Equivalent concentration

The equivalent concentration , outdated normality ( unit symbol  N and formula symbol N ), is a special substance concentration in chemistry . ${\displaystyle c_{\mathrm {eq} }}$ 

definition

The equivalent concentration is defined as:

${\displaystyle c_{\mathrm {eq} }=z\cdot c=z\cdot {\frac {n}{V}}}$

With

• the stoichiometric valency z , also called the equivalent number
• the molar concentration c of the solution
• the amount of substance ${\displaystyle n}$
• the volume  V .

In the case of z  = 3, the equivalent concentration is three times as large as the molar concentration, because every whole particle is counted z times. The expression 1 / z is also called equivalent particles or equivalents. c _eq is a measure of how many equivalents of a substance are in a certain volume of the solution, hence the term "equivalent concentration".

The stoichiometric valency - and thus also the equivalent concentration of a certain solution - can depend on the chemical reaction, i.e. the use of the solution, without the solution itself changing. The equivalent concentration also depends on the temperature: ${\displaystyle c_{\mathrm {eq} }=f(T)}$

Another possibility is the definition of normality  N via the number of equivalents or Val dissolved in one liter of a solution :

${\displaystyle N={\frac {n_{val}}{V}}={\frac {z\cdot n}{V}}}$

With

• the amount of equivalents. ${\displaystyle n_{val}}$

unit

The usual unit of the equivalent concentration is mol / liter. Solutions with c _eq = 1 mol / l used to be called "normal solutions". If c _eq = 0.1 mol / l, one spoke of “0.1 N solutions” etc. (see also standard solution ).

The use of normal solutions with an equivalent concentration of 1 mol / l ("one-normal solution") or 0.1 mol / l was introduced into analytical chemistry in particular by Friedrich Mohr (1806 to 1879) , especially in his from 1855 in several editions published textbook "Chemisch-Analytische Titrirmethode".

Examples

The equivalent concentration is particularly important in ion, neutralization and redox reactions as well as in dimensional analysis .

Saline solutions

Sodium carbonate (Na ₂ CO ₃ ) consists of two sodium ions (Na ⁺ ) and one carbonate ion. A 1 molar (M) sodium carbonate solution corresponds to a 2 normal (N) sodium carbonate solution based on the sodium ions (z = 2).

Acid-base reactions

In acid-base reactions , equivalent particles are protons (H ⁺ ) in acidic solutions or hydroxide ions (OH ^- ) in basic solutions. For example, two protons can attach to a sulfate ion (SO ₄ ²⁻ ), which corresponds to the valency of the acid ion. As a result, the solution contains twice as many equivalent particles (here protons) as the molecules of the substance itself.

${\displaystyle \mathrm {H_{2}SO_{4}\longrightarrow SO_{4}^{2-}+2\ H^{+}} }$,

d. i.e. 1 mol / l (H ₂ SO ₄ ) = 2 N (H ₂ SO ₄ ), or in other words: a 1-normal H ₂ SO ₄ solution is ½ molar (1 N corresponds to ½ M).

In acid / base titrations there are acids with one, two (e.g. sulfuric acid) or three protons (e.g. phosphoric acid). If these acids are titrated with caustic soda, one, two or three parts of caustic soda are needed to neutralize an acid, depending on the number of equivalents of the acid. Therefore, in acid / base titrations, the molar mass is divided by the number of protons that can be released or absorbed and this amount of substance is distilled in one liter. Dissolved water to get the equivalent concentration of 1 mole n ^eq protons. Then exactly:

n ^eq (H ₂ SO ₄ ) = n ^eq (NaOH)

Redox reactions

In redox reactions, on the other hand, the equivalent is the amount of substance of the oxidizing or reducing agent that can accept or release exactly 1 mol of electrons. An example:

${\displaystyle \mathrm {\ MnO_{4}^{-}+8\ H^{+}+5\ e^{-}\longrightarrow \ Mn^{2+}+4\ H_{2}O} }$

In this reaction, permanganate is the oxidizing agent, and 1 mole of manganese (VII) accepts 5 moles of electrons. Consequently, 1/5 mole of manganese (VII) accepts exactly 1 mole of electrons. The equivalent particle here is 1/5 MnO ₄ ^- .

In a redox reaction, a permanganation of MnO can accept ₄ ^- 5 electrons, but a chloride ion can only give off one electron. The molar mass of the potassium permanganate has to be divided by 5, then the amount has to be dissolved in exactly one liter of distilled water in order to get the equivalent concentration of this oxidizing agent of 1  n ^eq (mol of electron uptake) / liter. 1  n ^eq (= 1 ^eq ) electron uptake corresponds to 1/5 molar mass of KMnO ₄ and this is described as:

n ^eq (KMnO ₄ ) = n (1/5 KMnO ₄ ).

swell

• Hans R. Christen, Gerd Meyer: Basics of general and inorganic chemistry. Salle + Sauerländer, 1997, ISBN 3-7935-5493-7
• Frank H. Stepheson: Mathematics in the laboratory. Elsevier Verlag, Munich 2004, ISBN 3-8274-1596-9

Web links

Wiktionary: normality  - explanations of meanings, word origins, synonyms, translations

Individual evidence

1. ^ Wilhelm Strube: The historical way of chemistry . Aulis Verlag Deubner & Co KG, Cologne 1989, ISBN 3-7614-1180-4 , p. 220 .