Equivalent (chemistry)

An equivalent (symbol: officially equiv;[1] unofficially but often Eq[2]) is the amount of a substance that reacts with (or is equivalent to) an arbitrary amount of another substance in a given chemical reaction. It is an archaic unit of measurement that was used in chemistry and the biological sciences (see Equivalent weight#In history). The mass of an equivalent is called its equivalent weight.

Not to be confused with Electrochemical equivalent.

In a more formal definition, the equivalent is the amount of a substance needed to do one of the following:

By this definition, an equivalent is the number of moles of an ion in a solution, multiplied by the valence of that ion. If 1 mol of NaCl and 1 mol of CaCl2 dissolve in a solution, there is 1 equiv Na, 2  equiv Ca, and 3 equiv Cl in that solution. (The valency of calcium is 2, so for that ion 1 mole is 2 equivalents.)

An earlier definition, used especially for chemical elements, holds that an equivalent is the amount of a substance that will react with 1 g (0.035 oz) of hydrogen, 16 g (0.56 oz) of oxygen, or 35.5 g (1.25 oz) of chlorine—or that will displace any of the three.[5]

In medicine and biochemistry

In biological systems, reactions often happen on small scales, involving small amounts of substances, so those substances are routinely described in terms of milliequivalents (symbol: officially mequiv; unofficially but often mEq[2] or meq), the prefix milli- denoting a factor of one thousandth (10−3). Very often, the measure is used in terms of milliequivalents of solute per litre of solution (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 (0.001 mol) of Na+ is equal to 1 meq, while 1 mmol of Ca++ is equal to 2 meq.

References
  1. "CAS Standard Abbreviations & Acronyms". www.cas.org. Retrieved 2017-07-26.
  2. American Medical Association, "14.12: Units of Measure", AMA Manual of Style, retrieved 2019-10-23.
  3. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "equivalent entity". doi:10.1351/goldbook.E02192
  4. 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. "Archived copy" (PDF). Archived from the original (PDF) on July 26, 2011. Retrieved 2009-05-10.CS1 maint: archived copy as title (link)
  5. "Atome", Grand dictionnaire universel du XIXe siècle (in French), 1, Paris: Pierre Larousse, 1866, pp. 868–73
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