Hydronium is the common name for the cation H₃O⁺.

Nomenclature

According to IUPAC ion nomenclature, it should be referred to as "oxonium." Hydroxonium may also be used unambiguously to identify this ion. A draft IUPAC proposal recommends the use of oxonium and oxidanium in organic and inorganic chemistry contexts, respectively.

Acids and Acidity

Hydronium is the cation that forms from water in the presence of hydrogen ions. These hydrons do not exist in a free state: they are extremely reactive and are solvated by water. An acid is generally the source of these hydrons; however, since water can behave as an acid, hydronium exists even in pure water. This special case of water reacting with water to produce hydronium (and hydroxide) ions is commonly known as the self-ionization of water. The resulting hydronium ions are few and short-lived.

Hydronium is very acidic: at 25°C, its pKa=-1.7. It is also the most acidic species that can exist in water (assuming sufficient water for dissolution): any stronger acid will ionize and protonate a water molecule to form hydronium. The acidity of hydronium is the implicit standard used to judge the strength of an acid in water: strong acids must be better proton donors than hydronium, otherwise a significant portion of acid will exist in an unionized state. Unlike the hydronium that results from water's autodissociation, these hydronium ions are long-lasting and concentrated, in proportion to the strength of the dissolved acid.

The pH of a solution is a measure of its proton concentration. Since these protons react with water to form hydronium, the acidity of an aqueous solution is determined by its hydronium concentration.

Solvation

Researchers have yet to fully characterize the solvation of hydronium ion in water, in part because many different meanings of solvation exist. A freezing point depression study determined that the mean hydration ion in cold water is approximately H₃O⁺(H₂O)₆ (1): on average, each hydronium ion is solvated by 6 water molecules which are unable to solvate other solute molecules.

Some hydration structures are quite large: the H₃O⁺(H₂O)₂₀ magic ion number structure (called magic because of its increased stability with respect to hydration structures involving a comparable number of water molecules) might place the hydronium inside a dodecahedral cage (2). Because of the reactivity of the proton, it does not necessarily need to be associated with the water molecule in the centre of the cage. It could easily move around and be associated with any of the enclosing molecules.

Two other well-known structures are the Zundel and Eigen cations. Eigen placed the hydronium ion at the centre of an H₉O₄⁺ complex in which the hydronium is strongly hydrogen-bonded to 3 neighbouring water molecules (3). Eigen proposed an H₅O₂⁺ complex, in which the proton is shared equally by two water molecules (4). Recent work indicates that both of these complexes represent ideal structures in a more general hydrogen bond network defect (5).

References

1. Zavitsas, A. A. (2001) Properties of water solutions of electrolytes and nonelectrolytes. J. Phys. Chem. B 105 7805-7815.
2. Hulthe, G.; Stenhagen, G.; Wennerström, O. & C-H. Ottosson, C-H. (1997) Water cluster studied by electrospray mass spectrometry. J. Chromatogr. A 512 155-165.
3. Zundel, G. & Metzger, H. (1968) Energiebänder der tunnelnden Übershuß-Protenon in flüssigen Säuren. Eine IR-spektroskopische Untersuchung der Natur der Gruppierungen H₅O₂⁺ Z. Phys. Chem. 58 225-245.
4. Wicke, E.; Eigen, M. & Ackermann, Th. (1954) Über den Zustand des Protons (Hydroniumions) in wäßriger Lösung. Z. Phys. Chem. 1 340-364.
5. Marx, D.; Tuckerman, M. E.; Hutter, J. & Parrinello, M. (1999) The nature of the hydrated excess proton in water. Nature 397 601-604.