Desiccants: How Do These Beads Dry My Sample? 

Keeping samples dry is oftentimes necessary to maintain their applicability in further processing. For instance, metal samples are required to be dry to avoid corrosion damage. In addition, some dry (water-free) solvents are needed in some organic synthesis. Therefore, an appropriate water-attracting material is essential for these purposes. 

Desiccants are materials that can serve this purpose. But how does that work? 

Common desiccants work by adsorbing moisture to their surface, i.e., by adhering the moisture in the atmosphere to their surface without penetration to their porous structure (absorption). However, some desiccants absorb the water molecules in the air, but these desiccants are not common. 

Some desiccants also have a saturation indicator, which turns color if it cannot hold additional water molecules. For example, anhydrous cobalt(II) chloride (CoCl₂) is added to silica gel as an indicator. In the absence of water, it is blue-colored. When it bonds with two water molecules (CoCl₂.2H₂O), it turns purple. Therefore, periodic drying of silica gels is necessary to maintain their proper function. 

Table 1 summarizes the commercially available desiccants and their corresponding capacities. 

Desiccant	Composition	Appearance	Adsorption capacity
Clay	Bentonite and montmorillonite clay		30% of its weight at 25oC
Silica gel	Silica beads		40% of its weight up to 49oC
Calcium chloride	CaCl2 powder		200% of its weight Effective in a range of   -20-90oC

Suci Amelia

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