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| In [[chemistry]], the '''equivalent concentration''' or '''normality''' of a solution is defined as the [[molar concentration]] <math>c_i</math> divided by an equivalence factor <math>f_\mathrm{eq}</math>:
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| :Normality <math> = \frac {c_i}{f_\mathrm{eq}}</math>
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| == Units ==
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| 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.
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| == Usage ==
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| There are three common areas where normality is used as a measure of reactive species in solution:
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| *In acid-base chemistry, normality is used to express the concentration of protons (H<sup>+</sup>) or hydroxide ions (OH<sup>−</sup>) in a solution. Here, <math>1/f_\mathrm{eq}</math> is an [[integer]] value. Each solute can produce one or more equivalents of reactive species when dissolved.
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| *In [[redox]] reactions, the equivalence factor describes the number of [[electron]]s that an oxidizing or reducing agent can accept or donate. Here, <math>1/f_\mathrm{eq}</math> can have a fractional (non-integer) value.
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| *In [[Precipitation (chemistry)|precipitation]] reactions, the equivalence factor measures the number of ions which will precipitate in a given reaction. Here, <math>1/f_\mathrm{eq}</math> is an integer value.
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| Normal concentration of an ionic solution is intrinsically connected to the [[conductivity (electrolytic)]] through the equivalent conductivity.
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| == Examples ==
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| Normality can be used for acid-base titrations. For example, [[sulfuric acid]] (H<sub>2</sub>SO<sub>4</sub>) is a [[diprotic acid]]. Since only 0.5 mol of H<sub>2</sub>SO<sub>4</sub> are needed to neutralize 1 mol of OH<sup>-</sup>, the equivalence factor is:
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| :<math>f_\mathrm{eq}</math>(H<sub>2</sub>SO<sub>4</sub>) = 0.5
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| If the concentration of a sulfuric acid solution is ''c''(H<sub>2</sub>SO<sub>4</sub>) = 1 mol/L, then its normality is 2 N. It can also be called a "2 normal" solution.
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| Similarly, for a solution with c(H<sub>3</sub>PO<sub>4</sub>) = 1 mol/L, the normality is 3 N because [[phosphoric acid]] contains 3 acidic H atoms.
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| == Criticism ==
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| 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 [[Equivalent (chemistry)|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 MgCl<sub>2</sup> that is 2 N with respect to a Cl<sup>−</sup> ion, is only 1 N with respect to an Mg<sup>2+</sup> ion. Since <math>f_\mathrm{eq}</math> may not be unequivocal, [[IUPAC]] and [[NIST]] discourage the use of normality.<ref>http://old.iupac.org/publications/analytical_compendium/Cha06sec3.pdf</ref>
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| == References ==
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| {{reflist}}
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| {{Chemical solutions}}
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| {{DEFAULTSORT:Normality (Chemistry)}}
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| [[Category:Analytical chemistry]]
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| [[Category:Chemical properties]]
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