The ammonium cation is a positively charged polyatomic cation of the chemical formula NH4+. It has a formula weight of 18.05 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).
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.
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.
Reduction of the ammonium cation gives ammonia gas and hydrogen.
Ammonium radicals may dissolve in mercury to form an amalgam. Practically, it may be accomplished by the electrolysis of an ammonium solution with a mercury electrode. This amalgam spontaneously decomposes to give ammonia and hydrogen.
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, (CH3)2NH, with an acid to give the dimethylaminium cation, (CH3)2NH2+:
Ammonium is toxic to humans in high concentrations, and can cause injury to the mucosal lining of the lung, or alkali burns.