Encyclopedia
| Ammonia |
|---|
| |
| General |
|---|
| Systematic name | Ammonia
Azane |
| Other names | Hydrogen nitride
Spirit of hartshorn
Nitrosil
Vaporole |
| Molecular formula | NH3 |
| Molar mass | 17.0304 g/mol |
| Appearance | Colourless gas with
strong pungent odor |
| CAS number | |
| Properties |
|---|
| Density and phase | 0.6813 g/L, gas. |
| Solubility in water | 89.9 g/100 ml at 0 °C. |
| Melting point | -77.73 °C |
| Boiling point | -33.34 °C |
| Acidity | ˜34 |
| Basicity | 4.75 |
| Structure |
|---|
| Molecular shape | Terminus |
| Dipole moment | 1.42 D |
| Bond angle | 107.5° |
| Hazards |
|---|
| MSDS | External MSDS |
| Main hazards | Toxic and corrosive. |
| NFPA 704 | |
| Flash point | 11 °C |
| R/S statement | R: , , ,
S: , , ,
, |
| RTECS number | BO0875000 |
| Supplementary data page |
|---|
Structure and
properties | n, er, etc. |
Thermodynamic
data | Phase behaviour
Solid, liquid, gas |
| Spectral data | UV, IR, NMR, MS |
| Related compounds |
|---|
| Other ions | Ammonium
- hydroxide
- chloride
|
| Related compounds | Hydrazine
Hydrazoic acid
Hydroxylamine
Chloramine |
Except where noted otherwise, data are given for
materials in their standard state
|
|
Ammonia is a compound of
nitrogen and
hydrogen with the
formula NH3. At standard temperature and pressure, ammonia is a gas. It is
toxic and
corrosive to some materials, and has a characteristic pungent
odor. Ammonia used commercially is called
anhydrous ammonia to distinguish it from ammonium hydroxide
solution, which is
household ammonia.
An ammonia molecule has a trigonal pyramid shape, as predicted by
VSEPR theory. This shape gives the molecule an overall
dipole moment, and makes it
polar so that ammonia readily dissolves in
water. The nitrogen atom in the molecule has a lone electron pair, and ammonia acts as a base. That means that, when in aqueous solution, it can take a
proton from water to produce a hydroxide anion and an
ammonium cation , which has the shape of a regular
tetrahedron. The degree to which ammonia forms the ammonium ion depends on the
pH of the
solution—at "physiological" pH , about 99% of the ammonia molecules are protonated.
The main uses of ammonia are in the production of
fertilizers,
explosives and
polymers. It is also an ingredient in certain household glass cleaners. Ammonia is found in small quantities in the atmosphere, being produced from the putrefaction of
nitrogenous animal and vegetable matter. Ammonia and ammonium salts are also found in small quantities in rainwater, while
ammonium chloride and
ammonium sulfate are found in volcanic districts; crystals of
ammonium bicarbonate have been found in
Patagonian
guano. Ammonium salts also are found distributed through all fertile soil and in seawater. Substances containing ammonia, or that are similar to it, are called
ammoniacal.
History
Salts of ammonia have been known from very early times; thus the term
Hammoniacus sal appears in the writings of
Pliny, although it is not known whether the term is identical with the more modern
sal-ammoniac.
Liquid ammonia as a solvent
- See also: Inorganic nonaqueous solvent
Liquid ammonia is the best-known and most widely studied non-aqueous ionizing solvent. Its most conspicuous property is its ability to dissolve alkali metals to form highly coloured, electrically conducting solutions containing solvated electrons. Apart from these remarkable solutions, much of the chemistry in liquid ammonia can be classified by analogy with related reactions in aqueous solutions. Comparison of the physical properties of NH
3 with those of water shows that NH
3 has the lower melting point, boiling point, density,
viscosity, dielectric constant and electrical conductivity; this is due at least in part to the weaker H bonding in NH
3 and the fact that such bonding cannot form cross-linked networks since each NH
3 molecule has only 1 lone-pair of electrons compared with 2 for each H
2O molecule. The ionic self-dissociation constant of liquid NH
3 at −50 °C is approx. 10
-33 mol
2•l
-2.
Solubility of salts
Liquid ammonia is an ionizing solvent, although less so than water, and dissolves a range of ionic compounds including many
nitrates,
nitrites,
cyanides and
thiocyanates. Most
ammonium salts are soluble, and these salts act as acids in liquid ammonia solutions. The solubility of halide salts increases from fluoride to iodide. A saturated solution of
ammonium nitrate contains 0.83 mol solute per mole of ammonia, and has a vapour pressure of less than 1 bar even at 25 °C.
Solutions of metals
- See also: Solvated electron, metallic solution
Liquid ammonia will dissolve the alkali metals and other electropositive metals such as calcium, strontium, barium, europium and ytterbium. At low concentrations , deep blue solutions are formed: these contain metal cations and solvated electrons, free electrons which are surrounded by a cage of ammonia molecules.
These solutions are very useful as strong reducing agents. At higher concentrations, the solutions are metallic in appearance and in electrical conductivity. At low temperatures, the two types of solution can coexist as phases.
Redox properties of liquid ammonia
- See also: Redox.
| | E° | E° |
|---|
| + + e− Li | −2.24 | −3.04 |
| + + e− K | −1.98 | −2.93 |
| + + e− Na | −1.85 | −2.71 |
| 2+ + 2e− Zn | −0.53 | −0.76 |
| 4+ + e− ½ H2 + NH3 | 0.00 | – |
| 2+ + 2e− Cu | +0.43 | +0.34 |
| + + e− Ag | +0.83 | +0.80 |
|
The range of thermodynamic stability of liquid ammonia solutions is very narrow, as the potential for oxidation to
dinitrogen,
E° , is only +0.04 V. In practice, both oxidation to dinitrogen and reduction to
dihydrogen are slow. This is particularly true of reducing solutions: the solutions of the alkali metals mentioned above are stable for several days, slowly decomposing to the
metal amide and dihydrogen. Most studies involving liquid ammonia solutions are done in reducing conditions: although oxidation of liquid ammonia is usually slow, there is still a risk of explosion, particularly if transition metal ions are present as possible catalysts.
Detection and determination
Ammonia and ammonium salts can be readily detected, in very minute traces, by the addition of Nessler's solution, which gives a distinct yellow coloration in the presence of the least trace of ammonia or ammonium salts. Sulfur sticks are burnt to detect small leaks in industrial ammonia refrigeration systems. Larger quantities can be detected by warming the salts with a caustic alkali or with
quicklime, when the characteristic smell of ammonia will be at once apparent. The amount of ammonia in ammonium salts can be estimated quantitatively by distillation of the salts with
sodium or
potassium hydroxide, the ammonia evolved being absorbed in a known volume of standard
sulfuric acid and the excess of acid then determined
volumetrically; or the ammonia may be absorbed in
hydrochloric acid and the
ammonium chloride so formed precipitated as
ammonium hexachloroplatinate,
2PtCl
6.
Interstellar space
Ammonia was first detected in interstellar space in 1968, based on
microwave emissions from the direction of the
galactic core. This was the first polyatomic molecule to be so detected.
The sensitivity of the molecule to a broad range of excitations and the ease with which it can be observed in a number of regions has made ammonia one of the most important molecules for studies of
molecular clouds. The relative intensity of the ammonia lines can be used to measure the temperature of the emitting medium.
The following isotopic species of ammonia have been detected:
- NH3, 15NH3, NH2D, NHD2, and ND3
The detection of triply-
deuterated ammonia was considered a surprise as deuterium is relatively scarce. It is thought that the low-temperature conditions allow this molecule to survive and accumulate. The ammonia molecule has also been detected in the atmospheres of the
gas giant planets, including
Jupiter, along with other gases like
methane,
hydrogen, and
helium. The interior of Saturn may include frozen crystals of ammonia.
Safety precautions
Toxicity and storage information
The toxicity of ammonia solutions does not usually cause problems for humans and other mammals, as a specific mechanism exists to prevent its build-up in the bloodstream. Ammonia is converted to
carbamoyl phosphate by the enzyme carbamoyl phosphate synthase, and then enters the
urea cycle to be either incorporated into
amino acids or excreted in the urine. However
fish and
amphibians lack this mechanism, as they can usually eliminate ammonia from their bodies by direct excretion. Ammonia even at dilute concentrations is highly toxic to aquatic animals, and for this reason it is classified as
dangerous for the environment. Ammonium compounds should never be allowed to come in contact with bases , as dangerous quantities of ammonia gas could be released.
Household use
Solutions of ammonia are used as household cleaners, particularly for glass. These solutions are irritating to the eyes and mucous membranes , and to a lesser extent the skin. They should
never be mixed with chlorine-containing products or strong oxidants, for example household
bleach, as a variety of toxic and carcinogenic compounds are formed .
Laboratory use of ammonia solutions
The hazards of ammonia solutions depend on the concentration: "dilute" ammonia solutions are usually 5–10% by weight ; "concentrated" solutions are usually prepared at >25% by weight. A 25% solution has a density of 0.907 g/cm³, and a solution which has a lower density will be more concentrated. The European Union classification of ammonia solutions is given in the table.
Concentration
by weight | Molarity | Classification | R-Phrases |
|---|
| 5–10% | 2.87–5.62 mol/L | Irritant | |
| 10–25% | 5.62–13.29 mol/L | Corrosive | |
| >25% | >13.29 mol/L | Corrosive
Dangerous for
the environment | , |
|
- S-Phrases: , , , , .
The ammonia vapour from concentrated ammonia solutions is severely irritating to the eyes and the respiratory tract, and these solutions should only be handled in a fume hood. Saturated solutions can develop a significant pressure inside a closed bottle in warm weather, and the bottle should be opened with care: this is not usually a problem for 25% solutions.
Ammonia solutions should not be mixed with
halogens, as toxic and/or explosive products are formed. Prolonged contact of ammonia solutions with
silver, mercury or iodide salts can also lead to explosive products: such mixtures are often formed in qualitative chemical analysis, and should be acidified and diluted before disposal once the test is completed.
Laboratory use of anhydrous ammonia
Anhydrous ammonia is classified as
toxic and
dangerous for the environment . The gas is flammable and can form explosive mixtures with air . The permissible exposure limit in the United States is 50 ppm , while the IDLH concentration is estimated at 300 ppm. Repeated exposure to ammonia lowers the sensitivity to the smell of the gas: normally the odour is detectable at concentrations of less than 0.5 ppm, but desensitized individuals may not detect it even at concentrations of 100 ppm. Anhydrous ammonia corrodes
copper- and
zinc-containing
alloys, and so
brass fittings should not be used for handling the gas. Liquid ammonia can also attack rubber and certain plastics.
Ammonia reacts violently with the halogens, and causes the explosive polymerization of
ethylene oxide. It also forms explosive compounds with compounds of
gold,
silver, mercury,
germanium or
tellurium, and with
stibine. Violent reactions have also been reported with
acetaldehyde,
hypochlorite solutions,
potassium ferricyanide and peroxides.
See also
- Ammonia
- Ammonia production
- Chlorination
- Water purification
References
Bibliography
External links
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- for the Minnesota Department of Agriculture
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