Hydronium

Hydronium, also known as hydroxonium in traditional British English, is the cation H₃O⁺, a type of oxonium ion formed when water gains a proton. It is commonly considered the positive ion that appears when an Arrhenius acid dissolves in water, since such acid molecules release a proton to nearby water molecules. However, acids actually require more than one water molecule around them to ionize, producing aqueous H₃O⁺ and a conjugate base.

Three main structures for the aqueous proton have experimental support: the Eigen cation, a tetrahydrate (H₃O⁺(H₂O)₃); the Zundel cation, a symmetric dihydrate (H⁺(H₂O)₂); and the Stoyanov cation, an expanded Zundel cation that is a hexahydrate (H⁺(H₂O)₂(H₂O)₄). Spectroscopic evidence from well-defined infrared spectra strongly supports the Stoyanov cation as the dominant form, leading to the suggestion that the symbol H⁺(aq) should be used instead of hydronium whenever possible.

The molar concentration of hydronium (or H⁺) ions determines a solution’s pH, given by pH = –log([H₃O⁺]/M), where M = mol/L. The concentration of hydroxide ions similarly determines pOH. In pure water, molecules auto-dissociate into aqueous protons and hydroxide ions in an equilibrium, producing equal numbers of both ions, making the solution neutral. At 25 °C, pure water has a pH of 7 and a pOH of 7 (these values change with temperature; see self-ionization of water). A pH below 7 indicates an acidic solution, while a pH above 7 indicates a basic one.

According to IUPAC nomenclature for organic chemistry, the hydronium ion should be called oxonium. Hydroxonium can also be used unambiguously to identify it. An oxonium ion is any cation with a trivalent oxygen atom.

Because H₃O⁺ and NH₃ have the same number of electrons, they are isoelectronic. H₃O⁺ has a trigonal pyramidal molecular geometry with the oxygen atom at the apex. The H–O–H bond angle is about 113°, and the center of mass lies very close to the oxygen. Since the base of the pyramid consists of three identical hydrogen atoms, the molecule’s symmetric top configuration belongs to the C₃ᵥ point group. Due to this symmetry and its dipole moment, rotational selection rules are ΔJ = ±1 and ΔK = 0. The transition dipole lies along the c-axis, and because negative charge is localized near oxygen, the dipole moment points to the apex, perpendicular to the base plane.

The hydrated proton is highly acidic; at 25 °C, its pKa is approximately 0. Commonly given values for pKₐₐq(H₃O⁺) are 0 or –1.74. The former uses the convention that the activity of the solvent (water) in a dilute solution is 1, while the latter uses the concentration of water in pure liquid (55.5 M). Silverstein has shown the latter value is thermodynamically unsupportable. The disagreement arises because defining the pKa of H₃O⁺ in water requires water to act simultaneously as solute and solvent; IUPAC has not issued an official definition to resolve this. Burgot argues that the process H₃O⁺(aq) + H₂O(l) ⇄ H₂O(aq) + H₃O⁺(aq) is not thermodynamically well-defined. For an estimate, Burgot suggests using the measured pKₐEtOH(H₃O⁺) = 0.3 in ethanol and applying the correlation pKₐaq = pKₐEtOH – 1.0 (±0.3), yielding pKₐaq(H₃O⁺) = –0.7 (±0.3). Conversely, Silverstein shows that Ballinger and Long’s experimental results support a pKa of 0.0, and Neils and Schaertel provide additional arguments for that value.

The aqueous proton is the most acidic species that can exist in water (assuming enough water for dissolution); any stronger acid will ionize and yield a hydrated proton. The acidity of H₃O⁺(aq) is the implicit standard for judging acid strength in water: strong acids must be better proton donors than H₃O⁺(aq), or else a significant portion of the acid remains non-ionized (i.e., a weak acid). Unlike H₃O⁺(aq) in neutral solutions from water’s autodissociation, in acidic solutions H₃O⁺(aq) is long-lasting and concentrated, proportional to the dissolved acid’s strength. pH was originally conceived as a measure of hydrogen ion concentration in aqueous solution; virtually all free protons are quickly hydrated, so acidity is more accurately characterized by H₃O⁺(aq) concentration. In organic syntheses, such as acid-catalyzed reactions, hydronium (H₃O⁺) is used interchangeably with H⁺; choosing one over the other has no significant effect on the reaction mechanism.

The solvation of hydronium ion in water is not yet fully characterized, partly because “solvation” has many meanings. A freezing-point depression study found that the mean hydration ion in cold water is approximately H₃O⁺·6H₂O: on average, each hydronium is solvated by six water molecules that cannot solvate other solutes. Some hydration structures are quite large; the magic ion number structure (so called for its increased stability compared to hydration structures with a similar number of water molecules, analogous to the term in nuclear physics) might place hydronium inside a dodecahedral cage. However, more recent ab initio molecular dynamics simulations show that, on average, the hydrated proton resides on the surface of the H₃O⁺(H₂O)₂₀ cluster, and several features of these simulations agree with experimental counterparts.