- The limit test for lead is a qualitative test that determines whether the concentration of lead ions (Pb²⁺) in a sample is below a predefined limit.
- It is based on the colorimetric reaction between lead ions (Pb²⁺) and potassium chromate (K₂CrO₄), which forms a yellow-colored precipitate of lead chromate (PbCrO₄).
Chemical Reaction:
Pb2 + (aq) + CrO₄2−(aq) → PbCrO₄(s)
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Procedure of Limit test for Lead:
Prepare Reference Standard Solution:
- Prepare a reference solution containing a known concentration of lead ions, typically from a soluble lead salt like lead nitrate, representing the acceptable lead limit.
Prepare Test Solution:
- Prepare a test solution from the sample, ensuring that any lead ions present are in a soluble form.
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Add Potassium Chromate (K₂CrO₄):
- To both the test and reference solutions, add equal volumes of potassium chromate solution.
- This provides the chromate ions (CrO₄²⁻) required for the reaction.
Formation of Yellow Precipitate:
- Allow the solutions to stand for a few minutes, enabling the formation of lead chromate (PbCrO₄), a yellow-colored precipitate.
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Observation:
- Observe the intensity of the yellow color in both the test solution and the reference standard solution.
- The appearance of yellow color in the test solution indicates the presence of lead ions.
Compare Color Intensity:
- Compare the yellow color intensity in the test solution to that in the reference standard.
- If the test solution’s yellow color is less intense, the lead ion concentration is below the acceptable limit.
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