Environmental Engineering Reference
In-Depth Information
Nitric oxide (NO) and nitrogen dioxide (NO 2 ) : This is a classical colorimetric
method developed by Saltzman (1954). The process begins with bubbling air
through an impinge containing aqueous solution of a number of ingredients.
When NO 2 is bubbled through water, it forms HNO 2 , which will react with an
organic base (amine). The reaction mixture also contains acetic acid and
sufficient free protons to protonate the oxygen on the nitrosamine compound.
Water is then eliminated from the structure resulting in a diazonium ion, as
shown below.
2NO 2 ðgÞþH 2 OðlÞ!HNO 2 ðaqÞþH þ ðaqÞþNO 3 ðaqÞ
ð8
:
13Þ
O
OH
N
N
N
N +
NH 2
HN
N
H 2 O
ð8
:
14Þ
+ H +
+ HNO 2
O
S
O
O
SO
O
S
O
O
S
O
OH
OH
OH
OH
Diazonium ion
Sulfanilic acid
Nitrosamine
NH 2
H 2 C
NH 2
N
CH 2
H 2 C
N +
HN
CH 2
HN
(Pink)
+
ð8
:
15Þ
diazo coupling
N
O
S
O
OH
N
N -(1-naphthyl)ethylenediamine
O
SO
OH
The diazonium ions (characterized by N
N) are electrophiles, which will seek out
electron-containing species to share with. This species, known as N -(1-naphthyl)
ethylenediamine (N-NED), forms a diazo coupling ( N
N ) by a coupling
reaction at the position opposite to the amine group on the N-NED. Diazo
coupling always results in a strong colored compound. In this instance, the final
compound is pink with maximum absorption at 550 nm. Since nitrite solution is
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