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

Common desiccants


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.[1]
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 High No Reaction is very slow, rarely used; Mostly used for removing traces of water
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 Drying takes hours to days; 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 Low Low Yes Regenerating must be done in a stream of hydrogen chloride

1All compounds are considered anhydrous.


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  1. Ford, S. G.; Marvel, C. S., Organic Syntheses; Wiley: New York, 1943; Collect. Vol. 11, p 373.