Limit test for Heavy metals

• 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.