(Tue 09:38 23/Jun/09)The normal absorbent is Silica Gel .
Self indicating silica gel is blue when it is fresh then turns pink to show that it needs to be reactivated (that it can not absorb any more moisture)
To reactivate it (dry it} you just heat it in an oven at 100 C for a few hours, it will go back to blue and can be used over and over again.
theprof
(Tue 15:14 23/Jun/09)There is no such thing as a "normal absorbent". The selection of a desiccant is tailored to a particular application as they are not universal for all purposes.
Silica Gel is a highly porous form of Silicon Dioxide and consequently it is acceptable to consider it a "chemical".
However, I would suggest that if you asked a chemist this question, the first answer that would come to mind is anhydrous Calcium chloride, which is a highly hygroscopic compound in the form of white crystals, powder or flake. addition,
The compound and its solutions absorb moisture from the air at various rates depending on Calcium Chloride concentrations, relative humidity and vapour pressure of water in the air, temperature, surface area of exposed material, and the rate of air circulation. At 40% and 95% relative humidity and 25�C, one gram anhydrous calcium chloride may absorb about 1.4 g and 17 g water.
Yes, there are more modern dessicants available such as molecular sieves, but their selection can be problematical for particular applications due to the grade sizes and often, the absorbency factor is little different from that of Calcium Chloride. In addition, Calcium Chloride is probably the cheapest dessicant available, which is one of the reasons why so much of it is used for commercial applications.
Finally, Silica Gel is often doped with substances that enable the colour change outlined by EDDIE51. These substances along with additional compounds such as fungicides and mould inhibitors preclude the use of Silica Gel for many applications in a desiccator.
theprof
(Tue 15:17 23/Jun/09)I intended to add in the final paragraph that many of these mould inhibitors, fungicides etc are poisonous. This is why Silica Gel is not suitable as a universal absorbent.
EDDIE51
(Tue 18:57 23/Jun/09)Point taken Prof, prehaps I should have said that the most common desiccant is Silica Gel. Suitable for 'everyday' use.
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