Learn the modified limit test of chloride through a laboratory experiment or practical.
Aim:
To perform the modified limit test for chloride of the given samples as per IP 2010
Sample: Potassium permanganate (KMnO4).
Reference: Indian Pharmacopoeia (IP) 2010.
Requirements:
Glasswares and miscellaneous: Nessler cylinder, measuring cylinder, pipette, dropper, rubber stopper, test tube stand, glass rod, and balance.
Chemicals: Potassium permanganate (KMnO4), sodium chloride (NaCl), silver nitrate solution (AgNO3), nitric acid solution (HNO3),
Principle of modified limit test for chloride:
Before performing the limit test, the KMnO4 is de-colored since the purple color of potassium permanganate interferes with the limit test. The AgNO3 precipitate turns pink as it absorbs some of the permanganates.
This coloration hinders the identification of impurities in the test solution by comparing the opalescence which is formed in the standard solution of chloride. Potassium permanganate is reduced by heating with ethyl alcohol to decolorize it.
Filtration removes the precipitate, and the resulting colorless filtrate is obtained subject to a chloride limit test.
Preparation of chemicals:
How to prepare potassium permanganate test solution:
Take 01.50 gm of KMnO4 and dissolve in 50 ml of distilled water in a volumetric flask, and properly mixing it. Heat on a water bath and gradually add 06.00 ml of 95 % ethanol. Once cooling dilute to 60 ml by distilled water and filter it.
How to prepare 250 ppm of chloride standard solution:
Take 50 volume of 0.0824% w/v prepared solution of NaCl and dilute to 100 volume of distilled water in a volumetric flask, and properly mixing it.
How to prepare 0.1 M silver nitrate solution:
Take 17.00 gm of AgNO3 and dissolve in 500 ml of distilled water in a volumetric flask, and properly mixing it. Once it has completely dissolved, make up the volume with distilled water to 1000 ml.
How to prepare dilute nitric acid:
Click here to get the procedure of preparation dilute nitric acid.
Procedure of modified limit test for chloride:
Standard:
- Take 10.00 ml of 250 ppm sodium chloride solution using a pipette into a Nessler cylinder and label it as standard.
- Add 05.00 ml of distilled water.
- Add 10.00 ml of dilute nitric acid to the above preparation and properly mixed it.
- Dilute it to 50.00 ml with distilled water.
- Then add 01.00 ml of prepared 0.1 M silver nitrate solution.
- Stir immediately with a glass rod and let stand for five minutes by protecting from light.
- Observe this standard preparation under a black background.
Test:
- Take 40.00 ml of prepared potassium permanganate test solution using a pipette into a Nessler cylinder and label it as a test.
- Add 10.00 ml of dilute HNO3 solution.
- Then add 01.00 ml of prepared 0.1 M AgNO3 solution.
- Stir immediately with a glass rod and let stand for five minutes by protecting from light.
- Observe this test preparation under a black background.
- Compare the opalescence produced to the opalescence produced by a standard solution.
Observation:
The KMnO4 test solution produces less or same or more opalescence than the reference solution.
Result:
The sample passes/fails the modified limit test as per Indian Pharmacopoeia.
Precautions:
- The use of distilled water is required as chloride in tap water will interfere with the result.
- Do not use the same glass rod for the standard and sample, as this will affect the result.
- Store silver nitrate in an amber-colored container as it is photosensitive.
- If silver nitrate is in contact with skin it will be produced black dots.
FAQ (Frequently Asked Questions):
Modified limit test for chloride is done for which compound?
Modified limit test of chloride is done to test colored compounds which are not possible with normal tests.
Is observed in the limit test of chloride and sulphate?
Turbidity is observed in the limit test of chloride and sulphate.
State the reason for use of acidic conditions in the chloride limit test.
Nitric acid is added to the chloride limit test to acidify the solution and aid in silver chloride precipitation, which causes the solution to become turbid at the end of the process.
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