Hydronium

Hydronium is a polyatomic ion with the chemical formula H3O+. It is the type of oxonium ion produced by protonation of water. This cation is often used to represent the nature of the proton in aqueous solution, where the proton is highly solvated (bound to a solvent).

Formation

Hydronium ions are formed from the reaction between water and a proton (H+). This reaction is reversible, meaning that hydronium ions can also react to form water and a proton. The equilibrium constant for this reaction, known as the ion product, is extremely high, which means that at room temperature, nearly all protons in water exist as hydronium ions.

Properties

Hydronium ions are highly reactive and are the most common cause of acidic behavior in water. They can donate a proton to a base in a Brønsted-Lowry acid-base reaction, or they can act as a Lewis acid and accept a pair of electrons. Hydronium ions also play a key role in the self-ionization of water.

Role in pH

The concentration of hydronium ions in a solution is used to calculate the pH of the solution. The pH is defined as the negative logarithm (base 10) of the hydronium ion concentration. Therefore, a low pH indicates a high concentration of hydronium ions, and the solution is acidic. Conversely, a high pH indicates a low concentration of hydronium ions, and the solution is basic.

See also

• Water (molecule)
• Acid dissociation constant
• Acid–base reaction
• Protonation