Equivalent concentration

In chemistry, the equivalent concentration or normality of a solution is defined as the molar concentration c_i divided by an equivalence factor f_eq:

Normality = c_i/f_eq

Unit symbol N

The unit symbol "N" is used to denote "eq/L" (equivalent per liter) which is normality. Although losing favor, medical reporting of serum concentrations in "meq/L" (= 0.001 N) still occurs.

Usage

There are three common areas where normality is used as a measure of reactive species in solution:

• In acid-base chemistry, normality is used to express the concentration of hydronium ions (H₃O⁺) or hydroxide ions (OH⁻) in a solution. Here, 1/f_eq is an integer value. Each solute can produce one or more equivalents of reactive species when dissolved.
• In redox reactions, the equivalence factor describes the number of electrons that an oxidizing or reducing agent can accept or donate. Here, 1/f_eq can have a fractional (non-integer) value.
• In precipitation reactions, the equivalence factor measures the number of ions which will precipitate in a given reaction. Here, 1/f_eq is an integer value.

Normal concentration of an ionic solution is intrinsically connected to the conductivity (electrolytic) through the equivalent conductivity.

Examples

Normality can be used for acid-base titrations. For example, sulfuric acid (H₂SO₄) is a diprotic acid. Since only 0.5 mol of H₂SO₄ are needed to neutralize 1 mol of OH⁻, the equivalence factor is:

f_eq(H₂SO₄) = 0.5

If the concentration of a sulfuric acid solution is c(H₂SO₄) = 1 mol/L, then its normality is 2 N. It can also be called a "2 normal" solution.

Similarly, for a solution with c(H₃PO₄) = 1 mol/L, the normality is 3 N because phosphoric acid contains 3 acidic H atoms.

Criticism

Normality is an ambiguous measure of the concentration of a solution. It needs a definition of the equivalence factor, which depends on the definition of equivalents. The same solution can possess different normalities for different reactions. The definition of the equivalence factor varies depending on the type of chemical reaction that is discussed: It may refer to equations, bases, redox species, precipitating ions, or isotopes. For example, a solution of MgCl₂ that is 2 N with respect to a Cl⁻ ion, is only 1 N with respect to an Mg²⁺ ion. Since f_eq may not be unequivocal, IUPAC and NIST discourage the use of normality.^[1]

References