Limit test for Sulfates and modified limit test for sulfates

• Liit test for Sulphates:

Limit test for sulphates also like limit test for chlorides is a comparative test as well as a quantitative test. For comparison, we need to prepare a standard solution with a definite amount of impurity and that standard solution is compared for the turbidity developed with the solution of the substance of interest/sample/analyte. This test gives us the limit/range of sulphates present in the given sample quantitatively.

Aim of this test is to identify the impurity (sulphates) in the given sample/analyte. To perform this test, we need 25% W/V solution of Barium Chloride solution, 0.1089% W/V potassium sulphate (K₂SO₄) solution, dilute hydrochloric acid, distilled water and barium sulphate reagent (contain barium sulphate and ethyl alcohol).

Principle of the test is as follows.

Limit test for sulphates depends on the reaction of soluble sulphates with Barium chloride in acidic medium (usually, dilute hydrochloric acid or dilute citric acid is used) which results in the formation of barium sulfate. This Barium sulfate will appear as solid particles (produces turbidity) in the solution.

The test proceeds as follows.

Standard and test solutions are prepared as follows.

Table 7.3.: Limit test for sulphates

S.No.	Standard Solution	Test Solution
1.	Take 1ml of 25% W/V solution of Barium Chloride (BaCl2) solution in a Nessler cylinder.	Take 1ml of 25% W/V solution of Barium Chloride (BaCl2) solution in a Nessler cylinder.
2.	Add 1.5ml of ethanolic sulfate standard solution (10ppm SO42-) and mix. Allow to stand for 1min.	Add 1.5ml of ethanolic sulfate standard solution (10ppm SO42-) and mix. Allow to stand for 1min.
3.	Add 15ml of potassium sulfate solution (10ppm SO42-) and 0.15ml of 5M acetic acid (CH3COOH) or hydrochloric acid (HCl) to the Nessler cylinder.	Add 15ml of solution of the given sample (prepared as per the requirement given or as per the monograph given in Pharmacopoeia) and 0.15ml of 5M acetic acid (CH3COOH) or hydrochloric acid (HCl) to the Nessler cylinder.
4.	Make up the volume to 50ml by adding sufficient amount of distilled water.	Make up the volume to 50ml by adding sufficient amount of distilled water.
5.	Immediately stir with a glass rod and allow the solution to stand for 5min.	Immediately stir with a glass rod and allow the solution to stand for 5min.
6.	The Nessler cylinder is viewed against a black background transversely.	The Nessler cylinder is viewed against a black background transversely.

Observation: The turbidity produced in test mixture is compared against that of standard mixture.

Result/Conclusion: If the turbidity produced in the test mixture is equal with or more than that of the standard mixture that indicates the presence of impurity (sulphates) in excess amounts or out of the allowed/acceptable limits. Hence, the sample has declared as failed the test and it is not safe to use. If the turbidity observed in the test mixture is less than that of standard mixture, then this indicates that the impurities are present within the acceptable/allowed limits and the sample passed the test. Hence, the sample is safe to use.

Preparation of solutions:

• Ethanolic sulphate standard solution (10ppm SO₄^2-): 1.0ml of 0.181% w/v solution in ethanol of potassium sulphate is diluted to 100ml with same solvent (ethanol).
• Barium sulphate reagent: 10ml of 25% w/v solution of barium chloride is mixed with 15ml of ethanolic sulphate standard solution (10ppm SO₄^2-) and allowed to stand for 1min. We should always use freshly prepared reagent.
• Sulphate standard solution (10ppm SO₄^2-): 1.0ml of 0.181% w/v solution of potassium sulphate is diluted to 100ml with distilled water.
• 0M acetic acid: 28.5ml of glacial acetic acid is dissolved in sufficient quantity of distilled water and the volume is made up to 100ml.

Reason for using different solutions/reagents:

• Potassium sulphate helps to increase the sensitivity of the test by giving ionic concentration in the reagent.
• Alcohol is used to prevent supersaturation and keeps barium sulfate (which is precipitated) in the form of turbidity.
• Hydrochloric acid is used to make the solution acidic.

Limitation:

• This test is not used of the liquids that are not miscible in water.

• Modified limit test for Sulphates:

Modified limit test for sulphates is performed for the samples which cannot be tested under the normal process. Modified limit test for sulphates is done for the compounds which are:

• Alkaline in nature: These samples are dissolved in dilute acids like dilute HNO₃ or dilute HCl instead of distilled water to prepare solutions.
• Water – insoluble: Substances like light – kaolin, magnesium trisilicate are not soluble in water. Hence, these substances are generally boiled with acid (dilute HNO₃ or dilute HCl) and distilled water and filtered. The filtrate is used for the test.
• Metallic salts of organic acids (like sodium benzoate, sodium salicylate etc.): When such compound is shaken with the mineral acid (HCl or HNO₃) prescribed in the limit test, the free acid gets liberated which is filtered out. Then the filtrate is used for the limit test.
• Coloured sample: If the sample is coloured [like potassium permanganate (KMnO₄)], it is boiled with alcohol to decolourize before performing the test and in case of other coloured substances like crystal violet, malachite green, these substances are carbonated to ash and the ash is extracted with distilled water before performing the test. The extract is filtered and the filtrate is used for the test.

Let us consider potassium permanganate (KMnO₄) which is a coloured substance as the sample and see the procedure for the limit test of sulphates for that sample. For performing limit test for sulphates in potassium permanganate (KMnO₄), we need potassium sulphate (K₂SO₄), barium chloride (BaCl₂), potassium permanganate (KMnO₄), ethanol (CH₂COOH), hydrochloric acid (HCl), glacial acetic acid (CH₃COOH).

Principle:

Potassium permanganate gives purple colour to the solution which may interfere with the limit test. The precipitate of barium sulphate (BaSO₄) turns pink or violet as it may absorb some of the permanganates. This colouration may hinder the identification of impurities in the test solution by comparing the turbidity with that of the standard solution. To avoid this, the potassium permanganate is decolourized (a reduction reaction) by boiling with ethanol and the precipitated manganese dioxide (MnO₂) is filtered out and the colourless filtrate is used for the test.

2KMnO₄ + 3C₂H₅OH → 2KMnO₄ + 3C₂H₅OH + 3CH₃CHO 2KOH + 2MnO₂ + 2H₂O

The procedure for performing the test is as follows.

Table 7.4.: Modified Limit test for sulphates

S.No.	Standard Solution	Test Solution
1.	15.00 ml of prepared standard sulphate solution is transferred using a pipette into a Nessler cylinder and label it as standard	10.00 ml of prepared KMnO4 test solution is transferred using a pipette into a Nessler cylinder and label it as a test. And 15.00 ml of distilled water is added.
2.	Add 1 ml of 25 % w/v barium chloride (BaCl2) solution	Add 1 ml of 25 % w/v barium chloride (BaCl2) solution
3.	Then add 1.5 ml of ethanolic sulphate standard solution and allow one minute to stand.	Then add 1.5 ml of ethanolic sulphate standard solution and allow one minute to stand.
4.	Now add sufficient water to make volume up to 50 ml.	Now add sufficient water to make volume up to 50 ml.
5.	Then add 0.15 ml of 5M acetic acid to the reaction mixture.	Then add 0.15 ml of 5M acetic acid solution to the reaction mixture.
6.	Stir immediately with a glass rod and allow to stand for five minutes by protecting from light.	Stir immediately with a glass rod and allow to stand for five minutes by protecting from light.
7.	The Nessler cylinder is viewed against a black background transversely.	The Nessler cylinder is viewed against a black background transversely.

Observation: The opalescence produced in test mixture is compared against that of standard mixture.

Result/Conclusion: If the opalescence produced in the test mixture is equal with or more than that of the standard mixture that indicates the presence of impurity (sulphates) in excess amounts or out of the allowed/acceptable limits. Hence, the sample has declared as failed the test and it is not safe to use. If the opalescence observed in the test mixture is less than that of standard mixture, then this indicates that the impurities are present within the acceptable/allowed limits and the sample passed the test. Hence, the sample is safe to use.