Equivalent (chemistry)

The equivalent (Eq or eq) is a reasonably common measurement unit used in chemistry and the biological sciences.

The equivalent is formally defined as the amount of a substance (in moles) which will react with 6.022 x 10²³ electrons. (This is Avogadro's Number, which is the number of particles in a mole). For example, 1 mol of Na⁺ will react with 1 mol of electrons, and so 1 Eq of Na⁺=1 mol Na⁺. Using a different example, since it only takes 0.5 mol of Ca²⁺ to react with 1 mol of electrons, 1 Eq of Ca²⁺= 0.5 mol Ca²⁺. In a sense, the 0.5 mol of Ca²⁺ and the 1 mol of Na⁺ are 'equivalent' in that they react with the same number of electrons. 1 Eq of any substance reacts with the same number of electrons as 1 Eq of any other substance.

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 multiplied by the valence of the substance. For example, one mole of a divalent substance, such as oxygen, would equal 2 Eq.

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 (or milliNormal) ( (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.

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 (1 Eq of a monovalent ion will react with 1 mol of electrons)
• For divalent ions, 1 mol = 2 Eq (1 Eq of a divalent ion will react with 0.5 mol of electrons)
• For trivalent ions, 1 mol = 3 Eq (1 Eq of a trivalent ion will react with 0.33 mol of electrons)

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⁺.

External links

• A dictionary of units of measurement