Concentrated sulfuric acid is not used to dry ammonia because it undergoes an acid-base reaction with ammonia to form ammonium sulfate. This reaction releases water molecules and results in the formation of a white solid, ammonium sulfate, instead of drying the ammonia.

The chemical equation for the reaction between concentrated sulfuric acid (H2SO4) and ammonia (NH3) is:

H2SO4 + 2 NH3 → (NH4)2SO4 + H2O

This reaction is highly exothermic, meaning it releases a significant amount of heat. The water molecules produced during this reaction can also contribute to the wetness or dampness in the surroundings, rather than drying the ammonia.

Therefore, concentrated sulfuric acid is not a suitable drying agent for ammonia due to its reactive nature and the formation of ammonium sulfate as a byproduct.