🧪 Limit Test of Arsenic – Principle, Procedure, Reaction, and Viva Questions

 

🧪 Limit Test of Arsenic – Principle, Procedure, Reaction, and Viva Questions

🔹 Aim:

To carry out the Limit Test for Arsenic as per the Indian Pharmacopoeia (IP) standards.

🔹 Principle:

The limit test for arsenic is based on the formation of a yellow stain when arsine gas (AsH₃), generated from arsenic in the sample, reacts with mercuric chloride paper.

The intensity of the yellow stain produced is compared with a standard to determine if the sample meets the permissible limit.

🔹 Apparatus Required:

• Arsenic Test Apparatus (Arsenic Test Bottle)

• Glass Tube with a narrow end

• Mercuric Chloride Paper

• Nessler Cylinder (2) for comparison

🔹 Chemicals Required:

• Arsenic-free zinc granules

• Lead acetate solution

• Hydrochloric acid

• Potassium iodide

• Stannous chloride

• Standard arsenic solution

• Water

🔹 Reaction Involved:

1. Reduction of Arsenic compound:

   $$As_2O_3 + 6Zn + 12HCl → 2AsH_3 + 6ZnCl_2 + 3H_2O$$

2. Formation of Yellow Stain:

   $$AsH_3 + HgCl_2 → AsCl_3 + 2Hg + (Yellow stain)$$

🔹 Procedure:

1. In an arsenic test apparatus, add the test solution (sample).

2. Add 1 g of arsenic-free zinc and 5 mL of stannous chloride solution.

3. Add 10 mL hydrochloric acid, quickly attach the tube with mercuric chloride paper at its mouth.

4. The reaction releases arsine gas (AsH₃), which reacts with the mercuric chloride paper.

5. The paper gradually turns pale yellow to brownish-yellow, depending on arsenic content.

6. Compare the stain with that produced by the standard solution after 40 minutes.

🧾 Observation Table:

S.No	Sample Type	Observation (Color of Stain)	Result
1	Test Solution	Pale Yellow	Pass
2	Standard Solution	Pale Yellow	—
3	Above Limit Sample	Dark Yellow/Brown	Fail

📚 Inference:

If the intensity of the yellow stain on the test paper does not exceed that of the standard, the sample passes the test for arsenic.

💬 Viva Questions:

1. What is the principle of the limit test for arsenic?

2. Which gas is formed during the test?

3. Why is mercuric chloride paper used?

4. What is the color of the stain produced?

5. What is the permissible limit of arsenic according to IP?

🧠 Quick Revision Tip:

“Arsenic forms arsine gas, reacts with mercuric chloride paper, and gives a yellow stain proportional to arsenic content.”

🩺 Related Practicals:

👉 Limit Test for Iron – Principle and Observation

👉 Limit Test for Heavy Metals – Procedure & Reaction

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