Hydronium
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In chemistry, hydronium or hydroxonium, is the common name for the aqueous cation H3O+, the type of oxonium ion produced by protonation of water. It is often viewed as 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). In fact, acids must be surrounded by more than a single water molecule in order to ionize, yielding aqueous H+ and conjugate base.
The hydrated proton is very acidic: at 25 °C, its pKa is approximately 0.
Solid hydronium salts
For many strong acids, it is possible to form crystals of their hydronium salt that are relatively stable. These salts are sometimes called acid monohydrates. As a rule, any acid with an ionization constant of 109 or higher may do this. Acids whose ionization constants are below 109 generally cannot form stable H3O+ salts. For example, nitric acid has an ionization constant of 101.4, and mixtures with water at all proportions are liquid at room temperature. However, perchloric acid has an ionization constant of 1010, and if liquid anhydrous perchloric acid and water are combined in a 1:1 molar ratio, they react to form solid hydronium perchlorate (H3O+·ClO4-).
The hydronium ion also forms stable compounds with superacids, like the carborane superacid H(CB11H(CH3)5Br6).
