Question

The principle of the limit test for Arsenic is based on the Gutzeit test, where a yellow stain is produced on

• A. Lead acetate paper
• B. Mercuric chloride paper
• C. Filter paper
• D. Silver nitrate paper

Hint:

The limit test for arsenic checks that the amount of arsenic in a sample is below an allowed limit. It uses the Gutzeit reaction, where arsenic is turned into arsine gas and detected on a special test paper.

Answer 1

The Correct Option is B

Step 1: What is the Gutzeit test?
The Gutzeit test is a classical chemical test used in the limit test for arsenic, as specified in pharmacopoeias including the Indian Pharmacopoeia. It detects trace amounts of arsenic in pharmaceutical substances and excipients.

Step 2: Principle of the test.
Arsenic compounds are first reduced to arsine gas (AsH₃) using zinc and dilute sulfuric acid in the Gutzeit apparatus. The arsine gas then passes upward through a lead acetate cotton plug (to remove H₂S interference) and hits a strip of Mercuric chloride (HgCl₂) paper at the top.

Step 3: What happens at the Mercuric chloride paper?
Arsine reacts with mercuric chloride paper: AsH₃ + 3 HgCl₂ gives As(HgCl)₃ + 3 HCl. This reaction produces a yellow to brown stain on the mercuric chloride paper. The intensity of the stain is compared to a standard stain from a known arsenic standard.

Step 4: Rule out the distractors.
Lead acetate paper (Option 1): Used INSIDE the apparatus to trap H₂S gas, not where arsenic staining occurs. Filter paper (Option 3) and Silver nitrate paper (Option 4) are NOT used in the Gutzeit test for arsenic.

Step 5: Confirm the answer.
The yellow stain in the Gutzeit arsenic limit test is produced on Mercuric chloride paper. This is a pharmacopoeia-defined test specification.


Answer: Option (2) — Mercuric chloride paper