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.
Fertilizer
In addition to serving as a fertilizer ingredient, ammonia can also be used directly as a fertilizer by forming a solution with irrigation water, without additional chemical processing. This later use allows the continuous growing of nitrogen dependent crops such as maize (corn) without crop rotation but this type of use leads to poor soil health.
Refrigeration
Ammonia's thermodynamic properties made it one of the refrigerants commonly used in refrigeration units prior to the discovery of dichlorodifluoromethane in 1928, also known as Freon or R12.
But ammonia is toxic, gaseous, irritant, and corrosive to copper alloys, and over a kilo is needed for even a miniature fridge. With an ammonia refrigerant, the ever present risk of an escape brings with it a risk to life. However data on ammonia escapes has shown this to be an extremely small risk in practice, and there is consequently no control on the use of ammonia refrigeration in densely populated areas and buildings in almost all jurisdictions in the world.
Its use in domestic refrigeration has been mostly replaced by CFCs and HFCs in the first world, which are more or less non-toxic and non-flammable, and butane and propane in the 3rd world, which despite their high flammability do not seem to have produced any significant level of accidents. Ammonia has continued to be used for miniature and multifuel fridges, such as minibars and caravan refrigerators.
These ammonia absorption cycle domestic refrigerators do not use compression and expansion cycles, but are driven by temperature differences. However the energy efficiency of such refrigerators is relatively low. Today the smallest refrigerators mostly use solid state peltier thermopile heat pumps rather than the ammonia absorption cycle.
Ammonia continues to be used as a refrigerant in large industrial processes such as bulk icemaking and industrial food processing. Since the implication of haloalkanes being major contributors to ozone depletion, ammonia is again seeing increasing use as a refrigerant. Ammonia is increasingly popular in commercial applications, such as in grocery store freezer cases and refrigerated displays.
Disinfectant
It is also sometimes added to drinking water along with chlorine to form chloramines, a disinfectant. Unlike chlorine alone, chloramine does not combine with organic (carbon containing) materials to form carcinogenic halomethanes such as chloroform. However, chlorine and ammonia should never be mixed in an uncontrolled environment because they cause a chemical reaction that releases toxic gas. See Safety precautions for more information.
