Page 37 - Pharmaceutical Analytical Chemistry II - Pharm D Clinical- 07-PA202

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withdrawn by NaHCO3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

36