A measure of the acidity or alkalinity of a solution, numerically equal to 7 for neutral solutions, increasing with increasing alkalinity and decreasing with increasing acidity. The pH scale commonly in use ranges from 0 to 14.
It is defined as the negative common logarithm of the concentration of hydrogen ions [H+] in moles/litre: pH = -log10 [H+]. The letters of its name are derived from the absolute value of the power (p) of the hydrogen ion concentration (H). 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.
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