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 863.4mg of ferric ammonium sulfate [FeNH4(SO4)2·12H2O]in water,add 10mLof 2Nsulfuric acid,and dilute with water to 100.0mL.Pipet 10mLof this solution into a 1000-mLvolumetric flask,add 10mLof 2Nsulfuric acid,dilute with water to volume,and mix.This solution contains the equivalent of 0.01mg (10µg)of iron per mL.
AMMONIUM THIOCYANATE SOLUTION Dissolve 30g of ammonium thiocyanate in water to make 100mL.
Standard Preparation Into a 50-mLcolor-comparison tube pipet 1mLof Standard Iron Solution(10µg of Fe),dilute with water to 45mL,add 2mLof hydrochloric acid,and mix.
Test Preparation Into a 50-mLcolor comparison tube place the solution prepared for the test as directed in the individual monograph and if necessary dilute with water to 45mL;or,dissolve in water,and dilute with water to 45mLthe quantity,in g,of the substance to be tested,as calculated by the formula:
1.0/(1000L),in which Lis the Ironlimit in percentage.Add 2mLof hydrochloric acid,and mix.
Procedure To each of the tubes containing the Standard Preparationand the Test Preparationadd 50mg of ammonium peroxydisulfate crystals and 3mLof Ammonium Thiocyanate Solution,and mix:the color of the solution from the Test Preparationis not darker than that of the solution from the Standard Preparation.