Geology Reference
In-Depth Information
Oxidation
state
Free energy
of formation
∆
G
Oxides of nitrogen
dinitrogen oxide
nitric oxide
nitrogen dioxide
+1
+3
+4
N
2
O
NO
2
less stable
NO
positive
5
N
2
dinitrogen
0
0
5
NH
3
ammonia
5
6
−
3
2
negative
NO
2
nitrite
+3
3
2
more stable
NO
3
nitrate
+5
3
amino acids,
proteins
−
3
4
Reactions
1 fixation
2 nitrification
3 assimilation by piants
4 ammonification
5 denitrification
6 nitrate-containing precipitation often
as nitric acid in acid rain
micro-organisms play a part in reactions 1, 2, 4 and 5
Figure 10 Chemical forms and cycle of nitrogen
(From O'Neill.
23
)
tetraalkyl lead gasoline additives in which lead is present as Pb
IV
. The
predominant compounds used were tetramethyl lead, Pb(CH
3
)
4
, and
tetraethyl lead, Pb(C
2
H
5
)
4
. These are lost to the atmosphere as vapour
from fuel evaporation and exhaust emissions from cold vehicles, but
comprised only 1-4% of lead in polluted air.
2
Leaded gasoline also
contains the scavengers 1,2-dibromoethane, CH
2
BrCH
2
Br, and 1,2-di-
chloroethane, CH
2
ClCH
2
Cl, which convert lead within the engine to
lead halides, predominantly lead bromochloride, PbBrCl, in which lead
is in the Pb
II
valence state, its usual form in environmental media. About
75% of lead alkyl burned in the engine is emitted as fine particles of
inorganic lead halides. Atmospheric emissions of lead arise also from
industry; both these and vehicle-emitted lead have declined massively.
Figure 12 shows trends in United Kingdom emissions of lead to
