Equivalent concentration
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
[edit]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
[edit]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
[edit]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
[edit]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"
[edit]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
[edit]- Equivalent (chemistry)
- Normal saline, a solution of NaCl, but not a normal solution. Its normality is about 0.154 N.
References
[edit]- ^ 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.
- ^ "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
[edit]- Analytical Chemistry 2.1, by David Harvey (Open-source Textboox) | Chapter 16.1: Normality
- Normality: Definition, formula, equations, type, example,.[1]
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- ^ "Normality | Definition, Formula, Equations, Type, Example". 2022-11-09. Retrieved 2023-01-29.