Environmental Engineering Reference
In-Depth Information
First, hydrogen cyanide (HCN) is liberated from an acidified sample by
distillation and purging with air through a scrubbing solution containing NaOH.
Cyanide concentration in the scrubbing solution is then titrated with standard silver
nitrate (AgNO
3
) to form the soluble cyanide complex, Ag(CN)
2
. As soon as all
CN
has been complexed and a small excess of Ag
þ
has been added, the excess Ag
þ
combines with a silver-sensitive rhodanine indicator (p-dimethyl-aminobenzal-
rhodanine, C
12
H
12
N
2
OS
2
), which immediately turns solution from yellow to
brownish-pink. The titration is based on the following reaction:
2CN
þAg
þ
!½AgðCNÞ
2
ð6
:
39Þ
The concentration of CN
in
g/L in the original sample can be calculated as
follows. Again, as illustrated earlier in this chapter, the reader should be aware of the
operational principles and the stoichiometry behind this formula.
CN
m
m
2 mol CN
1eq
02 g CN
1 mol CN
110
6
g
L
¼
ð
A
B
C
D
E
F
26
:
m
g
AgNO
3
:
1g
ð6
:
40Þ
where A is the mL AgNO
3
for titration of sample, B the mL of AgNO
3
for titration of
blank, C the mL of sample titrated, D the actual normality of AgNO
3
, E the mL of
sample after distillation, F the mL of original sample used for distillation.
6.3.9 Sulfide in Water and Waste
Sulfide is often present in groundwater due to the prevalent reducing conditions.
In wastes and wastewaters, sulfide comes partly from the anaerobic decomposition
of organic matter, and mostly from the bacterial reduction of sulfate (SO
2
4
). The
term ''sulfide'' can include hydrogen sulfide (H
2
S, dissolved and unionized),
bisulfide (HS
) and sulfide (S
2
) - all in dissolved forms. At an environmentally
relevant pH, S
2
is often negligible. In addition, ''sulfide'' is also present as metallic
sulfides - some of them are ''acid-soluble'' (ZnS) and others are ''acid-insoluble''
such as CuS and SnS
2
. These metallic sulfides have very low water solubility, and
therefore are the precipitates associated with suspended matter in water, soils or
sediment.
Like several other parameters discussed previously, sulfide can also be
determined by titration-based iodometric method. Recall also from Chapter 4 that
sulfide samples are preserved by the addition of zinc acetate and NaOH to maintain
pH
9 (Table 4.1). The zinc sulfide precipitate is then oxidized to sulfur with the
addition of excess iodine (I
2
) in acidic solution. Then the remaining iodine is
determined by titration with a standard solution of sodium thiosulfate until the blue
iodine-starch complex disappears.
This titrimetric method is applicable to all aqueous, solid waste materials and
effluents. The specific procedures may differ slightly depending on the matrices and
the forms of sulfide species to be measured. Distillation is not needed for
clean aqueous samples, but in most cases, it is required. Sulfide is separated
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