Environmental Engineering Reference
In-Depth Information
ecology is presented (Section
6
). In view of the huge amount of data accumulating
in the field, we recommend comprehensive reviews as primary references
[
3
,
19
-
25
].
2 Ammonia and Its Role in the Environment
NH
3
is a colorless gas with a typical pungent odor. It has a pyramidal shape, with an
H-N-H angle of 106.7
(compared to 93.3
in phosphane, PH
3
), and contains a
single lone pair of electrons. Ammonia has a dipole moment of 1.47 D (compared to
0.57 D for PH
3
). The solubility of ammonia in water is greater than that of any other
gas because of hydrogen bond formation between NH
3
and H
2
O. In aqueous
solution, ammonia equilibrates with the ammonium ion (NH
4
) which is slightly
acidic (equations
7
and
8
). The equilibrium depends on pH and temperature.
NH
4
þ
OH
K
B
¼
10
5
NH
3
þ
H
2
O
!
1
:
8
p
K
B
NH
3
ð
Þ
¼
4
:
75
ð
7
Þ
¼
NH
4
þ
H
3
O
þ
þ
10
10
p
K
A
NH
4
H
2
O
!
NH
3
K
A
¼
5
:
6
9
:
25
ð
8
Þ
Ammonia can be obtained by the action of water on nitrides, e.g., lithium
nitride (Li
3
N), by heating of ammonium chloride (NH
4
Cl) with calcium hydroxide
[Ca(OH)
2
], or the reduction of nitrate with metallic zinc in basic solution. In the
so-called Ostwald process, ammonia is converted to nitric acid (HNO
3
)in
three stages. It is oxidized by heating with oxygen in the presence of a platinum-
rhodium catalyst to formNO and water, a strongly exothermic reaction (equation
9
).
NO is further oxidized to yield nitrogen dioxide (NO
2
) which is then readily
absorbed by the water, yielding the desired product HNO
3
(equations
10
and
11
).
Thus, the Ostwald process provides the main raw material for the most common
type of fertilizer production.
4NH
3
þ
5O
2
!
4NO
þ
6H
2
O
ð
9
Þ
þ
O
2
!
ð
Þ
2NO
2NO
2
10
3NO
2
þ
!
2 HNO
3
þ
ð
Þ
H
2
O
NO
11
The coordination chemistry of ammonia is well established. In metal ammine
complexes, NH
3
acts as
σ
-donor and is located in the middle of the
Spectrochemical
Series
:I
Br
<
SCN
<
Cl
<
NO
3
<
N
3
<
F
<
OH
<
NCS
<
H
2
O
<
2,2
0
-bipyridine
<
CH
3
CN
<
pyridine
<
NH
3
<
ethylenediamine
<
<
1,10-
CN
NO
2
<
phenanthroline
CO. Most metal ions will bind ammonia as a
ligand, e.g., the Cu(II) tetraammine complex [Cu(NH
3
)
4
]
2+
, or the Ag(I) diammine
complex [Ag(NH
3
)
2
]
+
. Ammine complexes of Cr(III) were key for Alfred
<
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