protonatedwater

Protonated water refers to water that has gained a proton, most commonly the hydronium ion, H3O+. The term also extends to protonated water clusters such as H+(H2O)n formed in hydrated environments. In aqueous systems, proton transfer among water molecules creates a dynamic network in which hydronium and various hydrated proton species coexist.

In acid–base chemistry, hydronium is the conjugate acid of water and is produced when an acid donates a proton to a water molecule. In pure water at 25°C, autoprotolysis yields equal concentrations of H3O+ and OH−, with [H3O+] about 1×10−7 M, corresponding to pH 7. Hydronium behaves as a relatively strong acid in water, readily donating a proton to bases to reform water and regenerate the neutral solvent.

Structurally, the H3O+ ion is typically described as a trigonal pyramidal species centered on oxygen, with three O–H bonds and a lone pair on the oxygen. In solution, the proton is often solvated by additional water molecules, leading to hydrated-proton motifs such as the Zundel cation H5O2+ (a proton shared between two water molecules) and the Eigen cation H9O4+ (a central H3O+ solvated by three water molecules). These clusters are important models for understanding proton transport in water, including proton hopping via the Grotthuss mechanism.

Protonated water species are central to acidity in aqueous media, proton conduction in electrolytes and membranes, and various analytical techniques that generate protonated water clusters as part of ionization processes.