Hydrogen ion potential, which is the log₁₀ of the reciprocal of hydrogen ion, H⁺, concentration. Mathematically, pH = log₁₀ (1/(H⁺)), where ( ) represents mole/L. PH is derived from the ion-product constant of water, which at room temperature is 1 x 10^-14 = (H⁺) x (OH^-). Pure water (at neutral pH) has equal concentrations of its two ions: (H⁺) = (OH^-) = 10^-7 mole/L. Log₁₀ 1/(H⁺) is 7, which is the pH of a neutral solution. The pH scale ranges from 0 to 14, and values below 7 are acidic and above 7 are basic.