JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD Pharm. D I Year PHARMACEUTICAL INORGANIC CHEMISTRY Previous Question Paper (June/July - 2019 - R08)

The limit test for iron is based on the reaction of iron present as an impurity in the sample with thioglycollic acid in the presence of citric acid and ammonia. This reaction produces a purple-colored complex, ferrous thioglycollate, which is compared against a standard iron solution.

Principle: Iron reacts with thioglycollic acid in the presence of ammonia to form a soluble, purple-colored coordination complex, ferrous thioglycollate. Citric acid is added to prevent the precipitation of iron by forming a stable complex with it in an alkaline medium.

Procedure:

1. Test Solution: Dissolve the specified quantity of the substance in 40 mL of water. Add 2 mL of 20% w/v citric acid and 0.1 mL of thioglycollic acid. Make the solution alkaline with ammonia and dilute to 50 mL with water.
2. Standard Solution: Prepare a standard by taking 2 mL of standard iron solution (20 ppm Fe) in a Nessler cylinder. Add 40 mL of water, 2 mL of 20% w/v citric acid, and 0.1 mL of thioglycollic acid. Make the solution alkaline with ammonia and dilute to 50 mL with water.
3. Observation: Allow the solutions to stand for 5 minutes. Compare the intensity of the purple color produced in the test solution with that of the standard solution.

If the color produced in the test solution is less intense than the standard, the sample passes the limit test for iron. If it is more intense, the sample fails.