Ammonium

From Wikipedia, the free encyclopedia

Ammonium is also an old name for the Siwa Oasis in western Egypt.

The ammonium cation is a positively charged polyatomic cation of the chemical formula NH₄⁺. It has a molecular mass of 18.04 and is formed by protonation of ammonia (NH₃). 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 (which could be symbolized as R).

[edit] Chemistry

In an ammonium ion, the positively charged nitrogen atom forms four covalent bonds, instead of three as in ammonia. This reaction is reversible. The ammonium ion can act as a very weak Brønsted-Lowry acid in the sense that it can protonate a stronger base using any one of its hydrogen ( H ) atoms and convert back to ammonia. This means that the ammonium ion is a conjugate acid of the base ammonia. In a solution, the degree to which ammonia forms the ammonium ion depends on the pH of the solution.

However, 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 water soluble. Ammonium ions are a toxic waste product of the metabolism in animals and are excreted unchanged in the urine by water animals. The ammonium ion behaves somewhat like an alkali metal ion. Boo!

[edit] 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.

An example of a reaction forming an ammonium ion is that between dimethylamine, (CH₃)₂NH, with an acid to give the dimethylaminium cation, (CH₃)₂NH₂⁺: