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Equivalent concentration

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

Definition

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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 N is normality, msol is the mass of solute in grams, EWsol is the equivalent weight of solute, and Vsoln is the volume of the entire solution in liters.

Usage

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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/feq 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/feq 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/feq is an integer value.

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

Medical

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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

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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:

feq(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"

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The normality of a solution depends on the equivalence factor feq for a particular reaction, which presents two possible sources of ambiguity – namely, feq 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

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References

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  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.
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