ammónium

Ammónium, in chemical terms ammonium, refers to the positively charged ion NH4+. It forms when ammonia (NH3) is protonated. In aqueous solution, the ammonium–ammonia system follows the equilibrium NH4+ ⇌ NH3 + H+, with a pKa around 9.25 at 25°C, so NH4+ dominates at lower pH and NH3 at higher pH.

Ammonium salts are compounds containing the NH4+ cation paired with various anions, such as chloride, sulfate, nitrate, and phosphate. Common examples include ammonium chloride (NH4Cl), ammonium sulfate ((NH4)2SO4), ammonium nitrate (NH4NO3), and ammonium phosphate ((NH4)3PO4) or more common dibasic and monobasic salts like (NH4)H2PO4 and (NH4)2HPO4.

Preparation and occurrence: Ammonium salts are typically prepared by reacting ammonia with a strong acid, for example NH3 + HCl → NH4Cl and NH3 + H2SO4 → (NH4)2SO4. Ammonium occurs naturally in soils and waters from the decomposition of nitrogenous organic matter and from fertilizers.

Uses: The largest use is as a fertilizer source of nitrogen. Ammonium salts are also used in chemical synthesis, in food additives (for example ammonium chloride as a leavening agent or acidity regulator in certain foods), and in pharmaceuticals. In biology, ammonium serves as a nitrogen source for plants and microorganisms; animals excrete excess ammonium via the urea cycle.

Safety and environmental: Ammonium salts are generally less volatile and less toxic than ammonia gas, but concentrated solutions can irritate skin and eyes. In the environment, ammonium can be oxidized to nitrate by bacteria (nitrification); excess ammonium in water can contribute to eutrophication.