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 in the era before researchers knew how to determine the chemical formula for a compound. 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 valence 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.

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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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