Equivalent (chemistry)

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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 (typically one mole) of another substance in a given chemical reaction. It is an archaic quantity that was used in chemistry and the biological sciences (see Equivalent weight § In history). The mass of an equivalent is called its equivalent weight.

Formula

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The formula from milligrams (mg) to milli-equivalent (mEq) and back is as follows:   where V is the valence and MW is the molecular weight.

For elemental compounds:  

Common examples

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mEq to milligram

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Compound Chemical formula Molecular weight (MW) Valencies (V) Sample
Reference Elemental mEq Elemental mEq to compound weight
Potassium (reference) K 39.098 g/mol 1 (K+) 20 mEq potassium 20*39.098/1=782 mg
Potassium citrate monohydrate C6H7K3O8 324.41 g/mol 3 (K+) Liquid potassium citrate/gluconate therapy for adults and teenagers taken two to four times a day[3] 20 mEq potassium 20*324/3=2160 mg
Potassium gluconate (anhydrous) C6H11KO7 234.245 g/mol 1 (K+) Liquid potassium citrate/gluconate therapy for adults and teenagers taken two to four times a day[3] 20 mEq potassium 20*234.245/1=4685 mg

Milligram to mEq

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Compound Chemical formula Molecular weight (MW) Elemental mass fraction Valencies (V) Sample
Reference Weight Compound weight to elemental mEq
Potassium (reference) K 39.098 g/mol 100% 1 (K+) 3000 mg 3000*1/39.098=77 mEq K+
Potassium citrate monohydrate C6H7K3O8 324.41 g/mol 36.16% 3 (K+) Tolerable DRI for potassium dietary supplements[4][5] 8.3 g (3000/0.3616) 8296*3/324.41=77 mEq K+
Potassium gluconate (anhydrous) C6H11KO7 234.245 g/mol 16.69% 1 (K+) Tolerable DRI for potassium dietary supplements[4][5] 18 g (3000/0.1669) 17975*1/234.245=77 mEq K+

Formal definition

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

The "hydrogen ion" and the "electron" in these examples are respectively called the "reaction units."

By this definition, the number of equivalents of a given ion in a solution is equal to the number of moles of that ion multiplied by its valence. For example, consider a solution of 1 mole of NaCl and 1 mole of CaCl2. The solution has 1 mole or 1 equiv Na+, 1 mole or 2 equiv Ca2+, and 3 mole or 3 equiv Cl.

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, 8 g (0.28 oz) of oxygen, or 35.5 g (1.25 oz) of chlorine—or that will displace any of the three.[8]

In medicine and biochemistry

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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 Ca2+ is equal to 2 meq.

References

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  1. ^ "CAS Standard Abbreviations & Acronyms". www.cas.org. Retrieved 2017-07-26.
  2. ^ a b American Medical Association, "14.12: Units of Measure", AMA Manual of Style, retrieved 2019-10-23.
  3. ^ a b "Potassium Supplement (Oral Route, Parenteral Route) Proper Use - Mayo Clinic". www.mayoclinic.org.
  4. ^ a b "Potassium - assessment of suggestd maximum limits in food supplements". VKM Report. 16 December 2016.
  5. ^ a b "Proposed maximum levels for the addition of potassium to foods including food supplements" (PDF). German Federal Institute for Risk Assessment. Archived (PDF) from the original on 2022-12-12.
  6. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "equivalent entity". doi:10.1351/goldbook.E02192
  7. ^ 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. "Chapter 6 - 3: The use of the equivalence concept" (PDF). Archived from the original (PDF) on July 26, 2011. Retrieved 2009-05-10.
  8. ^ "Atome", Grand dictionnaire universel du XIXe siècle (in French), vol. 1, Paris: Pierre Larousse, 1866, pp. 868–73
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