Redox Titrations

General applications of redox titrimetry

environment

public health

and industrial analyses

- chlorination of public water supplies.

Chlorination of public water supplies

Efficiency of chlorination depends on the form of the chlorinating species.

free chlorine residual, due to Cl2, HOCl, and OCl–

combined chlorine residual, formed by NH2Cl, NHCl2, and NCl3.

Total chlorine residual:

the oxidizing power of chlorine to convert I– to I3–

I3– formed is then determined by a redox titration using S2O32– as a titrant and starch as an indicator.

Chlorination of public water supplies

Procedure

sample volume: less than 20 mL of S2O32– to reach the end point.

pH 3 to 4 (glacial acetic acid)

add about 1 g of KI

titrate with Na2S2O3 until the yellow color due to I3– begins to disappear.

add 1 mL of a starch indicator solution,

continue titration until the blue color of the starch–I3– complex disappears.

Indirect analysis method

The redox half-reaction when I– is used as a titrant is

3I– = I3– + 2e–

Reaction product, I3–, is itself colored, the color of the solution containing the analyte changes with each addition of titrant. So it is difficult to find a suitable visual indicator for the titration’s end point.