Anhydrous methanol is methanol that is free of water. Methanol is hygroscopic, which means it absorbs moisture, including moisture from the air.
Pharmaceuticals and many other useful materials are made by reacting precursors in a process called chemical synthesis. Many reactions are conducted in liquids other than water. The presence of a small amount of water can produce undesirable results.
Chemists need to determine the identity of unknown substances. Contaminants in the solvents can interfere with these tests.
Storing Anhydrous Solvents
The air space in the bottle of an anhydrous solvent is usually filled with dry nitrogen or argon instead of air. Dry nitrogen or argon contain insignificant amounts of water vapor. When an anhydrous solvent is opened, the airspace above the solvent should be replaced with fresh dry gases.
Store Over Hygroscopic Solids
In the lab, hygroscopic solids such as anhydrous magnesium sulfate are sometimes added to bottles of solvent to remove any absorbed water.
Other Solvent Problems
Some solvents form a mixture with water that cannot be purified by single batch distillation. This is called an azeotrope. Complex distillation methods are used to make these solvents anhydrous. Methanol does not form an azeotrope with water, but solvents that do should be handled as described above.