Page 39 - Pharmaceutical Analytical Chemistry II - Pharm D Clinical- 07-PA202
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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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