Title: Limit Test for Iron
- Objective: The limit test for iron is provided to demonstrate that the content of iron does not exceed the limit given in the individual monograph in terms of micrograms of iron per gram of the test substance. The standard solution against which the comparison of colour is made contains 40 μg of iron.
- Principle: Limit test of Iron is based on the reaction of iron in ammonical solution with thioglycollic acid to form iron thioglycolate which is pink-reddish purple in colour.
3. Procedure:
- Preparation of Standard Solutions:
Note: All solutions should be prepared in Distilled/Purified Water.
- Ferric Ammonium Sulphate (0.1726%w/v): Dissolve 0.1726 g of Ferric Ammonium Sulphate in 05M Sulphuric acid and dilute to 100 ml with 0.05M Sulphuric acid.
- Iron Standard Solution (20 ppm Fe): Dilute 1 volume of Ferric Ammonium Sulphate (0.1726%w/v) to 10 volumes with water immediately before use.
- 05M Sulphuric acid: Add 2.7 ml of Sulphuric acid carefully with equal volume of water, cool and dilute to 1000 ml with water.
- Citric acid Solution (20% w/v): Dissolve 20.0 g of Citric acid in water and make up to 100 ml with water.
- Method:
Dissolve the specified quantity of the substance under examination in water, or prepare a solution as directed in the monograph, and transfer to a Nessler Cylinder. Add 2 ml of a 20% w/v solution of iron-free citric acid and 0.1 ml of Thioglycollic acid, mix make alkaline with iron-free ammonia solution, dilute to 50 ml with water and and allow to stand for 5 minutes.
Limit: Any colour produced is not more intense than that obtained by treating in the same manner, using 2.0 ml of Iron Standard Solution (20 ppm Fe) in place of the solution under examination.