| This limit test is provided to demonstrate
that the content of iron, in either the ferric or the ferrous forms, does
not exceed the limit for iron specified in the individual monograph. The
determination is made by concomitant visual comparíson with a control
prepared from a standard iron solution.
Special Reagents:
STANDARD IRON SOLUTION- Dissolve
863.4 mg of ferric ammonium sulfate [FeNH4(S04)2.12H20]
in water, add 10 mL of 2 N sulfuric acid, and dilute with water to 100.0
mL. Pipet 10 mL 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 mg)
of iron per mL
AMMONIUM THIOCYANATE SOLUTION- Dissolve
30 g of ammonium thiocyanate in water to make 100 mL.
Standard Preparation: Into
a 50-mL color-comparison tube pipet 1 mL of Standard Iron Solution (10
mg of Fe), dilute
with water to 45 ml, add 2 mL of hydrochloric acid, and mix.
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 to 45 mL the quantity 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 hydrochloric acid and mix.
Procedure:To each of the
tubes containing the Standard Preparation and the Test Preparation add
50 mg of ammonium peroxydisulfate crystals and 3 mL of Ammonium Thiocyanate
Solution, and mix: the color of the solution from the Test Preparation
is not darker than that of the solution from the Standard Preparation. |
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