Chemistry:Equivalent concentration

Short description: Molar concentration divided by equivalence factor

In chemistry, the equivalent concentration or normality (N) of a solution is defined as the molar concentration c_i divided by an equivalence factor or n-factor f_eq:

[math]\displaystyle{ N = \frac{c_i}{f_{\rm eq}} }[/math]

Definition

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

[math]\displaystyle{ N = \frac{m_{\rm sol}}{EW_{\rm sol} \times V_{\rm soln}} }[/math] where N is normality, m_sol is the mass of solute in grams, EW_sol is the equivalent weight of solute, and V_soln is the volume of the entire solution in litres.

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, 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 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 0.5 mol of H₂SO₄ are needed to neutralize 1 mol of OH⁻, the equivalence factor is:

f_eq(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 of the term "normality"

Normality is an ambiguous measure of the concentration of a given reagent in solution. It needs a definition of the equivalence factor, which depends on the definition of the reaction unit (and therefore 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 a reagent solution with a definite concentration may have different normality depending on which reaction is considered, IUPAC and NIST discourage the use of the terms "normality" and "normal solution".

See also

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

External links

• Analytical Chemistry 2.1, by David Harvey (Open-source Textboox) | Chapter 16.1: Normality
• Normality: Definition, formula, equations, type, example,.