Silica gel is silicon dioxide (SiO2). It is a naturally occurring mineral that
is purified and processed into either granular or beaded form. As a desiccant,
it has an average pore size of 24 angstroms and has a strong affinity for moisture
molecules. The silica gel will pull in moisture at temperatures up to 220°F
(105°C). As temperature goes above 100°F, the rate of moisture pickup
will slow down but the silica gel will still work.
Silica gel performs best at room temperatures (70° to 90°F) and high humidity
(60 to 90% RH) and will drop the relative humidity in a container down to around
40% RH. In the United States, silica gel is commonly used in food and pharmaceutical
applications as only silica gel has been approved by the FDA for direct contact
with these items.
As with clay, silica gel, with its wide range of pore sizes, has the capability
of adsorbing compounds other than water. The relative order of adsorbability is:
water, ammonia, alcohols, aromatics, diolefins, olefins and paraffins. When the
potential for multicomponent adsorption is present, expect the more strongly adsorbed
compounds, such as water, to displace the more weakly held ones.
Indicating Silica Gel
Indicating silica gel is a silica gel bead or granule that has been washed with
a concentration of cobalt chloride ( a heavy metal salt). The cobalt chloride
is a deep blue color when it is dry and turns from blue to purple to pink as
it becomes saturated with moisture. Typically, the color changes as the desiccant
goes past 8% moisture levels (by weight) and indicates it is time to replace
The most typical use for an indicating silica gel is for a moisture sensitive
product that will be inspected regularly as it gives a quick visual indication
of how well it is doing. Because of the addition of cobalt chloride, indicating
silica gel should not be used in contact with products for consumption such
as food or pharmaceuticals.
The Dri-Box canister is packed with an indicating silica gel that is regenerable.
Instead of replacing the desiccant inside, heating the canister will reverse
the adsorbing action and allow the material to be reused.
Calcium sulfate (better known commercially as Drierite) is created by
the controlled dehydration of gypsum. It is a general purpose desiccant geared
mainly toward laboratory use. It is chemically stable, non-disintegrating, nontoxic,
non-corrosive and does not release its adsorbed water easily when exposed to
higher ambient temperatures.
The low cost of calcium sulfate must be weighed against its equally low adsorptive
capacity; it adsorbs only up to 10% of its weight in water vapor. Calcium sulfate
also has regeneration characteristics that tend to limit its useful life. Although
available, it is not normally sold in package form.