Ammonium 69079 223129014 2008-07-02T18:23:06Z 128.147.28.1 /* Biology */ [[Image:Ammonium-3D-balls.png|thumb|right|150px|A [[ball-and-stick model]] of the ammonium cation]] :'''''Ammonium''' is also an old name for the [[Siwa Oasis]] in western [[Egypt]].'' The ''''''ammonium'''''' [[cation]] is a positively [[electric charge|charged]] [[polyatomic ion|polyatomic cation]] of the [[chemical formula]] NH<sub>4</sub><sup>+</sup>. It has a [[formula weight]] of 18.05 and is formed by [[protonation]] of [[ammonia]] (NH<sub>3</sub>). The resulting ion has a [[Acid dissociation constant|p''K''<sub>a</sub>]] of 9.25. Ammonium and '''aminium''' are also general names for positively charged or [[protonation|protonated]] [[Substitution|substituted]] [[amine]]s and [[quaternary ammonium cation]]s N<sup>+</sup>R<sub>4</sub>, where one or more [[hydrogen]] [[atoms]] are replaced by [[organic radical]] groups (which could be symbolized as '''R'''). ==Chemistry== [[Image:Hydrochloric acid ammonia.jpg|thumb|150px|Fumes from hydrochloric acid and ammonia forming a white cloud of [[ammonium chloride]]]] Ammonia is a weak base; it reacts with Bronsted acids (proton donors) to give the ammonium ion. When ammonia is dissolved in water, a significant amount of it reacts with the hydronium ions in water to give ammonium ions as well. The resultant ammonium ion is a comparatively strong conjugate acid, and reacts with any base, returning to the uncharged ammonia molecule. In [[aqueous solution]], the degree to which ammonia forms the ammonium ion depends on the [[pH]] of the solution. [[Image:Neutralization of Ammonia.PNG|thumb|center|400px|The [[lone pair|lone electron pair]] on the [[nitrogen]] (N) in ammonia is represented as a pair of dots. This electron pair forms the bond with a [[proton]] H<sup>+</sup>.]] In an ammonium ion, the [[nitrogen]] atom forms four [[covalent bond]]s, instead of three as in ammonia, forming a structure which is [[Isoelectronicity|isoelectronic]] to molecule of [[methane]] and so is energetically favorable. 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. Ammonium cations resemble [[alkali metal]] [[ion]]s like [[sodium|Na]]<sup>+</sup> or [[Potassium|K]]<sup>+</sup> and can be found in [[salt]]s such as [[ammonium bicarbonate]], [[ammonium chloride]], and [[ammonium nitrate]]. Most simple ammonium salts are very water [[soluble]]. Reduction of the ammonium cation gives ammonia gas and hydrogen.{{Fact|date=May 2008}} : 2NH<sub>4</sub><sup>+</sup> + 2e<sup>−</sup> &rarr; 2NH<sub>3</sub> + H<sub>2</sub> Ammonium radicals may dissolve in [[Mercury (element)|mercury]] to form an [[amalgam]]. Practically, it may be accomplished by the electrolysis of an ammonium solution with a mercury electrode.<ref>[http://www.hull.ac.uk/chemistry/intro_inorganic/Chap9.htm Pseudo-binary compounds]</ref> This amalgam spontaneously decomposes to give ammonia and hydrogen.<ref>{{cite encyclopedia | encyclopedia = VIAS Encyclopedia | title = Ammonium Salts | url = http://www.vias.org/encyclopedia/chem_ammonia_salts.html}}</ref> ==Substituted ammonium ions== Any hydrogen in the '''ammonium''' ion can be substituted with an alkyl (or other organic radical) group to form a ''substituted ammonium ion'', also called ''aminium ion''; see [[amine]] for details. Depending on the number of organic radical groups, it is called a ''primary'', a ''secondary'', a ''tertiary'', or a ''[[quaternary ammonium cation]]''. They exist in an equilibrium with the respective substituted amine, depending on the [[pH]]. Only quaternary ammonium cations are permanently charged. These cations, e.g. the [[tetra-n-butylammonium]] cation are sometimes used to replace sodium or potassium ions to increase the overall compound's solubility in organic solvents, based on [[HSAB]] principles. Quaternary ammonium salts are often used as [[phase-transfer catalysts]] for the same reason. An example of a reaction forming an ammonium ion is that between [[dimethylamine]], (CH<sub>3</sub>)<sub>2</sub>NH, with an acid to give the dimethylaminium cation, (CH<sub>3</sub>)<sub>2</sub>NH<sub>2</sub><sup>+</sup>: :[[Image:Dimethylammonium-formation-2D.png|400px]] ==Biology== {{main|Excretion}} Ammonium ions are a [[toxic]] waste product of the [[metabolism]] in [[animal]]s. In fish and aquatic invertebrates, it is excreted directly into the water. In mammals, sharks, and amphibians, it is converted in the [[urea cycle]] to [[urea]], because it is less toxic and can be stored more efficiently. In birds, reptiles, and terrestrial snails, metabolic ammonium is converted into [[uric acid]], which is solid, and can therefore be excreted with minimal water loss. <ref>{{cite book |last=Campbell |first=Neil A. |authorlink=Neil Campbell (scientist) |coauthors=[[Jane Reece|Jane B. Reece]] |title=Biology |edition=6th edition |year=2002 |publisher=Pearson Education, Inc. |location=San Francisco, California |language=English |isbn=0-8053-6624-5 |pages=937-938 |chapter=44}}</ref> Ammonium is toxic to humans in high concentrations, and can cause injury to the mucosal lining of the lung, or alkali burns. <ref>{{cite website |title = Ammonia Toxicity |url = http://www.emedicine.com/emerg/TOPIC846.HTM}}</ref> ==See also== * [[Ammonium transporter]] * [[f-ratio]] * [[Hydronium]] (H<sub>3</sub>O<sup>+</sup>) * [[Nitrification]] * [[Onium compounds]] * [[Quaternary ammonium cation]] (R<sub>4</sub>N<sup>+</sup>) == References == {{reflist}} [[Category:Cations]] [[Category:Ammonium compounds|*]] [[ar:أمونيوم]] [[bs:Amonijum]] [[ca:Amoni]] [[da:Ammoniumion]] [[de:Ammonium]] [[es:Amonio (química)]] [[eo:Amonio]] [[fr:Ammonium]] [[it:Ammonio]] [[nl:Ammonium]] [[nn:Ammonium]] [[pl:Jon amonowy]] [[pt:Amónio]] [[ru:Аммоний]] [[sv:Ammonium]] [[uk:Амоній]] [[zh:铵]]