Page 462 - Applied Pharmacognosy
P. 462
• At the end – point when no water is available, the brown colour of the
reagent persists.
• The basic reaction is a reduction of iodine by sulphur dioxide in
the presence of water (contained in the sample being examined).
•
• I2 + SO2 + H2O ↔ 2HI + SO3
•
• The reaction goes to completion by the removal of SO3 as
pyridine sulphur trioxide.
•
• Pyridine sulphur trioxide reacts with methanol to form the
pyridine salt of methyl sulphate.
