This is based on the following relationship:

* Kw = [H+][OH-] = 1.0 x 10^-14

Where:

* Kw is the ion product constant of water.

* [H+] is the hydrogen ion concentration.

* [OH-] is the hydroxide ion concentration.

Taking the negative logarithm of both sides of the equation:

* -log(Kw) = -log([H+][OH-])

* pKw = pH + pOH

Since pKw is equal to 14, the sum of pH and pOH will always be 14.