# 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 unit of measurement 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

milligrams to mEq = mg x V / MW
Example for elemental compounds: (mg element / elemental compound mass fraction) x V / MV
mEq to milligrams = mEq x MW / V

### 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.