Ammonium
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This article is about the cation. For the ancient city, see Siwa Oasis.
Ammonium
Systematic name[hide]
Ammonium
Properties
Molecular formula
NH+
4
Molar mass
18.03851 g mol−1
Except where noted otherwise, data are given for materials in their standard state (at 25 °C,
100 kPa)
Infobox references
The ammonium (more obscurely: aminium) cation is a positively charged polyatomic ion with the chemical
formulaNH4+.[1] It is formed by the protonation of ammonia (NH3). Ammonium is also a general name for
positively charged or protonated substituted amines and quaternary ammonium cations (NR4+), where one or
more hydrogen atoms are replaced by organic radical groups (indicated by R).
Contents
[hide]

1 Acid base properties

2 Ammonium salts

3 Structure and bonding

4 Organic ammonium ions

5 Biology

6 Ammonium metal

7 See also

8 References
Acid base properties [edit]
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):
H+ + NH3 → NH4+
The acid dissociation constant (pKa) of NH4+ is 9.25.
The ammonium ion is mildly acidic, reacting with Brønsted bases to return to the uncharged ammonia
molecule:
NH4+ + B− → HB + NH3
Thus, treatment of concentrated solutions of ammonium salts with strong base gives ammonia. When
ammonia is dissolved in water, a tiny amount of it converts to ammonium ions:
H3O+ + NH3
H2O + NH4+
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), 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 asolid in a thin white layer on surfaces.
The conversion of ammonium back to ammonia is easily accomplished by the addition of strong
base.
Ammonium salts [edit]
Formation of ammonium
Ammonium cation is found in a variety of salts such as ammonium carbonate, ammonium
chloride, and ammonium nitrate. 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 perchlorateare highly explosive, in
these cases ammonium is the reducing agent.
In an unusual process, ammonium ions form an amalgam. Such species are prepared by
the electrolysis of an ammonium solution using a mercury cathode.[2] This amalgam eventually
decomposes to release ammonia and hydrogen.[3]
Structure and bonding [edit]
The lone electron pair on the nitrogen atom (N) in ammonia, represented as a pair of dots, forms
the bond with a proton (H+). Thereafter, all four N-H bonds are equivalent, being polar covalent
bonds. The ion is isoelectronic with methane and borohydride. In terms of size, the ammonium
cation (rionic = 175 pm) resembles the caesium cation (rionic = 183 pm).
Organic ammonium ions [edit]
See also: Amine
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 (IUPACnomenclature": aminium ion).
Depending on the number of organic groups, the ammonium cation is called
a primary, secondary, tertiary, or quaternary. With the exception of the quaternary ammonium
cations, the organic ammonium cations are weak acids.
An example of a reaction forming an ammonium ion is that between dimethylamine, (CH3)2NH,
and an acid, to give the dimethylaminium cation, (CH3)2NH+
2:
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-nbutylammonium 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 andsurfactants.
Biology [edit]
Main article: Excretion
Ammonium ions are a waste product of the metabolism of animals. 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 urea 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. [4]
Ammonium is an important source of nitrogen for many plant species, especially those
growing on hypoxic soils. However, it is also toxic to most crop species and is rarely applied
as a sole nitrogen source.[5]
Ammonium metal [edit]
The ammonium ion has very similar properties to the heavier alkali metals and is often
considered a close relative.[6][7][8] Ammonium is expected to behave as a metal (NH+
4
ions in a sea of electrons) at very high pressures, such as inside gas giant planets such
as Uranus and Neptune.[7][8]