pOH Definition (Chemistry)

Chemistry Terminology: Definition of pOH

pOH is a measure of hydroxide ion concentration or alkalinity.
pOH is a measure of hydroxide ion concentration or alkalinity. Stephan Zabel / 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°C 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 when the pKw value should be used instead.