# 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 (H
_{3}O^{+}) 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 (H_{2}SO_{4}) is a diprotic acid. Since only 0.5 mol of H_{2}SO_{4} are needed to neutralize 1 mol of OH^{−}, the equivalence factor is:

*f*_{eq}(H_{2}SO_{4}) = 0.5

If the concentration of a sulphuric acid solution is *c*(H_{2}SO_{4}) = 1 mol/L, then its normality is 2 N. It can also be called a "2 normal" solution.

Similarly, for a solution with *c*(H_{3}PO_{4}) = 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 MgCl_{2} that is 2 N with respect to a Cl^{−} ion, is only 1 N with respect to an Mg^{2+} ion. Since *f*_{eq} may not be unequivocal, IUPAC and NIST discourage the use of normality.^{[1]}

## References

- ↑ "6.3 The use of the equivalence concept" (PDF). Archived from the original (PDF) on July 26, 2011.