The pH of a solution is used to describe the acidity [local] (the H⁺ concentration ) of a solution. The p comes from the German word potenz (or power). Brackets are used to signify molarity; H⁺ concentration is written as [H⁺]

Notice that the exponent above is written as a negative. The pH is defined as the negative logarithm of the H⁺ concentration or the H₃O⁺ concentration. Except for extremely acid solutions, the negative log turns the pH into a positive value.

In practice, the pH ranges from 0 to 14, where below 7 is acidic and above 7 is basic. Strong acids or strong bases with a molarity greater than 0.1M generally are not discussed in terms of pH (the -log[H⁺]), but simply in terms of molarity [H⁺].

The [H⁺] and [OH^-] are related [local]as follows:

For the pH's of common substances, see ChemSource [local].