hydroniumionin

The hydronium ion, with the chemical formula H3O+, is the positive ion formed when a proton (H+) attaches to a water molecule in aqueous solutions. It is the conjugate acid of water and a central species in acid–base chemistry of water-based systems. In typical acid–water chemistry, acids donate protons to water, producing hydronium ions according to reactions such as HA + H2O ⇌ A− + H3O+. The hydronium ion is largely responsible for the acidic character of a solution, and its concentration determines the pH: pH = -log10 [H3O+].

The autoprotolysis (autoionization) of water, 2 H2O ⇌ H3O+ + OH−, sets the relationship between hydronium and hydroxide ions in pure water at a given temperature. At 25°C, the equilibrium constant Kw is about 1.0 × 10−14, implying that [H3O+] and [OH−] are constrained by this product.

Structure and solvation are often summarized by describing H3O+ as a water molecule with an added proton, yielding a trigonal pyramidal arrangement around the oxygen. In liquid water, the proton is highly solvated and can participate in extended hydrated forms and proton-transfer networks; the simple H3O+ representation is a useful teaching model, while real solutions involve dynamic solvation structures such as Zundel and Eigen-type hydrated protons.

Hydronium ions occur in all aqueous acids and play a fundamental role in driving many chemical, biological, and electrochemical processes. They are typically short-lived intermediates in proton-transfer steps but exist in a steady-state concentration that governs reaction rates and acidity in water.