Learn the limit test for iron through a laboratory experiment or practical.
Aim:
To perform the limit test for iron of the given samples as per IP 2010.
Sample: Sodium chloride (NaCl)
Reference: Indian Pharmacopoeia (IP) 2010
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Requirements:
Glasswares and miscellaneous: Nessler cylinder, measuring cylinder, pipette, dropper, rubber stopper, test tube stand, pH paper, balance, and glass rod, etc.
Chemicals: Ferric ammonium sulphate (NH4Fe (SO4)2), sulphuric acid (H2SO4), citric acid (C6H8O7), thioglycolic acid (C2H4O2S), and ammonia solution (NH4OH).
Principle of limit test for iron:
The reaction between ferrous ions (Fe2+) and thioglycolic acid in the presence of citric acid is the basis for the limit test. By reacting with the same thioglycolic acid, iron in the ferric state (Fe3+) is converted to a ferrous state (Fe2+).
Fe2+ reacts with C2H4O2S to form a coordination complex called ferrous-thioglycolate. The formation of ferrothioglycolate ion gives this complex a deep reddish color under basic conditions.
In the presence of NH4OH, C6H8O7 forms a soluble complex with iron, preventing the precipitation of iron as Fe(OH)2 or Fe(OH)3.
Chemical
reaction involved in limit test for iron:
Preparation of chemicals:
How to prepare 20 ppm iron standard solution:
Take 1 volume of 0.1726 % w/v ferric ammonium sulphate solution in 0.05 M H2SO4 and dilute to 100 volume of distilled water in a volumetric flask, and properly mixing it. (Contains iron in the ferric state)
How to prepare 20 % w/v iron-free citric acid solution:
Take 20.00 gm of citric acid and dissolve in 50 ml of distilled water in a volumetric flask, and properly mixing it. Once it has completely dissolved, make up the volume to 100 ml.
Procedure of limit test for iron:
Standard:
- Take 02.00 ml of prepared 20 ppm iron standard solution using a pipette into a Nessler cylinder and label it as standard.
- Add 02.00 ml of prepared 20 % w/v iron-free citric acid solution.
- Then, add 0.10 ml of thioglycolic acid, and properly mixed it.
- Make it alkaline using an iron-free ammonia solution.
- Add sufficient water to make 50.00 ml and allow standing for five minutes.
Test:
- Take 02.00 gm of sodium chloride into a Nessler cylinder containing 20.00 ml of water and label it as a test.
- Add 02.00 ml of prepared iron-free citric acid solution (20 % w/v).
- Then, add 0.10 ml of thioglycolic acid, and properly mixed it.
- Make it alkaline using an iron-free ammonia solution.
- Add sufficient water to make 50.00 ml and allow standing for five minutes.
- Observe this test preparation and compare the opalescence of this solution to that of a standard solution.
Observations:
The sodium chloride test solution produces less or same or more opalescence than the standard solution.
Results:
The sample passes/fails the limit test as per Indian Pharmacopoeia.
FAQ (Frequently Asked Questions):
What is the importance of the iron limit test?
The purpose of a limit test is to find impurities in a substance and compare them to a standard. In general, it is a quantitative or semi-quantitative chemical test.
What is the role of citric acid in the limit test for iron?
Citric acid helps in the precipitation of iron by ammonia by forming a complex with it, which is why it is used in the limit test for iron.
Why thioglycolic acid is used in a limit test of iron?
Thioglycolic acid helps to oxidize iron-II to iron III, which is why it is used in the limit test for iron.
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