Formula; NH3; MW 17.03; tetrahedral planar geometry, H—N—H bond angle107.3°; N—H bond distance 1.016Å; dipole moment of the gas 1.46 x 10–18 esu;a Lewis base.
Occurrence and Uses
Ammonia occurs in nature, being constantly formed by putrefaction of theprotein of dead animals and plants. While some of it is washed away by therain into rivers and oceans where it is recycled and converted into proteins bymicroorganisms, much of it is rapidly absorbed from the earth by living plantsmaking new proteins. Ammonia occurs in urine from which it was producedearlier by chemists and alchemists for use as a soluble base. It occurs in gasliquor obtained from coal gas and producer gas plants and coke ovens. Gasliquor was a major source for producing ammonia before Haber-Bosch processwas developed. Combustion of coal, fuel oil, wood and natural gas, as well asforest fires produce ammonia in small amounts in the range 1 to 10 lb per ton.It occurs in many industrial effluents, wastewaters, and groundwaters attrace concentrations. It is also found at trace levels in varying concentrationsin the air in most metropolitan cities.
The single largest use of ammonia is its direct application as fertilizer, andin the manufacture of ammonium fertilizers that have increased world foodproduction dramatically. Such ammonia-based fertilizers are now the primarysource of nitrogen in farm soils. Ammonia also is used in the manufactureof nitric acid, synthetic fibers, plastics, explosives and miscellaneous ammoniumsalts.
Liquid ammonia is used as a solvent for many inorganic reactionsin non-aqueous phase. Other applications include synthesis of amines andimines; as a fluid for supercritical fluid extraction and chromatography; andas a reference standard in 15N–NMR.
Physical Properties
Colorless gas; pungent suffocating odor; human odor perception 0.5 mg/m3;liquefies by compression at 9.8 atm at 25°C, or without compression at–33.35°C (at 1 atm); solidifies at –77.7°C; critical temperature and pressure,133°C and 112.5 atm, respectively; vapor density 0.59 (air=1); density of liquidammonia 0.677 g/mL at –34°C; dielectric constant at –34°C is about 22;extremely soluble in water; solution alkaline; pKa 9.25 in dilute aqueous solutionat 25°C; the gas does not support ordinary combustion, but burns with ayellow flame when mixed in air at 16—27% composition.AnalysisAmmonia may be readily identified from its odor. It may be measured bytitrimetry. It is absorbed in an excess amount of a standard solution of dilutesulfuric acid and the excess unreacted acid is back titrated against a standardsolution of caustic soda using methyl orange indicator. Alternatively, potentiometrictitration may be used to find the end point. Concentrations at tracelevels in wastewaters, groundwaters, drinking waters, and air may be measuredby various colorimetric techniques or by the ammonia–selective electrodemethod (APHA, AWWA and WEF, 1999. Standard Methods for theExamination of Water and Wastewater, 20th ed. Washington, DC, AmericanPublic Health Association). Ammonia reacts with Nessler reagent under alkalineconditions, forming a yellow color. The intensity of color is measured byspectrophotometer, absorbance being proportional to concentration of ammoniain the solution. Alternatively, it may be analyzed by the indophenol bluemethod. Ammonia reacts with hypochlorite to form monochloramine whichreacts with phenol in the presence of manganous sulfate catalyst to produceblue indophenol (Patnaik, P. 1997. Handbook of Environmental Analysis.Boca Raton, FL, Lewis Publishers). Solutions at high concentrations may beappropriately diluted to measure ammonia within the calibration range in colorimetricand electrode methods.
Hazard
Ammonia causes intense irritation of eyes, nose and respiratory tract whichcan lead to tears, respiratory distress, chest pain, and pulmonary edema. Afew minutes exposure to 3,000 ppm can cause severe blistering of skin, lungedema, and asphyxia which can lead to death (Patnaik, P. 1992. AComprehensive Guide to the Hazardous Properties of Chemical Substances,p. 304. New York, Van Nostrand Reinhold). Contact with liquid ammonia cancause serious blistering and destruction of skin tissues. LC50 inhalation(mouse): 4,200 ppm/hr.Fire or explosion hazard may arise from the following ammonia reactions:Reaction with halogens produces nitrogen trihalides which explode on heating;its mixture with fluorine bursts into flame; reacts with gold, silver, ormercury to form unstable fulminate-type shock-sensitive compounds; similarly,shock-sensitive nitrides are formed when ammonia reacts with sulfur orcertain metal chlorides, such as mercuric, or silver chloride; liquid ammoniareacts violently with alkali metal chlorates and ferricyanides.