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The limit test for chloride is a qualitative test that detects whether the concentration of chloride ions (Cl⁻) in a sample is below a predefined acceptable limit.
It is based on the principle of a precipitation reaction between silver ions (Ag⁺) and chloride ions (Cl⁻), resulting in the formation of an insoluble white precipitate of silver chloride (AgCl).
Chemical Reaction
Ag+ (aq) + Cl- (aq) → AgCl (s)
Procedure of Limit test for Chloride
1) Preparation of Reference Standard Solution
Prepare a reference standard solution with a known concentration of chloride ions, usually from sodium chloride (NaCl) or potassium chloride (KCl).
This standard represents the acceptable limit of chloride ions.
2) Preparation of Test Solution
Prepare a test solution from the sample to be analyzed, ensuring any chloride ions present are in a soluble form.
3) Addition of Silver Nitrate (AgNO₃)
Add equal volumes of silver nitrate solution to both the test solution and the reference standard solution.
The silver nitrate provides the silver ions for the reaction.
Ensure the concentration of AgNO₃ is slightly in excess to react fully with chloride ions.
4) Acidification with Dilute Nitric Acid (HNO₃)
Add dilute nitric acid to both solutions to prevent the precipitation of other silver salts that could interfere with the test results.
5) Observation of Precipitate
Look for the formation of a white precipitate of silver chloride (AgCl) in both the test and reference solutions, indicating the presence of chloride ions.
6) Comparison of Precipitate
Compare the turbidity or intensity of the precipitate in the test solution with the reference standard solution.
If the test solution's precipitate is less intense or less turbid than the reference solution, the chloride concentration is below the acceptable limit.
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