Chrominfo: Modified limit test for sulphate

Learn the modified limit test of sulphate through a laboratory experiment or practical.

Aim:

To perform the modified limit test for sulphate 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 sulphate (K2SO4), potassium permanganate (KMn04), barium chloride (BaCl2), ethanol, hydrochloric acid (HCL), glacial acetic acid (CH3COOH).

Principle of modified limit test for sulphate:

The potassium permanganate is de-colored before the limit test because its purple color interferes with the limit test. The BaSO4 precipitate turns pink or violet 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.

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 600 ppm sulphate standard solution:

Take 60 volume of 0.181 % w/v prepared solution of K2SO4 and dilute to 100 volume of distilled water in a volumetric flask, and properly mixing it.

Preparation of potassium permanganate test solution:

Preparation of 10 ppm ethanolic sulphate standard solution:

Preparation of 25 % w/v barium chloride solution:

Preparation of 5M acetic acid:

We already discussed the preparation of the above solutions in the limit test for sulphate. Click here to get the procedure.

Procedure of limit test for sulphate:

Standard:

Take 15.00 ml of prepared 600 ppm standard sulphate solution using a pipette into a Nessler cylinder and label it as standard.
Add 1 ml of prepared 25 % w/v barium chloride solution.
Allow one minute to stand after adding 1.5 ml of ethanolic sulphate standard solution.
Then add 0.15 ml prepared 05 molar acetic acid.
Add sufficient water to make 50 ml.
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 10.00 ml of prepared KMnO4 test solution using a pipette into a Nessler cylinder and label it as a test.
Add 15.00 ml of distilled water.
Add 1 ml of prepared 25 % w/v barium chloride solution.
Allow one minute to stand after adding 1.5 ml of ethanolic sulphate standard solution.
Add sufficient water to make 50 ml.
Then add 0.15 ml of prepared 5 M acetic acid 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 and compare the opalescence of this solution to that of a standard solution.