Drying Chemicals (Removing Water)

Many organic compounds are isolated from aqueous solutions. When someone wants to remove all traces of water from these chemicals, an anhydrous salt (e.g., anhydrous sodium sulfate) is added to adsorb any water present in the solution.

The principle behind this process is that many ionic compounds exist as hydrates when isolated as solid crystals.  An example of this is sodium sulfate (Na₂SO₄). Normally when this salt forms crystals, it will bind as part of its ionic structure ten molecules of water.  However, you can isolate the anhydrous form that contains no water, and this is simply referred to as anhydrous sodium sulfate.  When added to a solution containing water, since the hydrated form is most stable, it will adsorb all of the water present in order to produce the hydrate. If the anhydrous salt is added to an organic liquid that contains some water, it will completely adsorb the water from the solution leaving the organic chemical completely dry.

Do not confuse "dry" with "liquid" for organic chemicals. The term "dry" refers to something without water, not something that does not contain any liquid. Examples you might be familiar with is dry ice which is simply solid carbon dioxide. Chemists think of chemicals which lack water as being dry, regardless whether the chemical is solid, liquid or gases.  Therefore, if you remove water, you dry that chemical.

The advantage of using anhydrous sodium sulfate is that it can bind almost twice its mass of water. Therefore, if you add 3.0 grams of anhydrous sodium sulfate, you can actually remove up to about 4.0 grams of water (about 1.4 times its mass) from the solution. This is an effective dehydrating agent since the salt that has bound the water usually adhers together and can be easily isolated via filtration.