The equivalent is formally defined as the amount of a substance which will either:
- react with or supply one mole of hydrogen ions (H+) in an acid–base reaction; or
- react with or supply one mole of electrons in a redox reaction.
A historical definition, used especially for the chemical elements, describes an equivalent as the amount of a substance that will react with one gram of hydrogen, or with eight grams of oxygen, or with 35.5 grams (1.25 oz) of chlorine, or displaces any of the three.
In practice, the amount of a substance in equivalents often has a very small magnitude, so it is frequently described in terms of milliequivalents (mEq or meq), the prefix milli denoting that the measure is divided by 1000. Very often, the measure is used in terms of milliequivalents of solute per litre of solvent (or milliNormal, where mEq/L = mN). This is especially common for measurement of compounds in biological fluids; for instance, the healthy level of potassium in the blood of a human is defined between 3.5 and 5.0 mEq/L.
- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "equivalent entity".
- International Union of Pure and Applied Chemistry (1998). Compendium of Analytical Nomenclature (definitive rules 1997, 3rd. ed.). Oxford: Blackwell Science. ISBN 0-86542-6155. section 6.3.
- "Atome", Grand dictionnaire universel du XIXe siècle 1, Paris: Pierre Larousse, 1866, pp. 868–73. (French)