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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.
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.
![{\displaystyle {\begin{aligned}{\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{aligned}}}](http://wikimedia.org/api/rest_v1/media/math/render/svg/6775af1dbe94461f27ad48a9ad5bcc7c16393612)
For elemental compounds:
![{\displaystyle {\text{mg}}\to {\text{mEq}}:\quad {\frac {\text{element mass [mg]}}{\text{mass fraction}}}\times {\frac {V}{MV}}}](http://wikimedia.org/api/rest_v1/media/math/render/svg/ca0723a8c5df70da3c356e7ec2a2126668a38105)
Common examples
mEq to milligram
| 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
| 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+ |
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 (H+) in an acid–base reaction
- react with or supply one mole of electrons in a redox reaction.[6][7]
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 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.
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