This is the ion formed by the
dissociation of an
acid in water; it is also known as a hydronium or hydroxonium ion. Its formula is H
3O
+
According to the Bronsted-Lowry definition of acids and bases, a species acts as an acid when it donates protons (hydrogen ions, H+) to something else. Acids in solution donate a proton to water to form the oxonium ion, H3O+. It is this ion which acts as the proton donor when an acid in solution reacts with a base.