In chemistry, hydronium is the common name for the aqueous cation H3O+ 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).

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 equillibrium:

2H2O is in equilibrium with OH− + H3O+

In pure water, there is an equal number of hydroxide and hydronium ions, and the pH is perfectly neutral (7.0). A pH value less than 7 indicates an acidic solution.

For more, visit the Wikipedia entry for hydronium, URL: http://en.wikipedia.org/wiki/Hydronium