Equivalent (chemistry)

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 Template:Section link). 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: $$\begin{array}{cc}\text{mg}\to \text{mEq}& :\text{mg }\times \frac{V}{MW}\\ \text{mEq}\to \text{mg}& :\text{mEq }\times \frac{MW}{V}\end{array}$$ where Template:Mvar is the valence and Template:Mvar is the molecular weight.

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

Common examples

mEq to milligram

Compound	Chemical formula	Molecular weight (Template:Mvar)	Valencies (Template:Mvar)	Sample
				Reference	Elemental mEq	Elemental mEq to compound weight
Potassium (reference)	K	39.098 g/mol	1 (Template:Chem2)		20 mEq potassium	20*39.098/1=782 mg
Potassium citrate monohydrate	Template:Chem2	324.41 g/mol	3 (Template:Chem2)	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)	Template:Chem2	234.245 g/mol	1 (Template:Chem2)	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 (Template:Mvar)	Elemental mass fraction	Valencies (Template:Mvar)	Sample
					Reference	Weight	Compound weight to elemental mEq
Potassium (reference)	K	39.098 g/mol	100%	1 (Template:Chem2)		3000 mg	3000*1/39.098=77 mEq Template:Chem2
Potassium citrate monohydrate	Template:Chem2	324.41 g/mol	36.16%	3 (Template:Chem2)	Tolerable DRI for potassium dietary supplements	8.3 g (3000/0.3616)	8296*3/324.41=77 mEq Template:Chem2
Potassium gluconate (anhydrous)	Template:Chem2	234.245 g/mol	16.69%	1 (Template:Chem2)	Tolerable DRI for potassium dietary supplements	18 g (3000/0.1669)	17975*1/234.245=77 mEq Template:Chem2

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 (Template:Chem2) 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 Template:Chem2 and 1 mole of Template:Chem2. The solution has 1 mole or 1 equiv Template:Chem2, 1 mole or 2 equiv Template:Chem2, and 3 mole or 3 equiv Template:Chem2.

An earlier definition, used especially for chemical elements, holds that an equivalent is the amount of a substance that will react with Template:Convert of hydrogen, Template:Convert of oxygen, or Template:Convert 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 Template:Nowrap). 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.

External links

• A dictionary of units of measurement Template:Webarchive