(Formerly p, pH+, PH, PH) The degree of acidity or alkalinity (basicity or baseness) of a solution. The measurement was introduced as p by Danish biochemist Søren Sørensen in 1909 in Biochemische Zeitschrift, the p representing the German potenz, "power" and the H· representing the hydrogen ion. It is the negative of the common logarithm of the concentration of hydrogen ions (or protons) in moles per litre of solution (pH = -log[H+]). For example, the common logarithm of .0000001 (1 × 10-7) mole of hydrogen ion per litre equals -7, the negative of which is 7. Therefore, 7 is the pH.

The pH of an aqueous solution normally lies between 0 and 14. A pH of 7, the value for pure water, is regarded as neutral. pH values from 7 to 0 indicate increasing acidity and from 7 to 14 indicate increasing alkalinity. A decrease of one unit of pH (an increase in acidity) indicates a tenfold increase in hydrogen ion concentration. An increase of one unit of pH (an increase in alkalinity) indicates a tenfold decrease in hydrogen ion concentration.

Litmus can be used as a pH indicator; it is red in acid solutions and blue in alkaline solutions. A pH meter translates into pH readings the difference in electromotive force between suitable electrodes placed in the solution to be tested.

See also: acid, base, buffer