Silica gel is an amorphous form of silicon dioxide, which is synthetically produced in the form of hard irregular granules (having the appearance of crystals) or hard irregular beads in Roorkee. A micro porous structure of interlocking cavities gives a very high surface area (800 square meters per gram). It is this structure that makes silica gel a high capacity desiccant. Water molecules adhere to the gels surface because it exhibits a lower vapour pressure than the surrounding air. When an equilibrium of equal pressure is reached, no more adsorption occurs. Thus the higher the humidity of the surrounding air, the greater the amount of water that is adsorbed before equilibrium is reached. It is in these higher humidity conditions (above 50% Relative Humidity) that stored or in-transit items are susceptible to damage.
The beauty of silica gel is the physical adsorption of water vapour into its internal pores. There is no chemical reaction, no by products or side effects. Even when saturated with water vapour, silica gel still has the appearance of a dry product, its shape unchanged.
Advantages of silica gel as a desiccant
Silica gel has many other properties that recommend it as a desiccant.
1. It will adsorb up to one third of its own weight in water vapour. This adsorption efficiency is approximately 35% greater that typical desiccant clays, making silica gel the preferred choice where weight or efficiency are important factors.
2. It has an almost indefinite shelf life if stored in airtight conditions.
3. It can be regenerated and reused if required. Gently heating silica gel will drive off the adsorbed moisture and leave it ready for reuse.
4. It is a very inert material, it will not normally attack or corrode other materials and with the exception of strong alkalis and hydrofluoric acid is itself resistant to attack.
5. It is non-toxic and non-flammable.
6. It is most frequently and conveniently used packed in a breathable sachet or bag. These are available in a wide range of sizes suitable for use with a wide range of applications.
Standard white silica gel is referred to as being non indicating. As it adsorbs moisture it remains physically unchanged. Non-indicating silica gel is both cheap and effective, available loose in bulk packs or packed in sachets.
indicating silica gels are colored gels whose colour changes as they adsorb moisture.
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