Equivalent (chemistry)

The equivalent (Eq or eq) is a reasonably common measurement unit used in chemistry and the biological sciences. It is a measure of a substance's ability to combine with other substances. It is frequently used in the context of normality.

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The equivalent is formally defined as the amount of a substance (moles) which will react with 6.022 x 10²³ electrons. (This is Avogadro's Number, which is the number of particles in a mole).

Another, slightly less precise, definition describes the equivalent as the number of grams of a substance that will react with a gram of free hydrogen. (This is practically true, since a gram of hydrogen is very close to a mole of hydrogen, and free hydrogen has one spare electron; hence one gram of hydrogen is effectively equivalent to 6.022 x 10²³ electrons.)

Hence, the amount of a given substance in equivalents is equal to the amount of the substance in moles divided by the valence of the substance.

Note that the equivalent weight is the mass of one equivalent of a substance.

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 (mEq/L). 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.

Equivalents have advantage over moles in quantitative analysis of reactions. The best feature of using equivalents is that there is no need to study much about the nature of reaction, i.e. no need to analyse and balance chemical equations. Equivalents of reactants react in equal numbers to yield the equal equivalents of products.

Conversion guide

• For monovalent ions, 1 equivalent = 1 mole
• For divalent ions, 1 Eq = 0.5 mol
• For trivalent ions, 1 Eq = 0.333 mol

Use in biochemistry and medicine

The composition of drug preparations, such as intravenous fluids, is usually stated in mmol/Litre rather than mEq/Litre. This is conceptually easier because the molarity refers to the number of dissolved particles, not the number of available charges. This simplifies understanding the composition of physiological solutions which contain the divalent ions Mg²⁺ and Ca²⁺.