Chemistry Terminology: Definition of pOH

How pOH values relate to acidity and basicity

female environmental engineer using PH probe

Nicola Tree / Getty Images

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.

Format
mla apa chicago
Your Citation
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 4, 2021).