Limit Test For Iron

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Table of Contents

Aim:

To perform the limit test for iron for the given sample.

Requirements: Nessler cylinder, 2ml bulb pipette, 20% w/v iron-free citric acid, thioglycolic acid, ammonia solution, iron standard solution (20 ppm Fe), glass rods, red-litmus paper.

Principle:

In this experiment, the test color (purple) obtained by the reaction of iron impurities with mercaptoacetic acid (thioglycolic acid) is compared with the standard color obtained by the reaction of a known quantity of
iron with mercaptoacetic acid.
Citric acid (iron-free) is used for complex metal cations other than iron if any present.

In addition to forming a complex with Fe2+, thioglycolic acid acts as a reducing agent and converts Fe3+ to Fe2+ if any present. The ferrous mercapto acetate formed gives purple color in presence of citric acid. In addition to the formation of a metal complex other than iron, citric acid forms ammonium citrate buffer when ammonia is added to make alkaline, which in turn stabilizes the complex formed.

Procedure:

Test color: Dissolve the given sample in 20 ml of water and transfer to a Nessler cylinder. Add 2 ml of a 20% w/v solution of iron-free citric acid and 0.1 ml of thioglycolic acid, mix, make alkaline with iron-free ammonia
the solution, dilute to 50 ml with water and allow to stand for 5 minutes and observe the color transversely

Standard color: Transfer 2.0 ml of iron standard solution (20 ppm Fe) to a Nessler cylinder. Dilute with 20 ml water. Add 2 ml of a 20% 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, and observe the color transversely.

Observation:

Test color is not more intense than standard color.
or
Test color is more intense than standard color.

Report/Result:

The given sample passes the limit test for iron.
or
The given sample fails the limit test for iron

Preparation of Reagents:

1. 0.05 M sulphuric acid: Solutions of any molarity xM may be prepared by carefully adding 54x ml of sulphuric acid to an equal volume of water and diluting to 1000 ml with water.
2. 20% w/v iron-free citric acid: Dissolve 20 g of iron-free citric acid in 100 ml water.
3. Iron-free ammonia solution: Contains approximately 10% w/w of NH3 (iron-free). Dilute 425 ml of strong ammonia solution to 1000 ml.
4. Iron standard solution (20 ppm Fe): Dilute 1 volume of a 0.1726% w/v solution of ferric ammonium sulfate in 0.05 M sulphuric acid to 10 volumes with water. Contains iron in the ferric state.

References

http://www.bspublications.net/downloads/059cc8f84560f2_Ch-1_Subba%20Rao_Practical%20Pharmaceutical%20In-organic%20Chemistry.pdf
https://www.pharmaguideline.com/2011/07/limit-tests.html
http://rambabu-pharmaceutical-qa.blogspot.com/
https://www.bspublications.net/downloads/059cc8f84560f2_Ch-1_Subba%20Rao_Practical%20Pharmaceutical%20In-organic%20Chemistry.pdf