Page 45 - Pharmaceutical Analytical Chemistry II - Pharm D- 02-06-07102

b) for I2 content
Direct titration by S2O32− , starch Ind (iodimetry)

c) I- is determined by difference

Lange´s modification

Lange's method involves Andrew's method principles except with
the following modifications,

(a) using 1M HCl in titration instead of 5M HCl
(b) using KCN (CN–) to stabilize I+ instead of Cl– and,
(c) using starch to detect the end point instead of CHCl3

[1] Determination of I–

1M HCl, KCN
IO3– + 2I– + 3CN– + 6H+ ⎯⎯⎯⎯⎯⎯→ 3ICN + 3H2O

Starch

[2] Determination of I2 / I- mixture

a) for total content:
Direct titration by st. IO3− , 1M HCl, starch Ind.

IO3− + 2I2 + 5CN− + 6H+ 5ICN + 3H2O
Brown Yellow

IO3– + 2I– + 3CN– + 6H+ ⎯⎯⎯→ 3ICN + 3H2O

b) for I2 content
Direct titration by S2O32− , starch Ind (iodimetry)

c) I- is determined by difference

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