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 (pKa) 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.
Read more about this topic: Ammonium
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