Quantitative measure of the strength of the acidity or alkalinity (see acid , base ) of a solution .
It is defined as the negative common logarithm of the concentration of hydrogen ion s [H + ] in mole s/litre: pH = -log 10 [H + ]. The letters of its name are derived from the absolute value of the power ( p ) of the hydrogen ion concentration ( H ). The product of the concentrations in water of H + and OH - (the hydroxide ion) is always about 10 -14 . The strongest acid solution has about 1 mole/litre of H + (and about 10 -14 of OH - ), for a pH of 1. The strongest basic solution has about 10 -14 moles/litre of H + (and about 1 of OH - ), for a pH of 14. A neutral solution has about 10 -7 moles/litre of both H + and OH - , for a pH of 7. The pH value, measured by a pH meter, titration , or indicator (e.g., litmus ) strips, helps inform chemists of the nature, composition, or extent of reaction of substances, biologists of the composition and environment of organisms or their parts or fluids, physicians of the functioning of bodily systems, and agronomists of the suitability of soils for crops and any treatments needed. The pH is now defined in electrochemical terms (see electrochemistry ).