pH, range of numbers expressing the relative acidity or alkalinity of a solution. In general, pH values range from 0 to 14. The pH of a neutral solution, i.e., one which is neither acidic nor alkaline, is 7. Acidic solutions have pH values below 7; alkaline, or basic, solutions have pH values above 7. A pH value provides a measure of the hydrogen ion concentration of a solution. In pure water the concentration of hydrogen ions is equal to 0.0000001, or 10^-7, moles per liter. (A mole is the amount of a substance, expressed in grams, that is equal to the molecular weight, or formula weight, of the substance.) When an acid is added to pure water, the hydrogen ion concentration increases above this level. When an alkaline substance, or base, is added to pure water, the hydrogen ion concentration decreases below this level. Once the concentration is determined, the pH value is found by taking the exponent used in expressing this concentration and reversing its sign. This is expressed as pH=-log₁₀ [H⁺]. For example, if the hydrogen ion concentration of a solution is 10^-4, or 0.0001, moles per liter, the pH is 4. See indicators, acid-base.

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 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 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).

Learn more about pH with a free trial on Britannica.com.