AS PER IP

Requirements 

Nesselar’s cylinder, 

Measuring cylinder, 

Glass rod, 

Citric acid, 

Thioglycolic acid, 

Ammonia Solution, 

Standard Iron solution

Solution Preparation

Iron Standard Solution (20 ppm) 

Dilute 1 ml of a 0.1726 percent w/v solution of ferric ammonium sulfate in 0.05 M sulphuric acid to 10 ml with water. Contains iron in a ferric state.

0.05M Sulphuric acid 

Add slowly, with stirring, 30 ml of sulphuric acid to about 1000 ml of water, and allow to cool 25°.

Principle 

The limit test of Iron is based on the reaction of iron in an ammonical solution with thioglycolic acid in the presence of citric acid to form iron thioglycolate and is pale pink to deep reddish purple in color. 

Sample

Dissolve the specified quantity of the substance under examination in water, or prepare a solution as directed in the

monograph, and transfer it to a Nessler cylinder. Add 2 ml of a 20 percent w/v solution of iron-free citric acid and 0.1 ml of thioglycolic acid, mix, make alkaline with iron-free ammonia solution, dilute to 50 ml with water, and allow to stand for 5 minutes. 

Standard 

Place 2.0 ml of iron standard solution (20 ppm Fe) and transfer to a Nessler cylinder. Add 2 ml of a 20 percent w/v solution of iron-free citric acid and 0.1 ml of thioglycolic acid, mix, make alkaline with iron-free ammonia solution, dilute to 50 ml with water, and allow to stand for 5 minutes. 

Observation:  

The purple color produced in the sample solution should not be greater than the standard solution. If the purple color produced in the sample solution is less than the standard solution, the sample will pass the limit test of iron and vice versa.  

Note: 

1. Citric acid helps the precipitation of iron by ammonia by forming a complex with it.  

2. Thioglycolic acid helps to oxidize iron (II) to iron (III).  

3. Ammonia to make the solution alkaline.  

AS PER BP

Iron Standard Solution (1ppm):

Immediately before use, dilute iron standard solution (20 ppm Fe) R to 20 times its volume with water R. 

Iron Standard Solution (20 ppm Fe):

Immediately before use, dilute with water R to 10 times its volume a solution containing ferric ammonium sulfate R equivalent to 0.863 g of  FeNH4(SO4)2,12H2O and 25 mL of dilute sulfuric acid R in  500.0 mL. 

Sample

Dissolve the prescribed quantity of the substance to be examined in water R and dilute to 10 mL with the same solvent or use 10 mL of the prescribed solution in the Nessler cylinder. Add 2  mL of a 200 g/L solution of citric acid R and 0.1 mL of thioglycolic acid R. Mix, make alkaline with ammonia R, and dilute to 20 mL with water R.

Standard 

Use 10 mL of iron standard solution (1 ppm Fe) to Nessler cylinder,   Add 2  mL of a 200 g/L solution of citric acid R and 0.1 mL of thioglycolic acid R. Mix, make alkaline with ammonia R, and dilute to 20 mL with water R. 

Observation:  

After 5 min, any pink colour in the test solution is not more intense than that in the standard. 

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As per USP

This limit test is provided to demonstrate that the content of iron, in either the ferric or the ferrous form, does not exceed the limit for iron specified in the individual monograph. The determination is made by concomitant visual comparison with a control prepared from a standard iron solution.

STANDARD IRON SOLUTION

Dissolve 863.4 mg of ferric ammonium sulfate [FeNH4(SO4)2·12H2O] in water, add 10 ml of 2N sulfuric acid, and dilute with water to 100.0 mL Pipet 10 mL of this solution into a 1000-mL volumetric flask, add 10 mL of 2N sulfuric acid, dilute with water to volume, and mix. This solution contains the equivalent of 0.01 mg(10 µg) of iron per mL.

AMMONIUM THIOCYANATE SOLUTION

Dissolve 30g of ammonium thiocyanate in water to make 100 mL.

Test Preparation

Into a 50-mL color comparison tube place the solution prepared for the test as directed in the individual monograph and if necessary dilute with water to 45 mL; or, dissolve in water, and dilute with water to 45 mL the quantity, in g, of the substance to be tested, as calculated by the formula:

1.0/(1000L)

in which L is the Iron limit in percentage. Add 2 mL of Cool, dilute cautiously with a few mL of water, and rinse hydrochloric acid and mix.

Procedure

To each of the tubes containing the Standard Preparation and the Test Preparation add 50 mg of ammonium peroxydisulfate (Ammonium Persulfate )crystals and 3 mL of Ammonium Thiocyanate Solution, and mix: 

Interpretation 

the color of the solution from the Test Preparation is not darker than that of the solution from the Standard Preparation.