The equivalent (symbol: eq or Eq), sometimes termed the molar equivalent, is a unit of amount of substance used in chemistry and the biological sciences.

The equivalent is formally defined as the amount of a substance which will either:

• react with or supply one mole of hydrogen ions (H⁺) in an acid–base reaction; or
• react with or supply one mole of electrons in a redox reaction.^[1][2]

The mass of one equivalent of a substance is called its equivalent weight.

A historical definition, used especially for the chemical elements, describes an equivalent as the amount of a substance that will react with one gram of hydrogen, or with eight grams of oxygen, or with 35.5 grams (1.25 oz) of chlorine, or displaces any of the three.^[3]

In practice, the amount of a substance in equivalents often has a very small magnitude, so it is frequently described in terms of milliequivalents (mEq or meq), the prefix milli denoting that the measure is divided by 1000. Very often, the measure is used in terms of milliequivalents of solute per litre of solvent (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.

Use in biochemistry and medicine [edit]

The composition of drug preparations, such as intravenous fluids, is often stated in mmol/L rather than mEq/L. ( The molarity refers to the number of moles per liter, and does not account for the number of available charges. For that reason, for an element such as Na⁺, which has a valence of one, 1 mmol/L = 1 mEq/L, whereas for a divalent element (i.e. an element having a valence of two) such as Mg²⁺ or Ca²⁺, 1 mmol/L = 2 mEq/L.

References [edit]

External links [edit]

• A dictionary of units of measurement