Gram equivalent

In chemistry, a gram equivalent is the mass, in grams, of a compound's equivalent weight.

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More formally, a gram equivalent of a substance taking part in a given reaction is the number of grams of the substance associated with the transfer of n electrons or protons or with the neutralization of n negative or positive charges, where n is Avogadro's number, 6.02×10²³.

In a practical sense, a gram equivalent is the amount of a compound that is necessary to produce a given normality of a reactive species in a solution. Here, reactive species include ions that will precipitate, neutralize base (protons, H⁺), neutralize acid (hydroxide, OH^-), donate electrons, or accept electrons.

To make clear the distinction between grams and gram equivalents, consider the following:

When making a solution with a given molarity, you measure grams of a compound and dissolve them in a known volume.

When making a solution with a given normality, you measure gram equivalents of a compound, in grams, and dissolve them in a known volume.

It is important to note that gram equivalents are specific to a given ion in solution (e.g., Na⁺, OH^-, Ca²⁺, Fe³⁺, etc.), as measured in the solid state. As such, it can be a measure of ions participating in one of three general categories of reactions: oxidation-reduction reactions, acid-base reactions, and precipitation reactions:

• In acid-base chemistry, the gram equivalent expresses the mass of acid or base which will furnish a concentration of protons or hydroxide ions undergoing neutralization.

• In redox reactions, gram equivalents denote the mass of oxidizing or reducing agent that can accept or furnish a given molarity of electrons.

• In precipitation reactions, the gram equivalent is mass of a substance (typically a salt) which will produce a molar concentration of ions that participate in the precipitation.

References

• Meldrum, W.B. J. Chem. Ed., 1955, 32, 48.