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Chapter 9 Titrimetric Methods of Analysis 345

the DPD to its red-colored form. Titrating the oxidized DPD with ferrous am-
monium sulfate yields the amount of NH 2 Cl in the sample. The amount of
dichloramine and trichloramine are determined in a similar fashion.
The methods described earlier for determining the total, free, or combined
chlorine residual also are used in establishing the chlorine demand of a water sup-
ply. The chlorine demand is defined as the quantity of chlorine that must be
added to a water supply to completely react with any substance that can be oxi-
dized by chlorine while also maintaining the desired chlorine residual. It is deter-
mined by adding progressively greater amounts of chlorine to a set of samples
drawn from the water supply and determining the total, free, or combined chlo-
rine residual.
Another important example of redox titrimetry that finds applications in both
public health and environmental analyses is the determination of dissolved oxygen.
In natural waters the level of dissolved O 2 is important for two reasons: it is the
most readily available oxidant for the biological oxidation of inorganic and organic
pollutants; and it is necessary for the support of aquatic life. In wastewater treat-
ment plants, the control of dissolved O 2 is essential for the aerobic oxidation of
waste materials. If the level of dissolved O 2 falls below a critical value, aerobic bacte-
ria are replaced by anaerobic bacteria, and the oxidation of organic waste produces
undesirable gases such as CH 4 and H 2 S.
One standard method for determining the dissolved O 2 content of natural wa-
ters and wastewaters is the Winkler method. A sample of water is collected in a fash-
ion that prevents its exposure to the atmosphere (which might change the level of
dissolved O 2 ). The sample is then treated with a solution of MnSO 4 , and then with a
solution of NaOH and KI. Under these alkaline conditions Mn 2+ is oxidized to
MnO 2 by the dissolved oxygen.
2+
–
2Mn (aq) + 4OH (aq)+O 2 (aq) ® 2MnO 2 (s)+2H 2 O(l)
After the reaction is complete, the solution is acidified with H 2 SO 4 . Under the now
–
–
acidic conditions I is oxidized to I 3 by MnO 2 .
+
–
–
2+
MnO 2 (s)+3I (aq)+4H 3 O (aq) ® Mn (aq)+I 3 (aq)+6H 2 O(l)
–
2–
The amount of I 3 formed is determined by titrating with S 2 O 3 using starch as an
indicator. The Winkler method is subject to a variety of interferences, and several
–
modifications to the original procedure have been proposed. For example, NO 2 in-
–
–
terferes because it can reduce I 3 to I under acidic conditions. This interference is
–
eliminated by adding sodium azide, NaN 3 , reducing NO 2 to N 2 . Other reducing
2+
agents, such as Fe , are eliminated by pretreating the sample with KMnO 4 , and de-
stroying the excess permanganate with K 2 C 2 O 4 .
Another important example of a redox titration for inorganic analytes, which is
important in industrial labs, is the determination of water in nonaqueous solvents.
The titrant for this analysis is known as the Karl Fischer reagent and consists of a
mixture of iodine, sulfur dioxide, pyridine, and methanol. The concentration of
pyridine is sufficiently large so that I 2 and SO 2 are complexed with the pyridine (py)
I
as py × 2 and py × 2 . When added to a sample containing water, I 2 is reduced to
SO
–
I , and SO 2 is oxidized to SO 3 .
SO
HI+py
SO
py × 2 +py × 2 +py+H 2 O ® 2py × × 3
I
Methanol is included to prevent the further reaction of py × 3 with water. The
SO
titration’s end point is signaled when the solution changes from the yellow color of
the products to the brown color of the Karl Fischer reagent.