A substance’s equivalent is the quantity that interacts with (or is equivalent to) an arbitrary amount (usually one mole) of another substance in a specific chemical reaction.
It is an obsolete unit of measurement used in chemistry and biology (see Equivalent weight#In history). The weight of an equivalent is its equivalent mass.
Formally, the equivalent is the amount of a material required to do one of the following:
- One mole of hydrogen ions (H+) reacts with or is supplied in an acid-base process.
- One mole of electrons is consumed or supplied in a redox reaction.
According to this definition, the number of equivalents of a certain ion in a solution equals the number of moles of that ion multiplied by its valence.
Consider a solution containing 1 mol of NaCl and 1 mol of CaCl2. There are 1 mole or 1 equiv of Na+, 1 mole or 2 equiv of Ca2+, and 3 moles or 3 equiv of Cl– in the solution.
An older definition, particularly for chemical elements, states that an equivalent is the quantity of a material 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 replace any of the three.
In biological systems, reactions frequently occur on small scales and involve small amounts of substances; therefore, these substances are routinely described in milliequivalents, with milli- denoting a factor of one thousandth (103).
Frequently, the solute concentration is expressed in milliequivalents per liter of solution (or mill normal, where meq/L = mN). It is especially prevalent when measuring substances in biological fluids; for example, the normal potassium concentration in human blood is between 3.5 and 5.0 mEq/L.
The same quantity of univalent ions yields the same number of equivalents, but the same quantity of divalent ions yields twice as many equivalents. 1 mmol (0.001 mol) of Na+ is equivalent to 1 meq, whereas 1 mmol of Ca2+ is equivalent to 2 meq.
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