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Ammonium is a modified form of ammonia that has an extra hydrogen atom. It is a positively charged () polyatomic ion with the chemical formula or . It is formed by the protonation (a hydrogen nucleus) to ammonia (). Ammonium is also a general name for positively charged (protonated) substituted and quaternary ammonium cations (), where one or more hydrogen atoms are replaced by Organic compound or other groups (indicated by R). Not only is ammonium a source of nitrogen and a key metabolite for many living organisms, but it is an integral part of the global nitrogen cycle. As such, human impact in recent years could have an effect on the biological communities that depend on it.
The degree to which ammonia forms the ammonium ion depends on the pH of the solution. If the pH is low, the equilibrium shifts to the right: more ammonia molecules are converted into ammonium ions. If the pH is high (the concentration of is low and is high), the equilibrium shifts to the left: the hydroxide ion abstracts a proton from the ammonium ion, generating ammonia.
Formation of ammonium compounds can also occur in the vapor phase; for example, when ammonia vapor comes in contact with hydrogen chloride vapor, a white cloud of ammonium chloride forms, which eventually settles out as a solid in a thin white layer on surfaces.
In an unusual process, ammonium ions form an amalgam. Such species are prepared by the addition of sodium amalgam to a solution of ammonium chloride. This amalgam eventually decomposes to release ammonia and hydrogen.
To find whether the ammonium ion is present in the salt, first, the salt is heated in presence of alkali hydroxide releasing a gas with a characteristic smell, which is ammonia.
To further confirm ammonia, it is passed through a glass rod dipped in an solution (hydrochloric acid), creating white dense fumes of ammonium chloride.
Ammonia, when passed through (copper(II) sulfate) solution, changes its color from blue to deep blue, forming Schweizer's reagent.
Ammonia or ammonium ion when added to Nessler's reagent gives a brown color precipitate known as the iodide of Million's base in basic medium.
Ammonium ion when added to chloroplatinic acid gives a yellow precipitate of ammonium hexachloroplatinate(IV).
Ammonium ion when added to sodium cobaltinitrite gives a yellow precipitate of ammonium cobaltinitrite.
Ammonium ion gives a white precipitate of ammonium bitartrate when added to potassium bitartrate.
An example of a reaction forming an ammonium ion is that between dimethylamine, , and an acid to give the dimethylammonium cation, :
Quaternary ammonium cations have four organic groups attached to the nitrogen atom, they lack a hydrogen atom bonded to the nitrogen atom. These cations, such as the tetra- n-butylammonium cation, are sometimes used to replace sodium or potassium ions to increase the solubility of the associated anion in organic solvents. Primary, secondary, and tertiary ammonium salts serve the same function but are less lipophilic. They are also used as phase-transfer catalysts and .
An unusual class of organic ammonium salts is derivatives of amine radical cations, such as tris(4-bromophenyl)ammoniumyl hexachloroantimonate.
The amount of ammonium in soil that is available for nitrification by microbes varies depending on environmental conditions. For example, ammonium is deposited as a waste product from some animals, although it is converted into urea in mammals, sharks, and amphibians, and into uric acid in birds, reptiles, and terrestrial snails. Its availability in soils is also influenced by mineralization, which makes more ammonium available from organic nitrogen sources, and immobilization, which sequesters ammonium into organic nitrogen sources, both of which are mitigated by biological factors.
Conversely, nitrate and nitrite can be reduced to ammonium as a way for living organisms to access nitrogen for growth in a process known as assimilatory nitrate reduction. Once assimilated, it can be incorporated into and DNA.
Ammonium can accumulate in soils where nitrification is slow or inhibited, which is common in hypoxic soils. For example, ammonium mobilization is one of the key factors for the Symbiosis association between plants and fungi, called . However, plants that consistently utilize ammonium as a nitrogen source often must invest into more extensive root systems due to ammonium's limited mobility in soils compared to other nitrogen sources.
Under normal conditions, ammonium does not exist as a pure metal but does as an amalgam (alloy with mercury).
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