An equivalent (symbol: officially equiv;[1] unofficially but often Eq) 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 ). 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 is the valence and is the molecular weight.
For elemental compounds:
| Compound | Chemical formula | Molecular weight | Valencies | Sample | |||
|---|---|---|---|---|---|---|---|
| Reference | Elemental mEq | Elemental mEq to compound weight | |||||
| Potassium (reference) | K | 39.098 g/mol | 1 | 20 mEq potassium | 20*39.098/1=782 mg | ||
| Potassium citrate monohydrate | 324.41 g/mol | 3 | Liquid potassium citrate/gluconate therapy for adults and teenagers taken two to four times a day[2] | 20 mEq potassium | 20*324/3=2160 mg | ||
| Potassium gluconate (anhydrous) | 234.245 g/mol | 1 | Liquid potassium citrate/gluconate therapy for adults and teenagers taken two to four times a day | 20 mEq potassium | 20*234.245/1=4685 mg | ||
| Compound | Chemical formula | Molecular weight | Elemental mass fraction | Valencies | Sample | |||
|---|---|---|---|---|---|---|---|---|
| Reference | Weight | Compound weight to elemental mEq | ||||||
| Potassium (reference) | K | 39.098 g/mol | 100% | 1 | 3000 mg | 3000*1/39.098=77 mEq | ||
| Potassium citrate monohydrate | 324.41 g/mol | 36.16% | 3 | Tolerable DRI for potassium dietary supplements[3] [4] | 8.3 g (3000/0.3616) | 8296*3/324.41=77 mEq | ||
| Potassium gluconate (anhydrous) | 234.245 g/mol | 16.69% | 1 | Tolerable DRI for potassium dietary supplements | 18 g (3000/0.1669) | 17975*1/234.245=77 mEq | ||
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 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 1g of hydrogen, 8g of oxygen, or 35.5g of chlorine—or that will displace any of the three.
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.