Anhydrous ammonia is a colourless, gaseous compound (which is easily liquefied under pressure) with a pungent smell and the formula NH3.

It reacts with steel at temperatures above 450°C and imparts nitrogen into its surface. Ammonia is the main reactant gas in nitriding and nitrocarburising.

When broken down (dissociated) into its constituent gases, it provides a reducing gas that is frequently used in furnace atmospheres for bright processing. See, for example, bright annealing.

Anhydrous simply means without water. Ammonia is so hydroscopic (water loving) that one cubic foot of water will dissolve 1300 cubic feet of ammonia. When ammonia reacts with water the alkaline compound ammonium hydroxide (NH4OH) will form.

Ammonia gas is much lighter than air and leaks in the open air normally disperse readily into the atmosphere. Under situations of high humidity however, the gas from a leak may absorb water from the atmosphere and hug the ground appearing as a white cloud.

Ammonia is extremely toxic in high concentrations and is highly irritant to the respiratory tract, eyes and skin, even in low concentrations.

Properties: Melting point -77°C
Boiling point -33°C
Vapour density 0.6 (Air = 1)
Vapour pressure 8.6 bar at 20°C
Flash point 11°C
Auto-ignition temperature 651°C
Explosive limits 15 to 27% in air