Equivalent concentration
In chemistry, the equivalent concentration or normality of a solution is defined as the molar concentration ci divided by an equivalence factor feq:
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.
There are three common areas where normality is used as a measure of reactive species in solution:
Normal concentration of an ionic solution is intrinsically connected to the conductivity (electrolytic) through the equivalent conductivity.
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:
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.
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 feq may not be unequivocal, IUPAC and NIST discourage the use of normality.
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