An Etymological Dictionary of Astronomy and Astrophysics

A → logarithmic measure of → hydrogen ion concentration, originally defined

pH = log₁₀ (1/[H⁺]),

where [H⁺] is the concentration of hydrogen ions in → moles per liter of solution.

The hydrogen ion concentration in pure water around room temperature is about 1.0 × 10^-7 moles. Therefore, a pH of 7 is considered “neutral,” because the concentration of hydrogen ions is exactly equal to the concentration of → hydroxide (OH^-) ions produced by → dissociation of the → water. Increasing the concentration of hydrogen ions above 1.0 × 10^-7 moles produces a solution with a pH of less than 7, and the solution is considered → acidic. Decreasing the concentration below 1.0 × 10^-7 moles produces a solution with a pH above 7, and the solution is considered → alkaline or → basic. The neutral pH is different for each → solvent. For example, the concentration of hydrogen ions in pure ethanol is about 1.58 &times 10^-10 moles, so ethanol is neutral at pH 9.8. A solution with a pH of 8 would be considered acidic in ethanol, but basic in water.

See also: From Ger. PH, introduced by Danish biochemist S.P.L. Sørensen (1868-1939)
in 1909, from P, for Ger. Potenz “power, potency,” and H, symbol of → hydrogen.