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:
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.
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 dissolved particles, and does not account for the number of available charges. For that reason, for an element such as Na+, which has a valence of 1, 1 mmol/l = 1 mEq/l, whereas for a divalent element (i.e. an element having a valence of 2) such as Mg2+ or Ca2+, 1 mmol/l = 2 mEq/l.