Question

In which limit test is Thioglycolic acid used:

• A. Limit test for arsenic
• B. Limit test for sulphate
• C. Limit test for iron
• D. Limit test for chloride

Hint:

Thioglycolic acid is specifically used in the limit test for iron, where it reduces Fe\(^{3+}\) to Fe\(^{2+}\), forming a colored complex for detection.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Each pharmacopoeial limit test for a specific impurity depends on its own characteristic reagent and reaction. The question wants the test where thioglycolic acid is the key reagent.

Step 2: Key Fact:
Thioglycolic acid is a reducing agent, and in the limit test for iron it reduces ferric ions to ferrous ions, which then form a colored complex used to judge how much iron is present against a standard solution.

Step 3: Detailed Explanation:
The arsenic test relies on generating arsine gas from zinc and acid, with no role for thioglycolic acid.
The sulphate test relies on turbidity from barium chloride.
The chloride test relies on a white precipitate formed with silver nitrate.
None of these three involve thioglycolic acid as a reagent.

Step 4: Final Answer:
Thioglycolic acid is used in the limit test for iron.