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 Equivalent weight § In history). The mass of an equivalent is called its equivalent weight.

The formula from milligrams (mg) to milli-equivalent (mEq) and back is as follows: $${\displaystyle {\begin{aligned}{\text{mg}}\to {\text{mEq}}&:\quad {\text{mg }}\times {\frac {V}{MW}}\\[4pt]{\text{mEq}}\to {\text{mg}}&:\quad {\text{mEq }}\times {\frac {MW}{V}}\end{aligned}}}$$ where V is the valence and MW is the molecular weight.

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

In a more formal definition, the equivalent is the amount of a substance needed to do one of the following:

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 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.