Equivalent (chemistry)

Not to be confused with Electrochemical equivalent.

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 Equivalent weight § In history). 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: $${\begin{aligned}{\text{mg}}\to {\text{mEq}}&:\quad {\text{mg }}\times {\frac {V}{MW}}\\[4pt]{\text{mEq}}\to {\text{mg}}&:\quad {\text{mEq }}\times {\frac {MW}{V}}\end{aligned}}$$ where V is the valence and MW is the molecular weight.

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

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	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	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	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	18 g (3000/0.1669)	17975*1/234.245=77 mEq K+

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 (H⁺) 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 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.

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 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.

External links

• A dictionary of units of measurement Archived 2018-10-06 at the Wayback Machine