Chemistry Terminology: Definition of pOH

How pOH values relate to acidity and basicity

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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 October 24, 2019

pOH is a measure of hydroxide ion (OH-) concentration. It is used to express the alkalinity of a solution.

Aqueous solutions at 25 degrees Celcius with pOH less than 7 are alkaline, pOH greater than 7 are acidic and pOH equal to 7 are neutral.

How to Calculate pOH

pOH is calculated based on pH or hydrogen ion concentration ([H+]). Hydroxide ion concentration and hydrogen ion concentration are related:

[OH-] = Kw / [H+]

Kw is the self-ionization constant of water. Taking the logarithm of both sides of the equation:

pOH = pKw - pH

An approximation is that:

pOH = 14 - pH

While the approximation works well in many settings, there are exceptions for which the pKw value should be used instead.

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Helmenstine, Anne Marie, Ph.D. "Chemistry Terminology: Definition of pOH." ThoughtCo, Feb. 16, 2021, thoughtco.com/definition-of-poh-in-chemistry-605893. Helmenstine, Anne Marie, Ph.D. (2021, February 16). Chemistry Terminology: Definition of pOH. Retrieved from https://www.thoughtco.com/definition-of-poh-in-chemistry-605893 Helmenstine, Anne Marie, Ph.D. "Chemistry Terminology: Definition of pOH." ThoughtCo. https://www.thoughtco.com/definition-of-poh-in-chemistry-605893 (accessed March 23, 2023).