NH4

NH4, in chemical contexts, denotes the ammonium ion, NH4+. It is a positively charged polyatomic ion and the conjugate acid of ammonia (NH3). Ammonium forms when ammonia accepts a proton and can be found in solution as NH4+ or in solid salts.

Structure and properties. The ammonium ion has a tetrahedral arrangement around the central nitrogen atom, with four N–H bonds. It carries a +1 charge and is highly soluble in water, forming salts with many counterions. In aqueous solutions, NH4+ can participate in acid–base equilibria with water: NH4+ + H2O ⇌ NH3 + H3O+. Its acid dissociation constant is commonly cited with a pKa around 9.25, reflecting its status as a weak acid.

Common salts and applications. Ammonium salts are widely used in chemistry and industry. Examples include ammonium chloride (NH4Cl), ammonium sulfate ((NH4)2SO4), and ammonium nitrate (NH4NO3). These salts are typically soluble in water and serve as fertilizers, buffering agents, or precursors to other nitrogen compounds. In biological and environmental systems, ammonium is a key intermediate in the nitrogen cycle.

Occurrence and role. Ammonium is produced biologically by the deamination of amino acids and other nitrogen-containing compounds (ammonification) and is taken up by plants as a nitrogen source. Soil and water ecosystems rely on the interconversion of NH4+ with ammonia (NH3) via microbial processes such as nitrification and assimilation.

Safety. Many ammonium compounds are irritants and can be caustic in concentrated form. Some ammonium salts, notably ammonium nitrate, can pose explosion hazards under certain conditions. Proper handling and storage are important in laboratory and industrial settings.