Page 42 - Pharmaceutical Analytical Chemistry II - Pharm D- 02-06-07102

Cu2I2 + 2SCN– ⎯→ Cu2 (SCN)2 + 2I–

2- Determination of oxidizing agents

2MnO4− + 10I− + 16H+ 2Mn2+ + 5I2 + 8H2O

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

Cl2 + 2I− I2 + 2Cl−

BrO3− + 6I− + 6H+ Br– + 3I2 + 3H2O

IO3− + 5I− + 6H+ 3I2 + 3H2O

2Fe3+ + 2I− 2Fe2+ + I2

H3AsO4 + 2I− + 2H+ H3AsO3 + I2 + H2O

Titrate the liberated I2 ≠ st Na2S2O3 , starch /CCl4 ind

2S2O32− + I2 S4O62− + 2I−

Question: Why titrations involving iodine doesn’t occur in
strong alkaline medium?
If pH is too alkaline I2 will disproportionate to hypoiodide and
iodide

I2 + 2OH− IO− + I− + H2O

OI– is a stronger oxidant than I2

6- Sources of error in Iodometry

1- Decomposition of thiosulfate solution
2- Premature addition of starch and its decomposition
3- Due to I2

Lack of stability (volatility, air oxidation of iodide)

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