Ammonium

The ammonium cation is a positively charged polyatomic ion with the chemical formula NH₄⁺. It is formed by the protonation of ammonia (NH₃). Ammonium is also a general name for positively charged or protonated substituted amines and quaternary ammonium cations (NR₄⁺), where one or more hydrogen atoms are replaced by organic groups (indicated by R).

Acid base properties

The ammonium ion is generated when ammonia, a weak base, reacts with Brønsted acids (proton donors):

The ammonium ion is mildly acidic, reacting with Brønsted bases to return to the uncharged ammonia molecule:

Thus, treatment of concentrated solutions of ammonium salts with strong base gives ammonia. When ammonia is dissolved in water, a tiny amount of it converts to ammonium ions:

The degree to which ammonia forms the ammonium ion depends on the pH of the solution. If the pH is low, the equilibrium shifts to the right: more ammonia molecules are converted into ammonium ions. If the pH is high (the concentration of hydrogen ions is low), the equilibrium shifts to the left: the hydroxide ion abstracts a proton from the ammonium ion, generating ammonia.