Ammonia

Ammonia is a compound with the formula NH₃. It is normally encountered as a gas with a characteristic pungent odor. Ammonia contributes significantly to the nutritional needs of the planet as a precursor to foodstuffs and fertilizers. Ammonia, either directly or indirectly, is a building block for the synthesis of many pharmaceuticals. Although in wide use, ammonia is caustic and hazardous.

Ammonia, used commercially is usually named anhydrous ammonia. This term emphasizes the absence of water. Because NH₃ boils at -33 °C, the liquid must be stored under pressure or at low temperature. Its heat of vaporization is, however, sufficiently high that NH₃ can be readily handled in ordinary beakers in a fume hood. “Household ammonia” or “ammonium hydroxide” is a solution of NH₃ in water. The strength of such solutions is measured in units of baume (density), with 26 degrees baume (about 30 weight percent ammonia at 15.5 °C) being the typical high concentration commercial product. Household ammonia ranges in concentration from 5 to 10 weight percent ammonia. See Baumé scale.

Structure and basic chemical properties

The ammonia molecule has a trigonal pyramid shape, as predicted by VSEPR theory. The nitrogen atom in the molecule has a lone electron pair, and ammonia acts as a base, a proton acceptor. This shape gives the molecule an overall dipole moment and makes it polar so that ammonia readily dissolves in water. In water, a very small percentage of NH₃ is converted into the ammonium cation (NH₄⁺). Thus, the term ammonium hydroxide is a misnomer. The degree to which ammonia forms the ammonium ion increases upon lowering the pH of the solution— at “physiological” pH (~7), about 99% of the ammonia molecules are protonated. Temperature and salinity also affect the proportion of NH₄⁺. NH₄⁺ has the shape of a regular tetrahedron.

The main use of ammonia is for fertilizer (83% in 2003). Another major application is its conversion explosives, because nitric acid is made via oxidation of ammonia. The entire nitrogen content of all manufactured organic compounds is derived from ammonia.

Natural occurence

Ammonia is found in small quantities in the atmosphere, being produced from the putrefaction of nitrogenous animal and vegetable matter. Ammonia and ammonium salts are also found in small quantities in rainwater, whereas ammonium chloride (sal-ammoniac), and ammonium sulfate are found in volcanic districts; crystals of ammonium bicarbonate have been found in Patagonian guano. The kidneys secrete NH₃ to neutralize excess acid. Ammonium salts also are found distributed through all fertile soil and in seawater. Substances containing ammonia, or those that are similar to it, are called ammoniacal.