In aqueous solution, ammonia deprotonates a small fraction of the water to give ammonium and hydroxide according to the following equilibrium:

NH3 + H2O ⇌ NH4+ + OH−.

In a 1M ammonia solution, about 1.42% of the ammonia is converted to ammonium, equivalent to a pH of 11.63. The base ionization constant is

Kb = [NH4+][OH−]/[NH3] = 1.8×10−5