Limit test for Lead
- S-1-Pharmaceutical Inorganic Chemistry
- Feb 26
- 1 min read
Updated: Mar 2
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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)
Procedure of Limit test for Lead
1) 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.
2) Prepare Test Solution
Prepare a test solution from the sample, ensuring that any lead ions present are in a soluble form.
3) 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.
4) Formation of Yellow Precipitate
Allow the solutions to stand for a few minutes, enabling the formation of lead chromate (PbCrO₄), a yellow-colored precipitate.
5) 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.
6) 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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