Limit Test For Iron as per USP in pharma

Principle of Limit test for Iron:

The Limit test for Iron is based on the reaction of iron in ammoniacal solution with thioglycolic acid in the presence of citric acid to form a iron thioglycolate which produces pale pink to deep reddish purple color.

Limit Test For Iron As Per USP:

Reagents used for limit test for iron:

2N Sulfuric Acid: Add slowly with stirring 60ml of sulfuric acid to 1020ml of water. Allow cooling to 25°C.

Ammonium Thiocyanate Solution: Dissolve 30gm of Ammonium Thiocyanate in 100ml of water.

Preparation of standard Iron solution:

  1. Dissolve 863.4mg of ferric ammonium sulfate in water.
  2. Add 10ml of 2N sulfuric acid.
  3. Dilute to volume 100ml with water.
  4. Pipette 10ml of this solution into a 1000ml of volumetric flask.
  5. Add 10ml of 2N sulfuric acid.
  6. Dilute to volume 1000ml with water and mix well.

Preparation of sample Iron solution:

Use the below-given formula to take quantity of substance to be tested.

1.0 / (1000L)

where L is the Iron limit in percentage.

Test Procedure for iron test:

Standard Solution: Into a 50ml color comparison tube, pipette 1 ml of standard iron solution, dilute with water to 45ml, add 2ml of hydrochloric acid, and mix.

Sample Solution: Into a 50ml color comparison tube, add sample solution prepared as per individual specification. Dilute with water to 45ml.

To each of the tubes containing standard solution and sample solution, add 50mg of ammonium peroxydisulfate crystals and 3ml of Ammonium Thiocyanate solution and mix.


The color of the solution from the sample preparation is not darker than that of the standard solution.

Limit Test For Iron
Limit Test For Iron

Also Read:

Refer YT Channel: Pharmabeejpro