Environmental Engineering Reference
In-Depth Information
In the presence of ammonia (sometimes it is intentionally added), the following
reactions will occur to form mono-, di-, and tri-chloroamines:
NH
4
þHOCl ! NH
2
Cl þH
2
OþH
þ
ð6
:
22Þ
NH
2
Cl þHOCl ! NHCl
2
þH
2
O
ð6
:
23Þ
NHCl
2
þHOCl ! NCl
3
þH
2
O
ð6
:
24Þ
The chloroamines are called combined chlorine residuals. They are less effective in
disinfection but can last longer, which is essential for continued disinfection in water
distribution conduits.
The current EPA and APHA manuals list three titrimetric methods along with
several other instrumental methods for the measurement of residual chlorines. These
three titrimetric methods are (a) iodometric titration, (b) iodometric back titration,
and (c) DPD-FAS titration. The iodometric method is applicable to the measurement
of total chlorine residuals in natural and treated waters at concentrations greater than
0.1 mg/L. The iodometric back titration method is applicable to all types of waters but
is primarily used for wastewater. Both back titration method and DPD-FAS method
are suited to differentiating free chlorines and combined chlorines. Described below
are the chemical principles of these three wet chemical methods commonly employed
in laboratories because of their simplicities.
Iodometric Titration Free chlorine (OCl
and HOCl) and chloramines
stoichiometrically liberate iodine (I
2
) from potassium iodide (KI) at pH 4 or
lower. The I
2
is titrated with a standard reducing agent sodium thiosulfate (Na
2
S
2
O
3
)
using a starch indicator. The end point is the disappearance of the blue color.
Cl
2
þ2KI ! I
2
þ2KCl
ð6
:
25Þ
I
2
þ starch ! blue color
ð6
:
26Þ
I
2
þ2Na
2
S
2
O
3
! 2NaI þNa
2
S
4
O
6
ð6
:
27Þ
Iodometric Back Titration In this method, the liberated I
2
(Eq. 6.25) is
immediately reacted with an excess amount of Na
2
S
2
O
3
(Eq. 6.26). The remaining
Na
2
S
2
O
3
is back titrated with I
2
as the titrant (Eq. 6.28) instead of using Na
2
S
2
O
3
as
the titrant.
I
2
þ2Na
2
S
2
O
3
ðexcessÞ!2NaI þNa
2
S
4
O
6
ð6
:
28Þ
Compared with iodometric titration method, the iodometric back titration method
causes immediate reaction of the I
2
generated so that any contact between the
full concentration of liberated I
2
and the potential I
2
-reducing substance in waste-
water is minimized.
Note that molecular iodine (I
2
) is not soluble in water to any appreciable degree.
To obtain iodine in solution for titrations, the iodine is reacted with excess iodide (I
)
to form the triiodide ion (I
3
), which is very soluble and reacts as if it were molecular
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