Determination of Sulphate Content | Limit Test for Sulphate

The method covers sulphate analysis for impurities in raw materials. It is not intended to be used for Sulphate assays where there is a high Sulphate content. The following table details the tests that are covered: Use of barium chloride in a limit test of sulphate

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Suitable for soluble materials from 10 ppm up to 1200 p.p.m of sulphate (SO4) Test is described in the EP 97


2.1 EP 97 European Pharmacopoeia 1997

2.2 Health & Safety Data for Chemical Reagents used in this test method.

2.3 Health & Safety Sheets for samples analysed by this method

4. Principle of Sulphate Content

The acidic solution is treated with barium chloride solution under standard conditions to produce an opalescence (Barium Sulphate) white colour which is matched for its intensity of colour against a standard. Samples equal to or less than the standard pass. The colour is compared using a matching pair of Nessler tubes under good even lighting.


Potassium Sulphate - Analytical Reagent is used as the reference

10 ppm ethanolic sulphate solution: Dilute 1.0 ml of a 0.181% w/v solution of potassium sulphate in 30% ethanol to 100 ml with 30% ethanol.


6.1 Water - De-ionised or distilled water. - Used boiled and cooled water when preparing and diluting sample test solutions and standards (Carbon Dioxide Free)

6.2 Ethanol 30% (Dilute from 96%)

6.3 Barium Chloride Solution 25% w/v

6.4 Acetic Acid (5M) C2H4O2 (60x5) 300 g / litre

6.5 Hydrochloric Acid 36% w/v Analytical Reagent

6.6 2 M Hydrochloric Acid.


7.1 Balance weighing to the nearest 0.1 mg.

7.2 1 pair of matching Nessler tubes

7.3 Nessler holder with black background

7.4 Glass stirring rod.

7.5 Stopwatch

Limit test for sulphate content procedure

or clock

7.6 1ml glass ml pipette

7.7 2 ml graduated pipette

7.8 15 ml pipette

7.9 25 ml measuring cylinder

7.10 pH indicating papers.

7.11 100 ml volumetric flask.


It is important to adjust the test solution and sample so the pH is correct. The stirring and addition of the barium chloride need to be done in the same way so the opalescence occurs under the same conditions. Samples with EDTA salts cannot be compared as they will dissolve Barium Sulphate.


Precautions: Read Health & Safety Data sheets for raw materials before sampling.

When sampling powders care is required to ensure the sample represents the whole. It may be necessary to select several samples and pass them through a quartering device to ensure the final sample is representative of the whole. If the test uses a sample below 1 g it is advisable to prepare a 10% solution and use an appropriate volume to give the right weight.

Ethanol Formula, Molar Mass, Uses and Properties 

10. Procedure

Precautions: Read Health & Safety Data Sheets on Chemical

Reagents before carrying out this analysis.

10.1 Dissolve x* gm of sample in 6 ml of water.

For strong alkalies adjust to pH 5 - 8 with about 7.5 ml of concentrated hydrochloric acid then dilute to 15 ml with water. (measuring cylinder) - transfer to Nessler tube. For alkali salts add acid dropwise until pH is in range. Select the weight required to give the ppm (maximum)

25 = 6 g

50  = 3 g

100 = 1.5 g

150 =  1.0 g

1200 = 0.5 g plus dilution Dilution 55 ml water + 6.2 ml 2M hydrochloric acid. Take 15 ml for a test.

10.2 Prepare the following solutions simultaneously in matched Nessler cylinders:

Test Reference

25% w/v Barium chloride solution 1.0 ml 1.0 ml

10 ppm Ethanolic sulphate solution 1.5 ml Shake the contents of each cylinder, stand for 1 minute then add:

Prepared test solution 15.0 ml -

10 ppm Sulphate solution - 15.0 ml

Acetic acid (5M) 0.5 ml 0.5 ml

Mix the contents of each cylinder and allow to stand for 5 minutes.

Compare the solutions by viewing them against a black background.

The sample complies with the test if any opalescence produced in the sample solution is not more intense than that produced in the reference solution.

11 CALCULATION OF RESULTS - Not applicable


This is a limited test so the precision may vary up to 10%


The test report shall state the result as equal to, less than or greater than the test limit. If it is greater than the limit it fails the test otherwise it passes. Quote the test method 24027 and the limit of sulphate in ppm (maximum), the name of the analyst, the date of testing and the identification of the sample. Also, state any recommended attention.

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