Limit Test For Sulphates

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Table of Contents

Aim:

To perform limit test for sulphates for the given sample.

Requirements:

Nessler cylinders, glass rod, 1ml bulb pipette, 2 ml graduate pipette, 5 M acetic acid, sulphate standard solution (10 ppm SO4), test sample, barium chloride solution, ethanolic sulphate standard solution (10
ppm SO4), 1ml graduate pipette.

Principle:

In this experiment, the test opalescence/turbidity obtained by the reaction of sulphate impurities with barium chloride is compared with standard opalescence/turbidity obtained by the reaction of known quantity of sulphate with barium chloride.

Dilute acetic acid is used to dissolve other impurities if present.

The precipitate barium sulphate formed is insoluble in dilute acetic acid and gives opalescence/turbidity. A known amount of potassium sulphate is added both in test and standard in order to increase sensitivity, rapid and complete precipitation by seeding. Ethyl alcohol is used to prevent supersaturation and thus producing uniform opalescence/turbidity. In the earlier editions of Indian pharmacopoeia, barium sulphate reagent containing barium sulphate and ethyl alcohol was used instead of ethanolic sulphate standard solution. Instead of acetic acid, hydrochloric acid was used. The change in the reagent does not effect the objective of the test.

Procedure:

Test opalescence:

To 1.0 ml of a 25.0% w/v solution of barium chloride in a Nessler cylinder add 1.5 ml of ethanolic sulphate standard solution (10 ppm SO4), mix and allow to stand for 1 minute. Dissolve the given sample in 15 ml of water and add 0.15 ml of 5M acetic acid, pour the solution in the Nessler cylinder. Add sufficient water to produce 50 ml, stir immediately with a glass rod and allow to stand for 5 minutes and viewed transversely against a black back ground.

Standard opalescence:

To 1.0 ml of a 25.0% w/v solution of barium chloride in a Nessler cylinder add 1.5 ml of ethanolic sulphate standard solution (10 ppm SO4), mix and allow to stand for 1 minute. Add 15 ml of sulphate standard solution (10 ppm SO4) and 0.15 ml of 5 M acetic acid. Add sufficient water to produce 50 ml, stir immediately with a glass rod and
allow to stand for 5 minutes and viewed transversely against a black background.

Test Solution	Standard Solution
1. Transfer 1.0 ml of a 25 % w/v solution of barium chloride in to Nessler’s cylinder.	Transfer 1.0 ml of a 25 % w/v solution of barium chloride in to Nessler’s cylinder.
2. Add 1.5 ml of ethanolic sulphate standard solution (10 ppm SO4), mix and allowed to stand for 1 min.	2. Add 1.5 ml of ethanolic sulphate standard solution (10ppm SO4), mix and allowed to stand for 1 min.
3. Dissolve the given sample in 15 ml of water and add 0.15 ml of 5 M acetic acid and pour the solution into a Nessler’s cylinder.	3. Add 15 ml of standard sulphate solution (10 ppm SO4) and 0.15 ml of 5 M acetic acid to the Nessler’s cylinder
4. Add sufficient water to make up 50ml.	4. Add sufficient water to make up 50ml.
5. Stir immediately with a glass rod and allowed to stand for 5 min and viewed transversely against a black background	5. Stir immediately with a glass rod and allowed to stand for 5 min and viewed Transversely against a black background.

Observation:

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

Report/Result:

The given sample passes limit test for sulphates.
or
The given sample fails limit test for sulphates.

Preparation of Reagents:

1. 25% w/v barium chloride: Dissolve 25.0 g of barium chloride in 100 ml water.
2. 5 M acetic acid: Solution of any molarity xM may be prepared by diluting 57x ml (60x g) of glacial acetic acid to 1000 ml with water.
3. Ethanolic sulphate standard solution (10 ppm SO4): Dilute 1 volume of a 0.181% w/v solution of potassium sulphate in ethanol (30%) to 100 volumes with ethanol (30%).
4. Sulphate standard solution (10 ppm SO4): Dilute 1 volume of a 0.181% w/v solution of potassium sulphate in distilled water to 100 volumes with the same solvent