ammoniumioon

Ammonium ion (NH4+) is a positively charged polyatomic ion formed when ammonia (NH3) accepts a proton. In aqueous solution it is the conjugate acid of ammonia. The ion adopts a tetrahedral geometry, with four N–H bonds and no lone pair on nitrogen.

Its acid–base behavior is described by the equilibrium NH4+ ⇌ NH3 + H+. The pKa for this dissociation is about 9.25 at 25 °C, meaning NH4+ is the predominant species at most biological pH values, but NH3 becomes more common at higher pH.

Ammonium forms a wide range of salts with various anions, such as ammonium chloride (NH4Cl), ammonium nitrate (NH4NO3), ammonium sulfate ((NH4)2SO4), and ammonium phosphate ((NH4)3PO4). These salts are typically soluble in water and are widely used in fertilizers and chemical synthesis. In soils and water, ammonium is produced by microbial decomposition and fixation, and it can be taken up by plants. It can be oxidized to nitrite and nitrate by nitrifying bacteria, linking to the broader nitrogen cycle.

Biologically, ammonium plays a central role in nitrogen metabolism. In animals, excess ammonium is detoxified in the liver via the urea cycle. In plants and microbes, ammonium can be assimilated into amino acids through pathways involving glutamine synthetase and glutamate synthase. Safety considerations: many ammonium salts are irritants; ammonium nitrate is a strong oxidizer and can pose explosion hazards under certain conditions.