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Wikipedia / Equivalent concentration

In chemistry, equivalent concentration or normality () is the number of equivalence of solute (eq) per liter of solution (L). In contrast from molarity, normality uses the concept of equivalence. Molar can be converted to normality via multiplying Molar concentration an equivalence factor or -factor ().

Definition

Normality is defined as the number of gram or mole equivalents of solute present in one liter of solution. The SI unit of normality is equivalents per liter (Eq/L).

where is normality, is the mass of solute in grams, is the equivalent weight of solute, and is the volume of the entire solution in liters.

Usage

There are three common types of chemical reaction 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, 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, 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, is an integer value.

Normal concentration of an ionic solution is also related to conductivity (electrolytic) through the use of equivalent conductivity.

Medical

Although losing favor in the medical industry, reporting of serum concentrations in units of "eq/L" (= 1 N) or "meq/L" (= 0.001 N) still occurs.

Examples

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

  1. feq(H2SO4) = 0.5

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

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

Criticism of the term "normality"

The normality of a solution depends on the equivalence factor eq for a particular reaction, which presents two possible sources of ambiguity namely, eq depends on the choice of reaction as well as which chemical species of the reaction is being discussed (e.g., acid/base species, redox species, precipitating salts, isotopes exchanged, etc.). That is to say, the same solution can possess different normalities for different reactions or potentially even the same reaction in a different context.

To avoid ambiguity, IUPAC and NIST discourage the use of the terms "normality" and "normal solution".

See also

References