Principle of Limit Test for Chloride IP
This test, which is mainly used to control chloride impurity in inorganic substances, depends upon the simple reaction between silver nitrate and soluble chlorides to give insoluble silver chloride in the presence of dilute nitric acid. The insoluble silver chloride makes the solution opalescent, and the extent of opalescence depending upon the amount of chloride present in the substance, is compared with a standard opalescence produced in a standard solution having a known amount of chloride by adding silver nitrate and the same volume of dilute nitric acid as used in the test solution.
If the opalescence produced in the test is less intense than that of standard opalescence, the sample passes the limit test for chloride and vice versa.
Dilute nitric acid is used to prevent the precipitation of other acid radicals with silver nitrate solution. It acts by providing a common ion i.e. nitrate.
Reaction:
Preparation of Reagents/Solutions:
1. Preparation of standard chloride solution (25 ppm Cl): Dilute 5 volumes of a 0.0824% w/v solution of sodium chloride to 100 volumes with water.
2. 0.1 M silver nitrate: Dissolve 1.7 g of AgNO3 in sufficient distilled water and make up the volume to 100 mL with distilled water.
3. Dilute nitric acid: It contains approximately 10% w/w of HNO3. Dilute 106 mL of concentrated nitric acid to 1000 mL with water.
Procedure for Limit Test for Chloride IP
Method (I.P. 2007):
Dissolve the specified quantity of the substance under examination in water, or prepare a solution as directed in the individual monograph and transfer it to a Nessler cylinder. Add 10 ml of dilute nitric acid, except when nitric acid is used in the preparation of the solution, dilute to 50 ml with water and add 1 ml of 0.1 M silver nitrate. Stir immediately with a glass rod and allow to stand for 5 minutes protected from light.
When viewed transversely against a black background any opalescence produced is not more intense than that obtained by treating a mixture of 10.0 ml of chloride standard solution (25 ppm Cl) and 5 ml of water in the same manner.
Note: HNO3 is added to prevent the precipitation of other acid radicals with AgNO3.
Example: Perform limit test for chloride in magnesium sulfate IP.
| Sr. No. | Test Sample | Standard |
| 1 | Dissolve 1 g of the MgSO4 sample in about 10 mL of distilled water, and transfer it to a Nessler cylinder for further testing. | Place 10.0 ml of chloride standard solution (25 ppm Cl) and 5 ml of water in the same manner. |
| 2 | Add 10 mL of dil. HNO3. | Add 10 mL of dil. HNO3. |
| 3 | Dilute to 50 mL with distilled water. | Dilute to 50 mL with distilled water. |
| 4 | Add 1 mL of 0.1 M AgNO3. | Add 1 mL of 0.1 M AgNO3. |
| 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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