pH plus pOH equals 14

Definition

At 25 degrees Celsius the sum of pH and pOH equals 14, the logarithmic form of the Kw relationship. Taking the negative log of [H+] times [OH-] equals 10 to the minus 14 yields pH plus pOH equals 14.

This lets an operator calculate hydroxide concentration directly from a pH reading at a known temperature. Each one-unit change in pH corresponds to a tenfold change in both ion concentrations in opposite directions. The relationship is why a single probe reading reveals both sides of the equilibrium.

Related concepts

pOH · pH definition formula · Self-ionization constant of water

---

Part of The Blueprint of Water · 📏 measurement