Equivalent concentration

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:

$${\displaystyle N={\frac {c_{i}}{f_{\rm {eq}}}}}$$

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).

$${\displaystyle N={\frac {m_{\rm {sol}}}{EW_{\rm {sol}}\times V_{\rm {soln}}}}}$$ 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 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, ⁠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"

The normality of a solution depends on the equivalence factor f_eq for a particular reaction, which presents two possible sources of ambiguity – namely, f_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^[1] and NIST^[2] discourage the use of the terms "normality" and "normal solution".

See also

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

References

1. International Union of Pure and Applied Chemistry (1998). Compendium of Analytical Nomenclature (definitive rules 1997, 3rd. ed.). Oxford: Blackwell Science. ISBN 0-86542-6155. section 6.3. Archived (PDF) from the original on July 26, 2011. Retrieved 2009-05-10.
2. "SI Unit rules and style conventions checklist". National Institute of Standards and Technology. September 2004 [February 1998]. Archived from the original on 2004-10-14. Retrieved 2024-10-08.

External links

• Analytical Chemistry 2.1, by David Harvey (Open-source Textboox) | Chapter 16.1: Normality
• Normality: Definition, formula, equations, type, example,.^[1]

1. "Normality | Definition, Formula, Equations, Type, Example". 2022-11-09. Retrieved 2023-01-29.