Organic solvents and bases absorb moisture especially polar ones. When dry reaction condition required, these solvents and bases need to be dry. To dry organic solvents and bases, one simple method is use of distillation but due to hygroscopic nature, water is being absorbed at ppm level. To dry properly, various water absorbing substances used. All these substances, specific for specific solvents and base are as given below.
You can store dried solvents over molecular sieves to maintain anhydrous condition as well as remove any traces of water. Protic solvents like ethanol and methanol can be dried by distillation and molecular sieves.