Ammonium - Acid Base Properties

Acid Base Properties

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

H+ + NH₃ → NH₄+

The acid dissociation constant (pK_a) of NH₄+ is 9.25.

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

NH₄+ + B- → HB + NH₃

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:

H₃O+ + NH₃ H₂O + NH₄+

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.

Formation of ammonium compounds can also occur in the vapor phase; for example, when ammonia vapor comes in contact with hydrogen chloride vapor, a white cloud of ammonium chloride forms, which eventually settles out as a solid in a thin white layer on surfaces.

The conversion of ammonium back to ammonia is easily accomplished by the addition of strong base.