Page 41 - Pharmaceutical Analytical Chemistry II - Pharm D- 02-06-07102
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WHY starch is added just before the eq point
- Iodine-starch complex is ONLY slowly dissociated, and
- a diffuse e.p. would result if a large amount of iodine is adsorbed
on starch
- Most iodometric titns are performed in strongly acidic solutions
and the starch has a tendency to hydrolyze in acid solution
CHCl3 or CCl4
-suitable for highly acidic solutions
-Iodine dissolves in such solvents giving intense purple color. Ep is
marked by the disappearance of purple color in the organic solvent
layer (vigorous shaken during titn is required)
5- Applications
1- Determination of Cu2+
Cu2+ + e− Cu+
E0 Cu2+/ Cu+ = + 0.15 V E0 I2/I− = + 0.536 V
To force the reaction---oxidation of I– by Cu2+--- you have to ↑E of
Cu2+/ Cu+ system to be > 0.53 V
BY:
adding SCN− OR I− reagent to ppt Cu+ --- [Cu+] ↓ → ↑E of Cu2+/Cu+
E = Eo – 0.059 [Cu+ ]
NOTE
log [Cu2+ ]
E↑
1
[Cu+] ↓
I– has 2 functions
(a) reduce Cu2+ to Cu+
(b) precipitate Cu+ ion
2Cu2+ + 4I- ⎯→ Cu2I2 ↓ + I2
But the precipitated Cu2I2 would adsorb I2 on its surface, so we add
(NH4)SCN to precipitate the more insoluble Cu2(SCN)2 and set free
the adsorbed I2.
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