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{{Short description|Molar concentration divided by equivalence factor}}

In [[chemistry]], the '''equivalent concentration''' or '''normality''' ({{mvar|N}}) of a [[Solution (chemistry)|solution]] is defined as the [[molar concentration]] {{mvar|c<sub>i</sub>}} divided by an [[Equivalent (chemistry)|equivalence]] factor or {{mvar|n}}-factor {{math|''f''<sub>eq</sub>}}:

<math display=block> N = \frac{c_i}{f_{\rm eq}}</math>

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== Definition ==
Normality is defined as the number of [[gram]] or [[Mole (unit)|mole]] [[Equivalent (chemistry)|equivalents]] of [[solute]] present in one liter of [[Solution (chemistry)|solution]]. The [[SI unit]] of normality is equivalents per [[liter]] (Eq/L).

<math chem display=block>N = \frac{m_{\rm sol}}{EW_{\rm sol} \times V_{\rm soln}}</math>
where {{mvar|N}} is normality, {{math|''m''{{sub|sol}}}} is the [[mass]] of solute in grams, {{math|''EW''{{sub|sol}}}} is the [[equivalent weight]] of solute, and {{math|''V''{{sub|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 (H<sub>3</sub>O<sup>+</sup>) or hydroxide ions (OH<sup>−</sup>) in a solution. Here, {{sfrac|1|''f''<sub>eq</sub>}} 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 [[electron]]s that an oxidizing or reducing agent can accept or donate. Here, {{sfrac|1|''f''<sub>eq</sub>}} can have a fractional (non-integer) value.
*In [[Precipitation (chemistry)|precipitation]] reactions, the equivalence factor measures the number of ions which will precipitate in a given reaction. Here, {{sfrac|1|''f''<sub>eq</sub>}} 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 titration|acid-base titrations]]. For example, [[sulfuric acid]] (H<sub>2</sub>SO<sub>4</sub>) is a [[diprotic acid]]. Since only 0.5&nbsp;mol of H<sub>2</sub>SO<sub>4</sub> are needed to neutralize 1&nbsp;mol of OH<sup>−</sup>, the equivalence factor is:

:''f''<sub>eq</sub>(H<sub>2</sub>SO<sub>4</sub>) = 0.5

If the concentration of a sulfuric acid solution is ''c''(H<sub>2</sub>SO<sub>4</sub>) = 1&nbsp;mol/L, then its normality is 2&nbsp;N. It can also be called a "2 normal" solution.

Similarly, for a solution with ''c''(H<sub>3</sub>PO<sub>4</sub>) = 1&nbsp;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 {{var|f}}<sub>eq</sub> for a particular reaction, which presents two possible sources of ambiguity &ndash; namely, {{var|f}}<sub>eq</sub> 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]]<ref name="OrangeBook">{{OrangeBook3rd|section=6.3|url=https://old.iupac.org/publications/analytical_compendium/Cha06sec3.pdf}} [https://web.archive.org/web/20110726000000/old.iupac.org/publications/analytical_compendium/Cha06sec3.pdf Archived] (PDF) from the original on July 26, 2011. Retrieved 2009-05-10.</ref> and [[NIST]]<ref>{{cite web |url=https://physics.nist.gov/cuu/Units/checklist.html |title=SI Unit rules and style conventions checklist |date=September 2004 |orig-date=February 1998 |publisher=[[National Institute of Standards and Technology]] |access-date=2024-10-08 |url-status=live |archive-url=https://web.archive.org/web/20041014002541/https://physics.nist.gov/cuu/Units/checklist.html |archive-date=2004-10-14 }}</ref> 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&nbsp;N.

== References ==
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== External links ==

* [https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Analytical_Chemistry_2.1_(Harvey)/16%3A_Appendix/16.01%3A_Normality Analytical Chemistry 2.1, by David Harvey (Open-source Textboox) | Chapter 16.1: Normality]
* ''Normality: Definition, formula, equations, type, example'',.<ref>{{Cite web |date=2022-11-09 |title=Normality {{!}} Definition, Formula, Equations, Type, Example |url=https://clavick.com/chemistry/normality/ |access-date=2023-01-29 |language=en-US}}</ref>

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[[Category:Concentration]]