Desiccant

A desiccant is a chemical which is hygroscopic enough to absorb water from hydrated compounds in the same sealed environment.

Common desiccants

• Calcium
• Calcium chloride
• Calcium oxide
• Concentrated sulfuric acid
• Copper sulfate (anhydrous)
• Magnesium sulfate
• Phosphorus pentoxide
• Silica gel
• Sodium and other alkali metals
• Sodium hydroxide
• Sodium oxide

Comparison

Substance1	pH	Water capacity	Effectiveness	Reversible	Notes
Acetonitrile		High	High	Yes	Rarely used
Activated alumina	Basic or acidic	Medium	High	Yes	Can also be used to adsorb fluorides
Activated charcoal		Medium	Medium	Yes	Will also adsorb other gasses
Aerogel		High	High	Yes	Expensive
Aluminium nitrate	Slightly acidic	Medium	Medium	No
Bentonite clay
Calcium	Alkaline	High	Very high	No	Reaction with water releases large amounts of hydrogen
Calcium chloride	Neutral	High	Medium	Yes	Deliquescent; often used in drying tubes
Calcium hydride	Alkaline	High	Very high	No
Calcium nitrate	Neutral	Medium	Medium	Yes
Calcium oxide	Alkaline	High	High	No	Calcium oxide will only remove water from ethanol until 5000 ppm.
Calcium sulfate	Neutral	Low	High	Yes	Very fast and efficient drying agent, but a lot of drying agent might be necessary
Cement (Portland)	Alkaline	Medium	Medium	No	Used in desiccators, cannot be used directly
Cobalt(II) chloride				Yes	Mostly used as water indicator
Copper(II) sulfate	Neutral	Low	Medium	Yes	Mostly used as water indicator
Magnesium				No	Reaction is very slow, rarely used
Magnesium sulfate	Neutral	High	Medium	Yes	Good multipurpose drying agent; exists in powder and granular form; has the ability to absorb a lot of water
Magnesium chloride	Neutral	High	Medium	Yes	Deliquescent
Molecular sieves	Weakly basic	High	High	Yes	Unsuitable for drying ketones
Potassium carbonate	Acidic	Medium	High	No	Good for thoroughly drying predried compounds
Potassium carbonate	Alkaline	Low	Medium	Yes	Only for alkaline compounds
Potassium hydroxide	Alkaline	High	High	Yes
Silica gel	Weakly acidic	High	Medium	Yes
Sodium	Alkaline	High	Very High	No	More often used to remove traces of water from aprotic solvents
Sodium hydroxide	Alkaline	High	High	Yes	Very effective for basic compounds, such as amines; caustic
Sodium oxide	Alkaline	High	Very High	No	More effective when used to dry compounds predried with another desiccant
Sodium sulfate	Neutral	High	Low	Yes	Used to dry solvents; Requires lots of it; only good for predrying;
Sulfur trioxide	Acidic	High	Very high	No	Tends to form a mist of sulfuric acid in contact with moist air
Sulfuric acid (concentrated)	Acidic	High	High	Yes, difficult	Used in desiccators, cannot be used to dry solutions directly
Zinc chloride	Acidic			Yes	Regenerating must be done in a stream of hydrogen chloride

¹All compounds are considered anhydrous.

Gallery