Oxonium is very acidic: at 25 °C, its pKa is -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 oxonium. The acidity of oxonium is the implicit standard used to judge the strength of an acid in water: strong acids must be better proton donors than oxonium, otherwise a significant portion of acid will exist in a non-ionized state. Unlike the oxonium that results from water's autodissociation, these oxonium 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 hydrogen ion concentration. Since free protons react with water to form oxonium cation, the acidity of an aqueous solution is determined by its oxonium concentration.
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 oxonium inside a dodecahedral cage . However, more recent ab initio molecular dynamics simulations have shown that, on average, the hydrated proton resides on the surface of the H3O+(H2O)20 cluster. Further, several disparate features of these simulations agree with their experimental counterparts suggesting an alternative interpretation of the experimental results.
Two other well-known structures are the Zundel cations and Eigen cations. The Eigen solvation structure has the oxonium ion at the center of an H9O4+ complex in which the oxonium is strongly hydrogen-bonded to 3 neighbouring water molecules . In the Zundel H5O2+ complex the proton is shared equally by two water molecules . Recent work indicates that both of these complexes represent ideal structures in a more general hydrogen bond network defect .
Isolation of the oxonium ion monomer in liquid phase was achieved in a nonaqueous, low nucleophilicity superacid solution (HF-SbF5SO2). The ion was characterized by high resolution O-17 nuclear magnetic resonance..
In 2007, Markovitch & Agmon have calculated for the first time ever the enthalpies and free energies of the various hydrogen bonds around the hydronium cation in liquid protonated water at room temperature and discussed the implementation for the proton hopping mechanism. Using molecular dynamics they were able to show that the hydrogen-bonds around the hydronium ion (formed with the three water ligands in the first solvation shell of the oxonium) are quite strong compared to those of bulk water.
The oxonium cation also forms stable compounds with the carborane superacid H(CB11H(CH3)5Br6) . X-ray crystallography shows a C3v symmetry for the oxonium ion with each proton interacting with a bromine atom each from three carborane anions 320 pm apart on average. The [H3O][H(CB11HCl11)] salt is also soluble in benzene. In crystals grown from a benzene solution the solvent co-crystallizes and a H3O.(benzene)3 cation is completely separated from the anion. In the cation three benzene molecules surround oxonium forming pi-cation interactions with the hydrogen atoms. The closest (nonbonding) approach of the anion at chlorine to the cation at oxygen is 348 pm.