Amonyum

Ammonium, NH4+, is a positively charged polyatomic ion formed when ammonia (NH3) accepts a proton. It is the conjugate acid of ammonia and is a common constituent of many inorganic salts. In aqueous solution the equilibrium NH3 + H2O ⇌ NH4+ + OH− governs the relative amounts of ammonia and ammonium, depending on pH.

Its geometry is tetrahedral, with four N–H bonds. The ammonium ion is highly soluble in water and forms a wide range of salts. The pKa of NH4+ is about 9.25, reflecting that ammonium is a weak acid; its conjugate base is ammonia.

Occurrence and production: In nature ammonium arises from the biological decomposition of nitrogen-containing organic matter and from mineral weathering. Industrially it is produced by acid-base neutralization of ammonia with acids (for example NH3 + HCl → NH4Cl). Ammonium salts are used extensively as nitrogen fertilizers, often after conversion to nitrate and other plant-available forms. Soil microbes convert ammonium to nitrite and nitrate through nitrification.

Common ammonium salts include ammonium chloride (NH4Cl), ammonium sulfate ((NH4)2SO4), ammonium nitrate (NH4NO3), and monoammonium phosphate NH4H2PO4. Ammonium hydroxide (NH4OH) is the solution of ammonia in water; it behaves as a weak base and releases ammonia gas on heating. Handling ammonium compounds requires avoiding inhalation of dust and solutions and protecting skin and eyes; some salts can be irritant or caustic.

Note: In Turkish and other languages, the term amonyum is used for ammonium.