The pH scale:

The equation: pKw=14.00=pH+pOH is obtained by taking the negative logarithm to the base 10 of the autoprotolysis equation, where

In general pX denotes -log10 of any given variable X, so that an increase in X results in a decrease in pX. This equation applies in all aqueous solutions. It allows the definition of the pH scale for water acidity from the pH of a solution, pOH ions can always be found from this equation, and the balance of these terms determines the water acidity. For example, when pH=7, then pOH=7 and the water solution is neutral (ph=7), as the activities of both H3O+ and of OH- are equal at 10-7. When pH+<7 and pOH->7, the solution has an excess of H3O+ and is described as an acidic solution. When pH+>7 and pOH_<7, the solution has an excess of OH- ions and is described as a basic solution.