Page 38 - Pharmaceutical Analytical Chemistry II - Pharm D- 02-06-07102
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Reduction half-reactions
I2 + 2e 2I− OR I3− + 2e 3I− E0 = 0.536 V
Change in oxidation state of iodine : 0 to -1X2
Eq wt = Mw / 2
Titrations involving Iodine
Iodimetric Iodometric
Direct method Indirect method
Iodimetric titrations
1- Used for analyzing strong reducing species
2- Titration of analyte with standard iodine solution (I3−)
NaHCO3/starch
H3AsO3 + I2 + H2O H3AsO4 + 2I− + 2H+
3- Titrations occur in neutral or weakly alkaline solutions by adding
NaHCO3 (Why?)
AsO33– + I2 + H2O AsO43– + 2I−+ 2H+
withdrawn by NaHCO3
In neutral solution:
- Eqm is shifted to right (ensure complete reaction),
- The potential of As(V) /As(III) couple is decreased sufficiently that
As(III) will reduce I2 ----------- ↑ reducing power of AsO33–
AsO43– + 2H+ + 2e AsO33– + H2O
E = Eo – 0.059 [AsO33–]
log
2
[AsO43–][H+]2
[H+] ↓ E↓
4- Ep detection Starch is used as ind. It is added at the beginning
of the titn. The first drop xss of I3− after the eqv point causes the
solution to turn blue
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