Normality (chemistry)

Normality (chemistry)

In chemistry, the normality of a solution is defined as the molar concentration ci divided by an equivalence factor feq:

normality  = \frac {c_i}{f_{eq}}

Contents

Units

The unit symbol "N" is used to denote "mol/L" when referring to normality. Alternatively, the symbol "Eq/L" is sometimes used. 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 protons (H+) 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.

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 produce 1 mol of H+, 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. For example, a solution of Mg2+ that is 2 N with respect to a Cl- ion, is only 1 N with respect to an O2- ion. Since feq may not be unequivocal, IUPAC and NIST discourage the use of normality. [1]

References


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