Ammonium hydroxide (NH4OH) is the name given to an aqueous solution of ammonia. Since ammonia ionizes to a small extent, ammonium and hydroxide ions will be present in an aqueous solution of ammonia. It is sometimes convenient to refer to these ions as "ammonium hydroxide". However this is a wrong way of describing the ionization. Instead, the following equation based on Bronsted-Lowry Theory should be used.
- NH3 + H2O ↔ NH4+ + OH-
With a base-ionization constant (Kb) of 1.8×10-5, in a 1M ammonia solution, about 0.4% of the ammonia will be ionized.
Ammonium hydroxide was an attempt by chemists to apply Arrhenius theory which did not seem to work earlier as Ammonia does not have any hydroxide ions (which Arhennius said was required for a substance to have alkaline properties).
Modern chemical analysis shows that hydroxide ions are rarely found in solution. Bronsted-Lowry theory completely explains this phenomenon without the introduction of this axiom, as shown in the above reaction, in which water acts as a base.