Ammonium is a modified form ofammonia that has an extra hydrogen atom. It is a positively charged (cationic)molecular ion with thechemical formulaNH+4 or[NH4]+. It is formed by theaddition of a proton (a hydrogen nucleus) to ammonia (NH3). Ammonium is also a general name for positively charged (protonated) substitutedamines andquaternary ammonium cations ([NR4]+), where one or morehydrogenatoms are replaced byorganic or other groups (indicated by R). Not only is ammonium a source ofnitrogen and a keymetabolite for many living organisms, but it is an integral part of the globalnitrogen cycle.[2] As such, human impact in recent years could have an effect on the biological communities that depend on it.
The ammonium ion is generated when ammonia, a weak base, reacts withBrønsted acids (proton donors):
H+ + NH3 → [NH4]+
The ammonium ion is mildly acidic, reacting with Brønsted bases to return to the uncharged ammonia molecule:
[NH4]+ + B− → HB + NH3
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:
H2O + NH3 ⇌ OH− + [NH4]+
The degree to which ammonia forms the ammonium ion depends on thepH 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 ofhydrogen ions is low andhydroxide ions is high), the equilibrium shifts to the left: thehydroxide ion abstracts a proton from the ammonium ion, generating ammonia.
Formation of ammonium compounds can also occur in thevapor 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 asolid in a thin white layer on surfaces.
In an unusual process, ammonium radicals (NH4) form anamalgam. Such species are prepared by the addition of sodium amalgam to a solution of ammonium chloride.[3] This amalgam eventually decomposes to release ammonia and hydrogen.[4]
To find whether the ammonium ion is present in the salt, first, the salt is heated in presence ofalkali hydroxide releasing a gas with a characteristic smell, which isammonia.
[NH4]+ + OH− → NH3 + H2O
To further confirm ammonia, it is passed through a glass rod dipped in anHCl solution (hydrochloric acid), creating white dense fumes ofammonium chloride.
NH3 + HCl → [NH4]Cl
Ammonia or ammonium ion when added toNessler's reagent gives a brown color precipitate known as the iodide of Million's base in basic medium.
Thelone electron pair on thenitrogen atom (N) in ammonia, represented as a line above the N, forms acoordinate bond with aproton (H+). After that, all fourN−H bonds are equivalent, being polarcovalent bonds. The ion has atetrahedral structure and isisoelectronic withmethane and theborohydride 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.[5]
Thevibrational spectrum consists of two main sets of absorptions, νN-H and δHNH. These bands are found near 3300 and 1400 cm-1.[6]
The hydrogen atoms in the ammonium ion can be substituted with analkyl group or some other organic group to form asubstituted ammonium ion (IUPAC nomenclature:aminium ion). Depending on the number of organic groups, the ammonium cation is called aprimary,secondary,tertiary, orquaternary. Except the quaternary ammonium cations, the organic ammonium cations are weak acids.
An example of a reaction forming an ammonium ion is that betweendimethylamine,(CH3)2NH, and an acid to give thedimethylammonium cation,[(CH3)2NH2]+:
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 thetetra-n-butylammonium cation, are sometimes used to replace sodium or potassium ions to increase thesolubility of the associated anion in organicsolvents. Primary, secondary, and tertiary ammonium salts serve the same function but are lesslipophilic. They are also used asphase-transfer catalysts andsurfactants.
Ammonium exists as a result of ammonification anddecomposers. Ammonium is eventually nitrified, where it contributes to the flow ofnitrogen through the ecosystem. Human impacts are not shown here, but can impact the globalnitrogen cycle.
The amount of ammonium in soil that is available fornitrification by microbes varies depending on environmental conditions.[9][10] For example, ammonium is deposited as a waste product from some animals, although it is converted intourea in mammals, sharks, and amphibians, and intouric acid in birds, reptiles, and terrestrial snails.[11] Its availability in soils is also influenced bymineralization, which makes more ammonium available from organicnitrogen sources, andimmobilization, which sequesters ammonium into organicnitrogen sources, both of which are mitigated by biological factors.[8]
Conversely,nitrate andnitrite can be reduced to ammonium as a way for living organisms to accessnitrogen for growth in a process known as assimilatory nitrate reduction.[12] Once assimilated, it can be incorporated intoproteins andDNA.[13]
Ammonium can accumulate in soils wherenitrification is slow or inhibited, which is common in hypoxic soils.[14] For example, ammonium mobilization is one of the key factors for thesymbiotic association between plants and fungi, calledmycorrhizae.[15] However, plants that consistently utilize ammonium as anitrogen source often must invest into more extensive root systems due to ammonium's limited mobility in soils compared to othernitrogen sources.[16][17]
Ammonium deposition from the atmosphere has increased in recent years due to volatilization from livestock waste and increased fertilizer use.[18] Because netprimary production is often limited bynitrogen, increased ammonium levels could impact the biological communities that rely on it. For example, increasingnitrogen content has been shown to increase plant growth, but aggravate soil phosphorus levels, which can impact microbial communities.[19]
The ammonium cation has very similar properties to the heavieralkali metal cations and is often considered a close equivalent.[20][21][22] Neutral ammonium is expected to behave as ametal ([NH4]+ ions in a sea ofelectrons) at very high pressures, such as insidegiant planets such asUranus andNeptune.[21][22]
Under normal conditions, ammonium does not exist as a pure metal but does as anamalgam (NH4 alloy withmercury).[23]
^Shannon, R. D. (1976). "Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides".Acta Crystallographica Section A.32 (5):751–767.doi:10.1107/S0567739476001551.
^Meeron, Emmanuel (1957). "Mayer's Treatment of Ionic Solutions".The Journal of Chemical Physics.26 (4):804–806.doi:10.1063/1.1743411.
^Campbell, Neil A.; Reece, Jane B. (2002).Biology. Internet Archive. San Francisco : Benjamin Cummings.ISBN978-0-8053-6624-2.{{cite book}}: CS1 maint: publisher location (link)
^Holleman, Arnold Frederik; Wiberg, Egon (2001), Wiberg, Nils (ed.),Inorganic Chemistry, translated by Eagleson, Mary; Brewer, William, San Diego/Berlin: Academic Press/De Gruyter,ISBN0-12-352651-5
