Hydronium Ion Definition

Chemistry Glossary Definition of Hydronium Ion

hydronium cation
The hydronium cation is the simplest type of oxonium ion. The ion is made by adding another hydrogen to water. Jacek FH/Wikipedia Commons/Public Domain

The hydronium ion or hydronium is the name given to the H3O+cation, derived from the protonation of water. The hydronium ion is the simplest type of oxonium ion. It is produced when an Arrhenius acid dissolves in water. Hydronium is also abundant in the interstellar medium and tails of comets.

Hydronium Ion and pH

In chemistry, pH is calculated as the ratio of hydronium ions to hydroxide (OH-) ions. When the hydrogen ion (H+) concentration is discussed, it's actually hydronium. The auto-dissociation of water at equilibrium is:

2 H2O ⇌ OH + H3O+

In pure water, the number of hydroxide and hydronium ions is equal and the pH is neutral or 7.


  • Meister, Erich; Willeke, Martin; Angst, Werner; Togni, Antonio; Walde, Peter (2014). "Confusing Quantitative Descriptions of Brønsted-Lowry Acid-Base Equilibria in Chemistry Textbooks – A Critical Review and Clarifications for Chemical Educators". Helv. Chim. Acta. 97 (1): 1–31. doi:10.1002/hlca.201300321
  • Rauer, H (1997). "Ion composition and solar wind interaction: Observations of comet C/1995 O1 (Hale-Bopp)". Earth, Moon, and Planets. 79: 161–178. doi:10.1023/A:1006285300913997EM&P...79..161R. doi:10.1023/A:100628530091
mla apa chicago
Your Citation
Helmenstine, Anne Marie, Ph.D. "Hydronium Ion Definition." ThoughtCo, Aug. 25, 2020, thoughtco.com/definition-of-hydronium-ion-604531. Helmenstine, Anne Marie, Ph.D. (2020, August 25). Hydronium Ion Definition. Retrieved from https://www.thoughtco.com/definition-of-hydronium-ion-604531 Helmenstine, Anne Marie, Ph.D. "Hydronium Ion Definition." ThoughtCo. https://www.thoughtco.com/definition-of-hydronium-ion-604531 (accessed May 31, 2023).