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). The resulting ion has a pKa 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 (which could be symbolized as R).
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. The ammonium ion behaves somewhat like an alkali metal ion.
At attempt of reception of metal ammonium the ion, receiving electron, breaks up to ammonia and hydrogen:
- 2NH4+ + 2e− → 2NH3 + H2
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+:
Ammonium ions are a toxic waste product of the metabolism in animals. In humans, it is converted in the urea cycle to urea, because it wastes less water to excrete. By water animals it is excreted unchanged in the urine.