monoammonium

Monoammonium is a descriptor used in inorganic chemistry to refer to salts that contain one ammonium ion (NH4+) per formula unit. The term emphasizes the presence of a single NH4+ cation balancing a corresponding monovalent anion. Such salts are formed when ammonia reacts with a strong acid to furnish the ammonium cation alongside a singly charged anion.

A well-known example is monoammonium phosphate, with the formula NH4H2PO4. It is widely used as a fertilizer to supply both nitrogen and phosphorus. Monoammonium phosphate is produced by neutralizing phosphoric acid with ammonia and crystallizes as a white, hygroscopic solid that dissolves readily in water. Other monoammonium salts exist, such as monoammonium bisulfate (NH4HSO4), though many common fertilizers are diammonium or contain more complex anions.

In aqueous solution, the ammonium cation behaves as a weak acid (pKa ≈ 9.25), capable of donating a proton to form ammonia under appropriate conditions. The solid salts are generally soluble in water, and their properties are influenced by the nature of the accompanying anion as well as ambient temperature and humidity. Safety considerations for monoammonium salts are similar to other ammonium salts: they are typically low in toxicity but can release ammonia at high temperatures or under strong basic conditions.

See also: ammonium, diammonium, ammonium salts, monoammonium phosphate.