An equivalent (symbol: Eq) is the amount of a substance that reacts with (or is equivalent to) an arbitrary amount of another substance in a given chemical reaction. It is an archaic unit of measurement that was used in chemistry and the biological sciences in the era before researchers knew how to determine the chemical formula for a compound. The mass of an equivalent is called its equivalent weight.
In a more formal definition, the equivalent is the amount of a substance needed to do one of the following:
By this definition, an equivalent is the number of moles of an ion in a solution, multiplied by the valence of that ion. If 1 mol of NaCl and 1 mol of CaCl2 dissolve in a solution, there is 1 Eq Na, 2 Eq Ca, and 3 Eq Cl in that solution. (The valence of calcium is 2, so for that ion you have 1 mole and 2 equivalents.)
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, 16 g (0.56 oz) of oxygen, or 35.5 g (1.25 oz) of chlorine—or that will displace any of the three.
Uses
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 has been divided by 0.001. 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.
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 1 meq, while 1 mmol of Ca++ is equal 2 meq.