Equivalent (chemistry)

Formula

The formula from milligrams (mg) to milli-equivalent (mEq) and back is as follows: \begin{align} \text{mg} \to \text{mEq} &: \quad \text{mg } \times \frac{V}{MW} \[4pt] \text{mEq} \to \text{mg} &: \quad \text{mEq } \times \frac{MW}{V} \end{align} where V is the valence and MW is the molecular weight.

For elemental compounds: \text{mg} \to \text{mEq} : \quad \frac{\text{element mass [mg]}}{\text{mass fraction}} \times \frac{V}{MW}

Common examples

mEq to milligram

Milligram to mEq

Formal definition

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

• react with or supply one mole of hydrogen ions () in an acid–base reaction
• react with or supply one mole of electrons in a redox reaction.

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 and 1 mole of . The solution has 1 mole or 1 equiv , 1 mole or 2 equiv , and 3 mole or 3 equiv .

An earlier definition, used especially for chemical elements, holds that an equivalent is the amount of a substance that will react with 1 g of hydrogen, 8 g of oxygen, or 35.5 g of chlorine—or that will displace any of the three.

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 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 ). 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

References

1. "CAS Standard Abbreviations & Acronyms".
2. American Medical Association. "AMA Manual of Style".
3. "Potassium Supplement (Oral Route, Parenteral Route) Proper Use - Mayo Clinic".
4. (16 December 2016). "Potassium - assessment of suggestd maximum limits in food supplements". VKM Report.
5. "Proposed maximum levels for the addition of potassium to foods including food supplements".
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7. ["Chapter 6 - 3: The use of the equivalence concept"](http://old.iupac.org/publications/analytical_compendium/Cha06sec3.pdf}} {{Cite web).
8. (1866). "[[Grand dictionnaire universel du XIXe siècle]]". [[Pierre Larousse]].