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 for which the pKw value should be used instead.