The Hydronium reference article from the English Wikipedia on 24-Apr-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Hydronium

Hydronium (also called oxonium) is a positively-charged ion that forms from water in the presence of hydrogen ions. The formula for hydronium is H₃O⁺.

Hydronium is present whenever an acid is dissolved in water; the hydrogen ions (H⁺ or protons) released by the acid do not exist in a free state. Water will also spontaneously dissociate into hydronium and hydroxide ions, in the following reaction:

2H₂O ↔ H₃O⁺ + OH⁻

The alkalinity of the OH⁻ ion released in this reaction balances hydronium's acidity, and water remains neutral at a pH of 7.0, i.e., at a hydronium ion concentration of 1 × 10^-7.0 mol/l at 298 K.

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