🧪 Limit Test for Lead (Heavy Metals)

Subject: Pharmaceutical Inorganic Chemistry

Experiment Type: Qualitative Analysis

Aim:

To carry out the Limit Test for Lead in a given sample using dithizone reagent and to ensure that lead impurities are within the permissible limit as per I.P. standards.

✨ Principle

• Lead (Pb²⁺) in solution reacts with dithizone (diphenylthiocarbazone) in chloroform.

• A pink to violet color develops if lead is present.

• The intensity of color in the sample is compared with that of a standard lead solution.

• If the test solution shows less or no color compared to the standard, the sample passes the test.

Reaction:

$$Pb^{2+} + Dithizone \longrightarrow Lead\text{-}Dithizonate\ Complex (Pink/Violet)$$

⚗️ Materials Required

• Test sample solution

• Standard lead solution (20 ppm Pb²⁺)

• Ammonia solution

• Dithizone reagent in chloroform

• Buffer solution (pH 9)

• Nessler cylinders

🔬 Procedure

1. Take the sample solution in a Nessler cylinder.

2. Add ammonia solution and buffer solution (pH ~9).

3. Shake well and add dithizone solution in chloroform.

4. Compare the color developed with that of the standard lead solution.

📊 Observation Table

S. No	Solution Type	Reagent Added	Observation (Color)	Inference
1	Standard (20 ppm Pb²⁺)	Dithizone in chloroform	Pink/Violet	Confirms lead
2	Sample	Dithizone in chloroform	No / Lighter color	Passes test / Safe

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✅ Result

The given sample solution passes the Limit Test for Lead if the color intensity is not deeper than the standard.

🎯 Viva Questions

Q1. Why is dithizone used in Limit Test of Lead?

👉 Because it forms a colored complex with lead ions.

Q2. What is the permissible limit of lead as per I.P.?

👉 20 ppm (parts per million).

Q3. Why is a buffer solution used?

👉 To maintain optimum pH (~9) for complex formation.

Q4. Which instrument can replace visual comparison?

👉 Colorimeter or Spectrophotometer.

📌 Previous Practical Blog: Limit Test of Iron

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