Page 40 - Pharmaceutical Analytical Chemistry II - Pharm D- 02-06-07102

Iodometric titrations
1- Used for analyzing oxidizing species

2- Titration of I2 produced by analyte ≠ st sodium thiosulphate
Add xss I− to a solution of analyte. I2 is produced in an amount
equivalent to the oxidizing agent. Liberated I2 is tit ≠ st Na2S2O3

Cr2O72− + 6I− (xss) + 14H+ 2Cr3+ + 3I2 + 7H2O

I2 + 2S2O32− 2I− + S4O62− (tetrathionate)

Cr2O72− ≡ 3I2 ≡ 6S2O32−

Na2S2O3 is a universal titrant for I2 in neutral or acidic solutions.
3- Titrations occur in acidic solutions (Why?)

- ↑E of oxidizing agent couple

MnO4− / Mn2+ Mn2+ + 4H2O
AsO43– / AsO33–
MnO4− + 8H+ + 5e

H3AsO4 + 2H+ + 2e H3AsO3 + H2O

E = Eo – 0.059 [Mn2+]
5
log
[MnO4–][H+]8

[H+] ↑ E↑

Acidity promotes oxidizing agent-iodide reaction

- eqm is shifted to right (ensure complete reaction)

4- Ep detection

Starch is used as ind. Starch should not be added at the beginning
of titn. Instead it is added just before the eqv point (fading of I3−

color, solution is straw yellow). The e.p is indicated by the

disappearance of blue starch-iodine color.

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