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.

The equivalent is formally defined as the mass in grams of a substance 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 equivalent weight of a given substance is effectively equal to the amount of substance in moles, divided by the valence of the substance.

In practise, the equivalent weight is often a very large unit, 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.



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Note added at 7 mins (2005-01-15 00:56:23 GMT)
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APENDICES
... Los prefijos más comunes suelen ser: kilo (kg), deci ... Por ejemplo, el peso molecular
del calcio es 40 ... Un miliequivalente se considera equivalente a un miliosmol ...
www.msd.es/publicaciones/mmerck_hogar/ seccion_app/seccion_app_01.html - 5k

>>> Parece que la unidad se escribe igual que en ingles, ya sea \"mEq\" o \"meq\" - creo que el punto despues de m. es un error...