An equivalent (symbol: officially equiv; 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.
Formula 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:
Common examples
mEq to milligram 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 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
Milligram to mEq 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 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
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 of hydrogen, of oxygen, or 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
External links A dictionary of units of measurement
pl:Równoważnik chemiczny