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
By
Anne Marie Helmenstine, Ph.D.
Anne Marie Helmenstine, Ph.D.
Chemistry Expert
  • Ph.D., Biomedical Sciences, University of Tennessee at Knoxville
  • B.A., Physics and Mathematics, Hastings College
Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels.
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Updated on February 04, 2019

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

This image depicts the distribution of electric potential across the hydronium cation.
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What Is Hydronium? Chemistry Definition
By Anne Marie Helmenstine, Ph.D.

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.

Sources

  • 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
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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 April 26, 2024).