🧪 Limit Test of Heavy Metals – Practical Guide

📌 Aim:

To carry out the limit test for heavy metals present in pharmaceutical substances as an impurity.

🎯 Principle

The test is based on the reaction of heavy metals (like lead) with hydrogen sulfide (H₂S) in an acidic medium, producing a brownish-black coloration of metal sulfides.

The intensity of this color is compared with a standard lead solution prepared simultaneously.

🧰 Materials Required

• Nessler cylinders

• Test sample solution

• Standard lead solution (Pb²⁺)

• Dilute acetic acid

• Hydrogen sulfide (H₂S) water or thioacetamide reagent

🧪 Procedure

1. Take the sample solution in a Nessler cylinder.

2. Add dilute acetic acid to adjust the pH.

3. Add a few drops of thioacetamide reagent (or pass H₂S gas).

4. Prepare a standard solution of lead in another Nessler cylinder.

5. Keep both cylinders side by side for comparison.

🔬 Reaction Involved

$$Pb^{2+} + H_2S \; \rightarrow \; PbS \downarrow \; (brownish-black precipitate)$$

Other heavy metals also form sulfide precipitates with H₂S.

📊 Observation Table

S. No.	Solution	Observation (Color)	Result
1	Test solution	Pale brown / No color	Pass
2	Standard (Pb²⁺)	Brownish-black	–

🎤 Viva Questions

1. What is the principle of the limit test for heavy metals?

2. Why is hydrogen sulfide used?

3. Which standard metal ion is used in this test?

4. What does the appearance of color indicate?

5. Name some heavy metals commonly tested in pharmaceuticals.

📎 Related Practical

👉 Limit Test of Iron (Previous Practical)

🚀 Coming Next

Stay tuned for Limit Test of Lead (Pb) in our next blog!

📌 Pro Tip for Blog Design:

• Add an infographic image showing Nessler cylinders with standard vs test solution.

• Highlight the reaction with color codes for brown/black precipitate.

• Interlink with previous and next practical posts for smooth navigation.