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Anhydrous ammonia is the compound formed by the combination
of the two gaseous elements, nitrogen and hydrogen,
in the proportion of one part of nitrogen to three
parts of hydrogen by volume. Since one volume of nitrogen
weighs fourteen times as much as one volume of hydrogen,
on a weight basis, the ratio is fourteen parts of
nitrogen to three parts of hydrogen, or about 82%
nitrogen and 18% hydrogen.
At atmospheric temperature
and pressures, anhydrous ammonia is a pungent colorless
gas. Anhydrous ammonia boils
at -28°F and freezes to a
white crystalline mass
at -108°F. When heated above its critical
temperature of 270.3°F ammonia exists only as a vapor
regardless of the pressure. Between the melting and
critical points, liquid ammonia exerts a vapor pressure
which increases with rising temperature. When liquid
ammonia is in a closed container, it is in equilibrium
with ammonia vapor and the pressure within the container
bears a definite relationship to the temperature.
Liquid anhydrous ammonia
is lighter than water, having a density of 42.57 pounds
per cubic foot at -28°F, while as a vapor, ammonia
is lighter than air, its relative density is 0.597
compared to air at atmospheric pressure and a temperature
of 32°F. Under the latter conditions, one pound of
ammonia vapor occupies a volume of 20.78 cubic feet.
At 70°F and at atmospheric pressure, one pound of
ammonia vapor occupies a volume of 22.5 cubic feet
and yields 45 cubic feet of dissociated gas at a ratio
of 25% nitrogen and 75% hydrogen.
of its great affinity for water, care must be taken
in the storage and handling of ammonia to keep it
dry. "Anhydrous" means "without water". When ammonia
gas is dissolved in water, the resulting material
is ammonium hydroxide or "aqua" ammonia. The two materials
should not be confused.
Molecular symbol: NH3
Molecular weight: 17.032
Boiling point at one atmosphere: -28°F
Freezing point at one atmosphere: -108°F
Critical temperature: 270.32°F
Critical pressure: 1657 psia
Vapor density at -28°F and one atmosphere:
0.056697 lb/cubic ft.
Heat of Combustion: 8001 BTU/lb
The common metals are not affected by dry ammonia.
Moist ammonia will not corrode iron or steel, but
will react rapidly with copper, brass, zinc and many
alloys, especially those containing copper. Only steel
or ductile iron should be used for ammonia containers,
valves, fittings and piping.
Under normal conditions,
ammonia is a very stable compound. It takes excessive
temperatures (about 840° to 930°F) to cause
it to dissociate slightly at atmospheric pressure.
When this happens, the dissociated products are nitrogen
and hydrogen. Ammonia gas burns in a mixture with
air within a limited range. The flammable limits at
atmospheric pressure are 15% to 28% by volume of ammonia
in air. Experiments conducted by Underwriters Laboratories
indicate that an ammonia-air mixture in a standard
quartz bomb will not ignite at temperatures below
1562°F. When an iron bomb, having a catalytic
effect, was used, the ignition temperature dropped
The following sources of information on anhydrous
ammonia are recommended.
Compressed Gas Association
4221 Walney Road, 5th Floor
Chantilly, VA 20151-2923
Handling of Anhydrous Ammonia
29 CFR 1910.111
Code of Federal Regulations, Title 29-Labor
Safety Requirements for the Storage & Handling
of Anhydrous Ammonia
American National Standard Institute
25 West 43rd Street, 4th Floor
New York, NY 10036
International Institute of Ammonia Refrigeration
1110 North Globe Road, Suite 250
Arlington, VA 22201
Dissociated ammonia is frequently used as a furnace
atmosphere for heat treating metals. Dryness is an
important factor. It is difficult to measure the moisture
content of the ammonia; however, the moisture content
of the dissociated gas can be readily determined by
measuring its dewpoint.