Ammonium

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Ammonium
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MeSH D000644
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Template:Longitem Ammonium radical •NH4

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Ammonium is a modified form of ammonia that has an extra hydrogen atom. It is a positively charged (cationic) molecular ion with the chemical formula Template:Chem2 or Template:Chem2. It is formed by the addition of a proton (a hydrogen nucleus) to ammonia (Template:Chem2). Ammonium is also a general name for positively charged (protonated) substituted amines and quaternary ammonium cations (Template:Chem2), where one or more hydrogen atoms are replaced by organic 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.[2] As such, human impact in recent years could have an effect on the biological communities that depend on it.

Acid–base properties

File:Hydrochloric acid ammonia.jpg
Fumes from hydrochloric acid and ammonia forming a white cloud of ammonium chloride

The ammonium ion is generated when ammonia, a weak base, reacts with Brønsted acids (proton donors):

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The ammonium ion is mildly acidic, reacting with Brønsted bases to return to the uncharged ammonia molecule:

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Thus, the treatment of concentrated solutions of ammonium salts with a strong base gives ammonia. When ammonia is dissolved in water, a tiny amount of it converts to ammonium ions:

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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 hydrogen ions is low and hydroxide ions 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.

Salts and characteristic reactions

File:Bildung Ammonium.svg
Formation of ammonium

Ammonium cation is found in several commercially important salts, dominated by ammonium nitrate and ammonium sulfate, which are components of fertilizers. Other ammonium compounds produced on a multi-ton scale are ammonium chloride, ammonium hydrogencarbonate, ammonium carbonate, and ammonium carbamate.[3]

Most simple ammonium salts are very soluble in water. An exception is ammonium hexachloroplatinate, the formation of which was once used as a test for ammonium.

The ammonium salts of nitrate and especially perchlorate are highly explosive, in these cases, ammonium is the reducing agent.

Addition of sodium amalgam to a solution of ammonium chloride gives an ammonium amalgam.[4] This amalgam eventually decomposes to release ammonia and hydrogen.[5]

Ammonium salts react with alkali hydroxide releasing ammonia.

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To further confirm ammonia, it is passed through a glass rod dipped in an Template:Chem2 solution (hydrochloric acid), creating white dense fumes of ammonium chloride.

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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).

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Ammonium ion when added to sodium cobaltinitrite gives a yellow precipitate of ammonium cobaltinitrite.

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Ammonium ion gives a white precipitate of ammonium bitartrate when added to potassium bitartrate.

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Structure and bonding

The lone electron pair on the nitrogen atom (N) in ammonia, represented as a line above the N, forms a coordinate bond with a proton (Template:Chem2). After that, all four Template:Chem2 bonds are equivalent, being polar covalent bonds. The ion has a tetrahedral structure and is isoelectronic with methane and the borohydride anion. In terms of size, the ammonium cation (rionic = 148 pm), it is intermediate in size between potassium (144 pm) and rubidium ions (152 pm) when in an octahedral environment.[6]

The vibrational spectrum consists of two main sets of absorptions, νN-H and δHNH. These bands are found near 3300 and 1400 cm−1.[7]

Organic ions

Script error: No such module "Labelled list hatnote". The hydrogen atoms in the ammonium ion can be substituted with an alkyl group or some other organic group to form a substituted ammonium ion (IUPAC nomenclature: aminium ion). Depending on the number of organic groups, the ammonium cation is called a primary, secondary, tertiary, or quaternary. Except the quaternary ammonium cations, the organic ammonium cations are weak acids.

An example of a reaction forming an ammonium ion is that between dimethylamine, Template:Chem2, and an acid to give the dimethylammonium cation, Template:Chem2:

File:Dimethylammonium-formation-2D.png

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 surfactants.

An unusual class of organic ammonium salts is derivatives of amine radical cations, Template:Chem2 such as tris(4-bromophenyl)ammoniumyl hexachloroantimonate.

Biology

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Ammonium exists as a result of ammonification and decomposers. Ammonium is eventually nitrified, where it contributes to the flow of nitrogen through the ecosystem. Human impacts are not shown here, but can impact the global nitrogen cycle.

Ammonium is utilized by living organism for biosynthesis and as an electron donor.[2]

In part related to the use of fertilizers, there is much interest in ammonium in soils.[8][9]

Ammonia oxidation

oxidation of ammonium occurs nitrification, which produces nitrate and nitrite.[10] This process is a form of autotrophy that is common amongst Nitrosomonas, Nitrobacter, Nitrosolobus, and Nitrosospira, amongst others.[10] Conversely, nitrate and nitrite can be reduced to ammonium in a process known as assimilatory nitrate reduction.[11]

When nitrification is slow, which is common in hypoxic soils, ammonium can accumulate in soils.[12]

Excretion

Ammonium is 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.[13] If transport or excretion of ammonium is interrupted, accumulation of ammonium can lead to hyperammonemia.[14] Toxic effects arise from high concentrations of glutamate, which can interfere with the TCA cycle.[15]

Biosynthesis with ammonium

Once assimilated, it can be incorporated into proteins and DNA.[16]

Ammonium transporters

transportors or carriers occur widely.[17] Common human transport proteins for ammonium include the Amt/Mep/Rh family.[18] [19][20] Glutamine synthetase is one participant.[21][22]

File:Urea cycle002.svg
Urea Cycle showing the formation of carbamoyl phosphate in the mitochondria and the rest of the Urea Cycle occurring in the cytoplasm of the liver cell.

Other mechanisms include the Cahill cycle, glutamate, and glutamine to transport ammonium into the mitochondria.[23] This ammonium combines with bicarbonate to form carbamoyl phosphate which then enters the urea cycle to be excreted as urea.[21]

Human impact

Ammonium deposition from the atmosphere has increased in recent years due to volatilization from livestock waste and increased fertilizer use.[24] Because net primary production is often limited by nitrogen, increased ammonium levels could impact the biological communities that rely on it. For example, increasing nitrogen content has been shown to increase plant growth, but aggravate soil phosphorus levels, which can impact microbial communities.[25]

Metal

The ammonium cation has very similar properties to the heavier alkali metal cations and is often considered a close equivalent.[26][27][28] Neutral ammonium is expected to behave as a metal (Template:Chem2 ions in a sea of electrons) at very high pressures, such as inside the giant planets Uranus and Neptune.[27][28]

Under normal conditions, ammonium does not exist as a pure metal but does as an amalgam (Template:Chem2 alloy with mercury).[29]

See also

References

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  1. ↑ International Union of Pure and Applied Chemistry (2005). Nomenclature of Inorganic Chemistry (IUPAC Recommendations 2005). Cambridge (UK): RSC–IUPAC. Template:ISBN. pp. 71,105,314. Electronic version.
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