Limit test for Heavy metals
- S-1-Pharmaceutical Inorganic Chemistry
- Feb 26
- 2 min read
Updated: Mar 2
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The limit test for heavy metals is a qualitative test that determines if the concentration of heavy metal ions (e.g., Pb²⁺, Cu²⁺, Cd²⁺) in a sample is below a predefined acceptable limit.
The test is based on the reaction between heavy metal ions and sulfide ions (S²⁻) from sodium sulfide (Na₂S), forming colored metal sulfide precipitates.
General Chemical Reaction
M2 + (aq) + S2−(aq) → MS(s)
Here, M²⁺ represents a heavy metal ion, (aq) refers to the aqueous state, and (s) indicates the solid precipitate.
Procedure of Limit test for Heavy metals
1) Prepare Reference Standard Solution
Prepare a reference solution with a known concentration of heavy metal ions, typically from a soluble heavy metal salt like lead acetate, representing the acceptable heavy metal limit.
2) Prepare Test Solution
Prepare a test solution from the sample, ensuring any heavy metal ions are in a soluble form.
3) Acidification with Dilute HCl
Acidify both the test solution and the reference standard solution using dilute hydrochloric acid (HCl) to maintain heavy metal ions in solution.
4) Add Sodium Sulfide (Na₂S)
Add equal volumes of sodium sulfide (Na₂S) solution to both the test and reference solutions.
This provides sulfide ions for the reaction.
5) Formation of Metal Sulfide Precipitate
Let the solutions stand for a few minutes, allowing colored metal sulfide precipitates to form.
6) Observe Color Formation
Check for the formation of a colored precipitate in both solutions. A colored precipitate in the test solution indicates the presence of heavy metal ions.
7) Compare Color Intensity
Compare the color intensity of the precipitate in the test solution with that of the reference standard solution.
If the test solution is less intense, the heavy metal concentration is below the acceptable limit.
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