Desiccants are compounds that maintain a state of dryness in an enclosed container. The desiccant absorbs the water in the air or solution. Many chemicals react with water and require storage in a dry environment. Some desiccants are added to an aqueous solution and bind the water in the solution. Several types of desiccants can be regenerated by heating to remove the water. After regeneration the desiccant is used again. Different types of desiccants dry specific types of chemicals and compounds.
Calcium Chloride
Calcium chloride, a salt, binds water when in a solution with any of the following chemicals; aryl halides, esters, ethers, alkyl halides and saturated hydrocarbons. It binds with water to form clumps. When the calcium chloride stops clumping, all the water is bound. This desiccant is frequently used with drying tubes, as a packaging aid for shipping and as a drying agent to produce sodium carbonate from kelp.
Anyhydrous Magnesium Sulfate
Anhydrous magnesium sulfate is used for drying nitril, ester, aldehyde, ketone and acid compounds. This is frequently used in solution preparation and works in the same way as calcium chloride. Once all the water binds to the magnesium sulfate, it is filtered out of the solution. Make your own anhydrous magnesium sulfate at home by purchasing Epsom salt and heating the salt to 200 degrees Celsius for one hour. Epsom salt is magnesium sulfate bound to water. Heating releases the water and results in the formation of the anhydrous desiccant.
Silica Gel
Silica gel is used to dry most organic compounds. It works by absorbing water vapor from the air. This desiccant comes in granular or small beads. When water vapor comes in contact with the granules, it binds to the gel. There are two forms of silica gel desiccant; indicating and non-indicating. Indicating silica gel changes color as it absorbs water vapor. When the gel completely changes color, it can no longer absorb water vapor and needs regeneration. Silica gel regenerates with heating to 100 to 350 degrees Celsius. Non-indicating silica gel will not change color. It is inexpensive and can be purchased in small sachets or bulk packs.
Anhydrous Sodium Sulfate
Anhydrous sodium sulfate comes in granular or powder form. It binds with water in solution with aryl halides, acids, ketones, aldehydes and alkyl halides. This is the sodium salt of sulfuric acid. It works in the same way as anhydrous magnesium sulfate. This desiccant is more efficient than anhydrous magnesium sulfate because it binds more water per mass. However this process of binding water takes longer than using magnesium sulfate and is only effective as a desiccant at temperatures below 30 degrees Celsius. Sodium sulfate cannot be regenerated.
Calcium Sulfate
Calcium sulfate is used to dry industrial gases, organic solids, organic liquids, refrigerants and air. Calcium sulfate is found naturally in gypsum. Heating gypsum to 180 degrees Celsius produces the desiccant calcium sulfate. Unlike other desiccants listed previously, calcium sulfate forms a crystal when it binds with water in a solution. Regenerate the desiccant by heating to 210 degrees Celsius for one hour. A color indicating form of calcium sulfate is made by adding calcium chloride. This indicating form works in the same way as indicating silica gel.
Clay
Clay, also known as activated clay, is a desiccant when used at temperatures below 120 degrees Celsius. At temperatures above 120 degrees Celsius, it give off moisture rather than absorb moisture. Clay desiccants are found as small gray pellets and remove water vapor from air. This desiccant is used for shipping and storage and is the least expensive of all available desiccants.
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