Equivalent (chemistry)

An equivalent is the amount of a substance that reacts with an arbitrary amount of another substance in a given chemical reaction. It is an archaic quantity that was used in chemistry and the biological sciences. The mass of an equivalent is called its equivalent weight.

Formula

The formula from milligrams to milli-equivalent and back is as follows:
where is the valence and is the molecular weight.
For elemental compounds:

Common examples

mEq to milligram

Milligram to 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, the prefix milli- denoting a factor of one thousandth. Very often, the measure is used in terms of milliequivalents of solute per litre of solution. 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 of Na⁺ is equal to 1 meq, while 1 mmol of Ca²⁺ is equal to 2 meq.