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340 Modern Analytical Chemistry

9D.3 Representative Method

Although every redox titrimetric method has its own unique considerations, the fol-
lowing description of the determination of total residual chlorine in water provides
an instructive example of a typical procedure. 13

Representative Methods Description of the Method. The chlorination of public water supplies results in
Method 9.3 Determination of Total Chlorine Residual

the formation of several chlorine-containing species, the combined concentration of
which is called the total chlorine residual. Chlorine may be present in a variety of
–
states including free residual chlorine, consisting of Cl 2 , HOCl, and OCl , and
combined chlorine residual, consisting of NH 2 Cl, NHCl 2 , and NCl 3 . The total chlorine
–
–
residual is determined by using the oxidizing power of chlorine to convert I to I 3 .
–
as a
2–
The amount of I 3 formed is then determined by a redox titration using S 2 O 3
titrant and starch as an indicator. Regardless of its form, the total chlorine residual is
million of Cl.
The photo in Colorplate 8c shows the calculated as if all the chlorine were available as Cl 2 , and is reported as parts per
2–
indicator’s color change for this titration. Procedure. Select a volume of sample requiring less than 20 mL of S 2 O 3 to reach
the end point. Using glacial acetic acid, acidify the sample to a pH in the range of 3
–
to 4, and add about 1 g of KI. Titrate with Na 2 S 2 O 3 until the yellow color due to I 3
begins to disappear. Add 1 mL of a starch indicator solution, and continue titrating
–
until the blue color of the starch–I 3 complex disappears. The volume of titrant
needed to reach the end point should be corrected for reagent impurities by
conducting a blank titration.
Questions
1. Is this an example of a direct or an indirect analysis?
This is an indirect method of analysis because the chlorine-containing species
do not react with the titrant. Instead the total chlorine residual oxidizes
–
–
–
I to I 3 , and the amount of I 3 is determined by the redox titration with
Na 2 S 2 O 3 .
2. Why is the procedure not carried out directly using KI as a titrant?
–
The redox half-reaction when I is used as a titrant is
–
–
3I t I 3 +2e –
–
Because the product of the reaction, I 3 , is itself colored, the color of the
solution containing the analyte changes with each addition of titrant. For this
reason it is difficult to find a suitable visual indicator for the titration’s end
point.
3. Both oxidizing and reducing agents can interfere with this analysis. Explain
what effect each of these interferents will have on the result of an analysis.
–
–
An interferent that is an oxidizing agent will convert additional I to I 3 . This
– 2–
extra I 3 requires an additional volume of S 2 O 3 to reach the end point,
overestimating the total chlorine residual. If the interferent is a reducing agent,
it reduces some of the I 3 , produced by the reaction between the total chlorine
–
–
residual and iodide, back to I . The result is an underestimation of the total
chlorine residual.