The ammonium (more obscurely: aminium) cation is a positively charged polyatomic cation with the chemical formula NH+
4. 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 (N+R4), where one or more hydrogen atoms are replaced by organic radical groups (indicated by R).
In the substitutive nomenclature NH4+ is denoted by the name azanium instead of ammonium.
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 hydronium 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.
Ammonium cation is found in a variety of salts such as ammonium carbonate, ammonium chloride, and ammonium nitrate. Most simple ammonium salts are very soluble in water. An exception is ammonium hexachloroplatinate, the formation of which was once used as a test for ammonium. The ammonium salts of nitrate and especially perchlorate are highly explosive, in these cases ammonium is the reducing agent.
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