Limits test for chloride is a semi-quantitative analytical test to check the chloride impurities in a given pharmaceutical sample.
It is performed to identify the traces of chloride impurities which are likely to be present in a substance.
Since the preparation of the medicines involves many reactions, there is chance of intrusion of impurities from water and other sources.
To comply with quality control, these impurities have to be kept under check and hence they are to be estimated.
The quantity of chloride impurity in an official substance is often small. So, the visible reaction response of any test for that substance is very small.
The test is done by the production of color and compared with that of the standard solution.
Principle of limit test for chlorides
- The chloride impurities in the substance are allowed to interact with silver nitrate solution in the presence of nitric acid.
- This leads to the formation of silver chloride which is white in color.
- Since the quantity of chloride impurity is low, it forms an opalescence instead of a precipitate.
- The formed opalescence (white color) is compared with that of a standard solution against a uniform illumination.
The two solutions are prepared in a Nessler cylinder.
Reaction
- Chlorides + AgNO3 (silver nitrate) ——————> AgCl (silver chlorides)+ nitrate. (Test solution reaction)
- NaCl (sodium chloride) + AgNO3 ————————> AgCl + NaNO3 (sodium nitrate). (standard solution reaction)
- The chlorides react with silver nitrate to form sliver chloride which is white in color.
- This follows the principle of ion displacement reaction. Here chloride displaces nitrate due to its greater electronegativity.
Procedure for the test
- Dissolve a specified quantity of a given sample in distilled water.
- Then add 10ml of dilute nitric acid (HNO3) and mix.
- Add 50ml of distilled water and 1ml of 5% silver nitrate solution (AGNO3).
- Stir the solution with a glass rod and allow it to stand for 5 minutes. Keep the solution protected from light.
- Then compare the opalescence produced with that of standard solution transversely.
Precautions:
♠ Avoid using tap water, as it can have chloride ions dissolved in it.
♠ Nitric acid should be used to prevent precipitation of other acid radicals with AgNO3.
♠ Keep the solutions developed away from light to prevent any light related interaction.
♠ Check the color crosswise (transversely) for a better view without errors.
Experimental protocol
This can be represented in two tabular forms. Take two Nessler’s cylinders and follow the procedure as in the below table
| Steps | Test solution | Standard solution |
| 1 | Dissolve sample in 10 ml of distilled water in a nesseler cylinder labelled as “Test” | Take 1ml of standard sodium chloride solution into Nesslers cylinder and label it as “Standard” |
| 2 | Add 10ml of dilute nitric acids | Add 10ml of dilute nitric acid |
| 3 | Make up the solution to 50 ml mark using distilled water | Dilute the solution up to 50ml mark with distilled water |
| 4 | Add 1ml of 5% silver nitrate solution | Add 1ml of 5% of silver nitrate solution |
| 5 | Stir with glass rod and allow it stand for 5min. | Stir with glass rods and wait for 5min. |
Check the opalescence of white color produced in both Nessler’s cylinders in a transverse angle.
Observation: The opalescence produced in the test solution should be less than the standard solution to pass the limit test for chlorides for the particular sample.
If the test solution passes the limit test, it would confirm the standard set by different pharmacopeias.
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