Normality

Normality

Normality only has meaning in the context of a chemical reaction. The number of equivalents is number of reacting units in a species times the species' concentration in Molarity. That is,

where n is the number of reacting units, such as H⁺, e^-, univalent cations, etc. For example, when dealing with acids and bases we consider the H⁺ or OH^- ions as reacting units. Thus, the acid H₂SO₄ contains two reacting H⁺. So, a 2 M H₂SO₄ solution is a 4 N H₂SO₄ solution.

In reduction/oxidation (redox) reactions it is the electrons that are counted as reacting units. For example, consider the oxidation of the oxalate ion by permanganate:

For this reaction the change in oxidation number of Mn is 5. This is known by looking at the reduction half-reaction involving Mn:

where it is clear that Mn accepts 5 electrons. Thus, a 1M KMnO₄ solution is a 5 N solution when used in the oxidation of oxalate ions.

It is important to emphasize that normality only has meaning in the context of a chemical reaction. For example, the permanganate ion could also be involved in chemical reactions with different half reactions:

and the normality of a KMnO₄ solution would be different in each case.