture can be released upon heating at 120 °C for extended periods of time. This makes it reusable multiple times with very little, if any, loss of efficiency.

Uses

Desiccant

Silica gel, as beads packed in a permeable bag, is a commonly used desiccant.

In many items, moisture encourages the growth of mold and spoilage. Condensation may also damage other items such as electronics and may speed the decomposition of chemicals, such as those in vitamin pills. Through the inclusion of silica gel packets, these items can be preserved longer. It may aid the drying out of electronic equipment during attempts to restore items that have been exposed to moisture accidentally.

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 can adsorb about 40 percent of its weight in moisture and can take the relative humidity in a closed container down to about 40 percent. Once saturated, you can propel the moisture off and reuse silica gel by heating it above 300 degrees F (150 C).

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 USA, 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.

Chromatography column  In chemistry, silica gel is used in chromatography as a stationary phase.

Cat litter

Silica gel is also used as cat litter

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 USA 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, chillproofing 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.

Water filtration

Given the water adsorption properties of silica gel, it is used in domestic water filters. 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)

Indicating 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.

Regeneration

Once saturated with water, the gel may be regenerated by heating it to 120 °C (248 °F) for 1–2 hours. Some types of silica gel will "pop" when exposed to enough water. This is caused by breakage of the silica spheres when contacting the water.

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