Ammonia: Anhydrous and Aqueous Forms, Uses and Hazards

Ammonia (NH3, CAS 7664-41-7) is a colourless gas with a sharp, suffocating smell. It is one of the most important industrial chemicals, produced in vast quantities by the Haber-Bosch process. In trade it comes in two very different forms: anhydrous ammonia, a liquefied gas under pressure, and aqueous ammonia (ammonia solution, ammonium hydroxide), a liquid containing ammonia dissolved in water at about 10 to 35 percent.

Uses of ammonia

• Fertiliser, by far the largest use: direct application of anhydrous ammonia, and manufacture of urea, ammonium nitrate and ammonium sulphate.
• Industrial refrigeration, because it is an efficient natural refrigerant used in cold stores and food plants.
• Cleaning products: glass cleaners and household ammonia, typically at low strength.
• Chemical manufacturing: nitric acid, nylon, plastics, dyes and explosives.
• Water treatment, forming chloramine disinfectants, and pH control.
• Metal treatment: nitriding steel and as a reducing atmosphere.
• Power plant emission control, as a reagent for removing nitrogen oxides.

Two forms, two SDS documents

The pressurised gas and the solution do not share a classification. Anhydrous ammonia is a flammable gas (though hard to ignite), a gas under pressure, acutely toxic by inhalation, corrosive to skin and eyes, and very toxic to aquatic life. Aqueous ammonia is commonly classified as causing severe skin burns and eye damage (H314), as very toxic to aquatic life (H400), and for solutions of sufficient strength with respiratory irritation (H335). Never use one SDS for both, and state the concentration of every solution in Section 3. The signal word is Danger in both cases.

Health hazards

Ammonia is intensely irritating to the eyes, nose, throat and lungs, and exposure to high concentration can cause chemical burns, pulmonary oedema and death. Liquid anhydrous ammonia causes frostbite as well as corrosive burns because it evaporates rapidly and chills tissue. Odour is detectable at very low concentration, well below the exposure limit, which is a useful warning, but at high concentration the sense of smell can fail. Occupational exposure limits are set in most countries, and Section 8 should list the relevant ones.

Handling and storage

Anhydrous ammonia requires pressure-rated systems, leak detection, ventilation and emergency procedures, and employers often must maintain an emergency plan. Equipment containing copper, brass, zinc or silver alloys is attacked by ammonia, so materials selection is critical. Aqueous ammonia should be stored in closed containers of steel or compatible plastic, in a cool area, away from heat because it releases gas as it warms. Keep it away from acids, halogens, hypochlorite bleach (forming toxic chloramines) and heavy metal salts. Read our bleach guide for the mixing hazard, and the storage compatibility guide.

Transport and documents

Anhydrous ammonia is a Class 2.3 toxic gas with a corrosive subsidiary hazard. Ammonia solutions are Class 8 corrosives, with separate entries by concentration (for example, above 10 percent and up to 35 percent, and above 35 percent). Check the current UN numbers. The SDS should include exposure limits, emergency measures for gas release, and environmental precautions, since ammonia is toxic to fish. The data we hold is on the ammonia page.