ammonium

Ammonium
other names Ammonium cation, Protonated ammonia
Identifiers
CAS number	14798-03-9 Y
SMILES	N(H)(H)(H)H
Properties
Molecular formula	NH+4
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

The ammonium cation (also known as ionized ammonia due to its electrical charge) is a positively charged polyatomic cation of the chemical formula NH+4. It has a formula weight of 18.05 and is formed by the protonation of ammonia (NH₃). The resulting ion has a pK_a of 9.25.

Ammonium and aminium are also general names for positively charged or protonated substituted amines and quaternary ammonium cations N⁺R₄, where one or more hydrogen atoms are replaced by organic radical groups (indicated by R).

Chemistry

Fumes from hydrochloric acid and ammonia forming a white cloud of ammonium chloride

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

H⁺ + :NH₃ → NH+4

The lone electron pair on the nitrogen atom (N) in ammonia is represented as a pair of dots. This electron pair forms the bond with a proton (H⁺).

The ammonium ion is a comparatively strong conjugate acid, reacting with Brønsted bases to return to the uncharged ammonia molecule:

NH+4 + :B → HB + NH₃

When ammonia is dissolved in water, a significant amount of it reacts with the hydronium ions in water to give ammonium ions:

H₃O⁺ + NH₃ H₂O + NH+4

The degree to which ammonia forms the ammonium ion depends on the pH of the solution. If the pH is low (there is a high concentration of hydronium ions), the equilibrium shifts to the right: more ammonia molecules are protonated into ammonium ions. If the pH is high (the concentration of hydronium 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 a solid in a thin white layer on surfaces. Ammonium cations resemble alkali metal ions like Na⁺ or K⁺ and can be found in salts such as ammonium bicarbonate, ammonium chloride, and ammonium nitrate. Most simple ammonium salts are very soluble in water.

Reduction of the ammonium cation gives ammonia gas and hydrogen.^{[citation needed]}

2 NH+4 + 2 e⁻ → 2 NH₃ + H₂

Ammonium ions may dissolve in mercury to form an amalgam. Practically, it may be accomplished by the electrolysis of an ammonium solution with a mercury electrode.^[1] This amalgam spontaneously decomposes to give ammonia and hydrogen.^[2]

Structure and bonding

In an ammonium ion, the nitrogen atom forms four covalent bonds (including one coordinate covalent bond), instead of three as in ammonia, forming a structure which is isoelectronic to molecule of methane and so is energetically favorable.^{[citation needed]}

Substituted ammonium ions

Each of 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, also called aminium ion. Depending on the number of organic radical groups, it is called a primary, secondary, tertiary, or quaternary ammonium cation, respectively. Except quaternary ammonium cations, they exist in equilibrium with their respective substituted amines, depending on the pH.

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 which can be abstracted by a base, and so are permanently charged. These cations, such as 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. They are also used as phase-transfer catalysts for the same reason.

Biology

Ammonium ions are a toxic waste product of the metabolism in 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.^[3]

Ammonia is toxic to humans in high concentrations, and can cause injury to the mucosal lining of the lung, or alkali burns.^[4]

Ammonium can be 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]

See also

• Ammonium transporter
• f-ratio
• Hydronium (H₃O⁺)
• Iminium
• Nitrification
• Onium compounds

References