Environmental Engineering Reference
In-Depth Information
distillate is then titrated with H
2
SO
4
in the presence of a mixed indicator (methyl
red and methylene blue). The sample is then titrated until it turns pale lavender
(Eq. 6.36).
NH
3
þH
3
BO
3
! NH
4
þH
2
BO
3
ð6
:
35Þ
2NH
4
þ2H
2
BO
3
þH
2
SO
4
! 2H
3
BO
3
þðNH
4
Þ
2
SO
4
ð6
:
36Þ
EXAMPLE 6.2.
Reaction stoichiometry. The Standard Method uses standard 0.02 N
H
2
SO
4
solution to titrate ammonia after distillation of water samples. The calculation formula
was provided as:
mg NH
3
N=L ¼
ð
A
B
Þ
280
mL Sample
ð6:37Þ
where A is the volume of H
2
SO
4
titrated for sample (mL), and B the volume of H
2
SO
4
titrated for blank (mL). Verify this formula using stoichiometry involved in distillation and
titration.
SOLUTION:
Combine reactions Eqs 6.35 and 6.36 by eliminating the reaction intermediate,
NH
4
H
2
BO
3
. This results in a balanced overall reaction as follows:
2NH
3
þH
2
SO
4
!ðNH
4
Þ
2
SO
4
ð6:38Þ
The above reaction indicates that 2 mol NH
3
will stoichiometrically react with 1 mol H
2
SO
4
.
Note that 0.02 N H
2
SO
4
is equal to 0.01 M (mol/L). We then have:
0:01
mol H
2
SO
4
2 mol NH
3
1 mol H
2
SO
4
14
;
000 mg NH
3
N
1 mol NH
3
L
ðABÞ mL H
2
SO
4
ml Sample
¼
ð
A
B
Þ
280
1
mg NH
3
N
L
mL Sample
In the above calculation, NH
3
-N denotes ''ammonia as nitrogen,'' hence 1 mol
NH
3
contains 14 g N or 14,000 mg N. This is a common practice in reporting as
''N basis'' or ''P basis'' for the concentration of anions such as NO
3
,NO
2
and
PO
3
4
. In addition, the titrant volume (B) for the blank sample must be subtracted
to minimize the error.
6.3.8 Cyanide in Water, Wastewater and Soil Extract
Cyanide refers to all of the cyanide groups in cyanide compounds that can be
determined as the cyanide ion (CN
), which is the conjugate base of the more toxic
HCN gas. Cyanide is often present in the form of a metal complex, which is stable
and less toxic. Organically bound cyanides, such as acrylonitriles (cyanoethene) are
important industrial chemicals. Cyanides are used in plastics, electroplating,
metallurgy, synthetic fibers, and chemicals. The titrimetric method is applicable if
cyanide concentration is above 1 mg/L in the aqueous sample or the distillate from
wastewater, soil or waste extract.
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