Principle for Limit Test for Sulphate IP
This test is designed to control sulphate impurity primarily in inorganic substances and is based upon the simple reaction between barium chloride and soluble sulphate in the presence of acetic acid to give insoluble barium sulphate.
The opalescence produced in the test is compared with that of standard opalescence obtained from a standard sulphate solution containing a known amount of sulphate produced in the same manner. If the test opalescence is less intense than that of standard opalescence, the sample passes the limit test for sulphate and vice versa.
Note: The solubility of barium sulphate precipitates is very much affected by the concentration of the acid. The acidity of the solution is controlled by using acetic acid. Ethanolic sulphate standard solution (10 ppm SO2−4) is added to increase the sensitivity of the test. The ionic concentration is adjusted in such a manner that the solubility product of barium sulphate gets exceeded and traces of barium sulphate present assist rapid and complete precipitation by seeding. Ethanol prevents supersaturation and helps in producing a more uniform opalescence.
Reaction:
Preparation of Reagents/Solutions:
1. 25% w/v barium chloride solution: Dissolve 25.0 g of barium chloride in sufficient distilled water to produce 100.0 mL.
2. Ethanolic sulphate standard solution (10 ppm SO2−4): Dilute 1 volume of a 0.181% w/v solution of potassium sulphate in ethanol (30%) to 100 volumes with ethanol (30%).
3. Sulphate standard solution (10 ppm SO2−4): Dilute 1 volume of a 0.181% w/v solution of potassium sulphate in distilled water to 100 volumes with distilled water.
4. Dilute HCl: Approximately 10% w/w of concentrated hydrochloric acid.
Procedure for Limit Test for Sulphate IP
Method (I.P. 2007):
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 SO2−4), mix and allow to stand for 1 minute. Add 15 ml of the solution prepared as directed in the monograph or a solution of the specified quantity of the substance under examination in 15 ml of water 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.
When viewed transversely against a black background any opalescence produced is not more intense than that obtained by treating in the same manner in 15 ml of sulphate standard solution (10 ppm SO2−4), in place of the solution under examination.
Example: Perform limit test for sulphate in sodium chloride IP.
| Sr. No. | Test Sample | Standard |
| 1. | Dissolve 2 g of NaCl sample in 10 mL of distilled water and transfer to Nessler cylinder. | Place 15 mL of sulphate standard solution (10 ppm SO4) in a Nessler cylinder. |
| 2. | Add 2 mL of dil. HCl. | Add 2 mL of dil. HCl. |
| 3. | Dilute to 45 mL with distilled water. | Dilute to 45 mL with distilled water. |
| 4. | Add 5 mL of barium sulphate reagent. | Add 5 mL of barium sulphate reagent. |
| 5. | Stir with a glass rod and allow to stand for 5 min. | Stir with a glass rod and allow to stand for 5 min. |
Interpretation: Compare the opalescence produced transversely against a dark background. For passing the test, the opalescence produced in the test sample should not be more than that of the standard.
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