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Formation of an ammonium ion by protonation of ammonia.
Ammonia gag when ammonia and hydrochloric acid vapors come together

The ammonium ion NH 4 + ( also azanium ion according to IUPAC ) is a cation that forms salts with anions similar to alkali metal ions . It is the conjugate acid to the base ammonia (NH 3 ). Ammonium salts are, for example, ammonium nitrate (NH 4 NO 3 ) or ammonium chloride (NH 4 Cl). In organic ammonium salts, the nitrogen atom also has four binding partners and a positive formal charge. However, at least one organic radical is bound to the nitrogen atom, such as. B. in hydrochlorides .


An ammonium ion has a tetrahedral structure. Ammonium forms a dissociation equilibrium with ammonia . Because of the involvement of an oxonium ion, this equilibrium is dependent on the pH value . The proportion of ammonia increases with increasing pH value and increasing temperature.

The pK s value of ammonium is 9.2. Ammonium salts (for example ammonium sulfate ) react in aqueous solution as weak acids and therefore form slightly acidic solutions.

The nitrogen atom in ammonium can be oxidized to nitrogen relatively easily. A typical example of this is the thermal decomposition of ammonium dichromate .

In the literature, ammonium salts are often described as pseudo- alkali metal salts, which results from their comparable properties to the alkali metal salts. This analogy can mainly be explained by an almost identical ionic radius and the same charge. Above all, they are similar to alkali salts in terms of their solubility in water.

See also: Grimm's hydride shift theorem

Ammonium in nature

In nature, ammonium is primarily formed when proteins break down. It is used by fish and most other aquatic organisms as an end product, e.g. B. via the gills, excreted. It is also released as an end product when dead biomass is rotted through bacteria. It plays an important role in the citric acid cycle , in which it reacts with α-ketoglutarate to form glutamic acid.

Ammonium is first oxidized to nitrite by bacteria (including Nitrosomonas ) and then further to nitrate by another type of bacteria (including Nitrobacter ) and is thus "detoxified" in the soil and in water with consumption of oxygen . In addition to bacteria, archaea also play an important role in ammonium oxidation in the soil. This process is called nitrification and is very desirable in the soil. Nitrification is also an important part of self-cleaning in water bodies .

Ammonia is toxic to fish even in low concentrations. Ammonium levels in the water of 0.5 to 1 mg / l are therefore classified as dangerous for fish , depending on the pH value of the water. If the ammonium content exceeds 1 mg / l, a body of water is not suitable for fishing purposes.

Physiological importance

The ammonium ion is similar to the potassium ion (K + ) both in size and charge and can therefore take its place in the organism. But since it reacts differently, e.g. B. synapses that are potassium-controlled cannot be split off again, it blocks them permanently. So it works in all organisms with potassium-controlled synapses as a nerve poison .

Ammonium in the urine indicates calculus .

In mammals, ammonium is converted into non-toxic urea via the urea cycle in the liver and partly in the kidneys and excreted via the kidneys. In birds and lizards living on land, uric acid is instead produced and excreted. Fish do not require any conversion of ammonia; in their case, the skin in direct contact with the water offers the simple route of osmosis .


Ammonium salt of thioglycolic acid in three formulas
Ammonium salt of thioglycolic acid in three formulas

Ammonium salts are the most important compounds in the inorganic chemical industry. They are produced on a megaton scale and are mainly used as fertilizers , and to a lesser extent in dry batteries and dyes.

The ammonium salt of thioglycolic acid is used as a reducing agent in the permanent wave . The ammonium salt of thiolactic acid is another example from organic chemistry and is also used in the hairdressing trade.


To test ( pre-test ) a substance for ammonium, it is mixed with a little sodium hydroxide solution or sodium hydroxide . The ammonia released can either be smelled or it can be detected by the basic discoloration of a moist pH test strip above the reaction mixture ( cross match ).

A sensitive proof is the reaction with Nessler's reagent , which however also responds to amines . A sensitive and selective quantitative determination takes place according to DIN mostly with the help of the Berthelot reaction with the formation of a blue indophenol , the concentration of which can be measured colorimetrically .

Organic chemistry

Tetramethylammonium chloride and triethylamine - hydrochloride are examples of organic ammonium salts, in which the nitrogen atom also has four binding partners, however, these organic radicals or partly are hydrogen atoms, as in hydrochlorides, hydrobromides or Hydroiodiden .

Individual evidence

  1. ^ GJ Leigh (Ed.): Principles of chemical nomenclature. A guide to IUPAC recommendations. The Royal Society of Chemistry, Cambridge 2011, p. 46.
  2. a b M. Binnewies et alii: Allgemeine und Anorganische Chemie. 2nd Edition. Spectrum, 2010, ISBN 3-8274-2533-6 . P. 478f.
  3. ^ AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 101st edition. Walter de Gruyter, Berlin 1995, ISBN 3-11-012641-9 , p. 654.
  4. Primordial bacteria as "ammonium killer" , accessed on January 31, 2013.
  5. Robert Guderian (Ed.): Handbook of Environmental Changes and Ecotoxicology - Volume 1A , Springer, Berlin 2000, ISBN 978-3-540-66184-9 .
  6. Wolfgang Legrum: Fragrances, between stink and fragrance , Vieweg + Teubner Verlag (2011) p. 165, ISBN 978-3-8348-1245-2 .
  7. ^ E. Schweda: Jander / Blasius: Inorganic Chemistry I - Introduction & Qualitative Analysis. 17th edition. Hirzel, 2012, ISBN 978-3-7776-2134-0 . P. 294.

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