Equivalent concentration

Normality is a measure of concentration equal to the gram equivalent weight per litre of solution. Gram equivalent weight is the measure of the reactive capacity of a molecule. The solute's role in the reaction determines the solution's normality. Normality is also known as the equivalent concentration of a solution. In chemistry, the equivalent concentration or normality of a solution is defined as the molar concentration ci divided by an equivalence factor feq:

Normality = ci/feq

Unit symbol N

The unit symbol "N" is used to denote "eq/L" (equivalent per litre) 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/feq 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/feq 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/feq is an integer value.

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

Derivatives

A normal solution contains a 1 normal concentration.

Decinormal is one tenth (​1⁄10) of the normal concentration.

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:

feq(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. Since feq may not be unequivocal, IUPAC and NIST discourage the use of normality.[1]

See also
  • Normal saline, a solution of NaCl, but not a normal solution. Its normality is about 0.154 N.

References
  1. "6.3 The use of the equivalence concept" (PDF). Archived from the original (PDF) on July 26, 2011.
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