The Equivalent (symbol: Eq), was an archaic unit used in chemistry and the biological sciences in the era before chemical formulas and equations were known. It represented the quantity of a substance which reacted exactly with (or was equivalent to) an arbitrarily fixed quantity of another substance in a particular reaction. As knowledge increased, the theoretical basis for these 'chemical equivalents' became understood and the need for a common standard using the concept of Equivalent weights developed. The equivalent weight of a chemical element is the gram atomic weight divided by its valence. The equivalent can also be formally defined through the amount of 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.
Given this definition, an equivalent may also be defined as the number of moles of a given ion in a solution multiplied by the valence of that ion. So, if 1 mol of NaCl and 1 mol of CaCl2 are dissolved in a solution, there is 1 Eq Na, 2 Eq Ca, and 3 Eq Cl in that solution. (Note that the valence of Ca is 2, so for that ion you have 1 mole and 2 equivalents.)
A historical definition, used especially for the chemical elements, describes an equivalent as the amount of a substance that will react with 1 g (0.035 oz) of hydrogen, or with 8 g (0.28 oz) of oxygen, or with 35.5 g (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 has been multiplied 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.
A certain amount of univalent ions provides the same amount of equivalents while the same amount of divalent ions provides twice the amount of equivalents. For example, 1 mmol of Na+ is equal 1 meq, while 1 mmol of Ca++ is equal 2 meq.
- 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)