Page 361 - Modern Analytical Chemistry

1400-CH09 9/9/99 2:13 PM Page 344

344 Modern Analytical Chemistry

Instead, an excess of KI is added, reducing the analyte and liberating a stoichiomet-
–
–
ric amount of I 3 . The amount of I 3 produced is then determined by a back titra-
tion using Na 2 S 2 O 3 as a reducing titrant.
2–
2–
2S 2 O 3 (aq) t S 4 O 6 (aq)+2e –
Solutions of Na 2 S 2 O 3 are prepared from the pentahydrate and must be stan-
dardized before use. Standardization is accomplished by dissolving a carefully
weighed portion of the primary standard KIO 3 in an acidic solution containing an
–
excess of KI. When acidified, the reaction between IO 3 and I –
–
–
+
–
IO 3 (aq)+8I (aq)+6H 3 O (aq) t 3I 3 (aq)+9H 2 O(l)
–
–
liberates a stoichiometric amount of I 3 . Titrating I 3 using starch as a visual indica-
tor allows the determination of the titrant’s concentration.
Although thiosulfate is one of the few reducing titrants not readily oxidized
by contact with air, it is subject to a slow decomposition to bisulfite and ele-
mental sulfur. When used over a period of several weeks, a solution of thiosul-
fate should be restandardized periodically. Several forms of bacteria are able to
metabolize thiosulfate, which also can lead to a change in its concentration.
This problem can be minimized by adding a preservative such as HgI 2 to the
solution.
6H
Another reducing titrant is ferrous ammonium sulfate, Fe(NH 4 ) 2 (SO 4 ) 2× 2 O,
in which iron is present in the +2 oxidation state. Solutions of Fe 2+ are normally
very susceptible to air oxidation, but when prepared in 0.5 M H 2 SO 4 the solution
may remain stable for as long as a month. Periodic restandardization with K 2 Cr 2 O 7
is advisable. The titrant can be used in either a direct titration in which the Fe 2+ is
3+
oxidized to Fe , or an excess of the solution can be added and the quantity of Fe 3+
produced determined by a back titration using a standard solution of Ce 4+ or
2–
Cr 2 O 7 .
Inorganic Analysis Redox titrimetry has been used for the analysis of a wide range
of inorganic analytes. Although many of these methods have been replaced by
newer methods, a few continue to be listed as standard methods of analysis. In this
section we consider the application of redox titrimetry to several important envi-
ronmental, public health, and industrial analyses. Other examples can be found in
the suggested readings listed at the end of this chapter.
One of the most important applications of redox titrimetry is in evaluating the
chlorination of public water supplies. In Method 9.3 an approach for determining
the total chlorine residual was described in which the oxidizing power of chlorine is
–
–
–
used to oxidize I to I 3 . The amount of I 3 formed is determined by a back titration
2–
with S 2 O 3 .
The efficiency of chlorination depends on the form of the chlorinating
species. For this reason it is important to distinguish between the free chlorine
–
residual, due to Cl 2 , HOCl, and OCl , and the combined chlorine residual. The
latter form of chlorine results from the reaction of ammonia with the free chlo-
rine residual, forming NH 2 Cl, NHCl 2 , and NCl 3 . When a sample of iodide-free
chlorinated water is mixed with an excess of the indicator N,N-diethyl-p-
phenylenediamine (DPD), the free chlorine oxidizes a stoichiometric portion of
DPD to its red-colored form. The oxidized DPD is then titrated back to its color-
less form with ferrous ammonium sulfate, with the volume of titrant being pro-
portional to the amount of free residual chlorine. Adding a small amount of KI
–
–
reduces monochloramine, NH 2 Cl, forming I 3 . The I 3 then oxidizes a portion of