Hydronium
In chemistry, hydronium is the common name for the aqueous cation H
3O+
, the type of oxonium ion produced by protonation of water. It is the positive ion present when an Arrhenius acid is dissolved in water, as Arrhenius acid molecules in solution give up a proton (a positive hydrogen ion, H+) to the surrounding water molecules (H2O).
Determination of pH
It is the presence of hydronium ion relative to hydroxide that determines a solution's pH. The molecules in pure water auto-dissociate into hydronium and hydroxide ions in the following equilibrium:
In pure water, there is an equal number of hydroxide and hydronium ions, so it has a neutral pH of 7. A pH value less than 7 indicates an acidic solution, and a pH value more than 7 indicates a basic solution.
Nomenclature
According to IUPAC nomenclature of organic chemistry, the hydronium ion should be referred to as oxonium.Hydroxonium may also be used unambiguously to identify it. A draft IUPAC proposal also recommends the use of oxonium and oxidanium in organic and inorganic chemistry contexts, respectively.
