Page 37 - Pharmaceutical Analytical Chemistry II - Pharm D- 02-06-07102

IV- Standard Iodine solution, I3−

Properties

It is a weak oxidizing agent used for determination of strong
reductants

Standard solution

Iodine is slightly soluble in water.

It is dissolved in aqueous KI solution as a consequence of the

reaction: I3−
I2(s) + I−

(Triiodide ion is very soluble in water, brown color)

Stability

Iodine solutions lack stability (WHY)

1. Volatility of iodine, losses of iodine from an open vessel
occur in a relatively short time

2. Air oxidation of iodide ion → ↑ M of iodine solution
4I− + O2(g) + 4H+ → 2I2+ 2H2O

This reaction is promoted by acids, light, heat and nitrogen oxides.

To keep stability of standard iodine solution:
It should be stored in closed dark glass stoppered bottles, keep
from elevated temp

Standardization

1. Using primary st reducing agent, arsenious oxide As2O3

(H3AsO3)

NaHCO3

H3AsO3 + I2 + H2O H3AsO4 + 2I− + 2H+

starch

2. using previously standardized sodium thiosulfate

I2 + 2S2O32− starch + 2I−
S4O62−

(tetrathionate)

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