Question

In which limit test, Thioglycolic acid is used:

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

Answer 1

The Correct Option is C

The correct option is (C): Limit test for iron.

Answer 2

Each limit test in pharmacopoeial analysis uses a specific reagent chosen because it forms a distinctive, detectable reaction product with the particular impurity being tested. Checking thioglycolic acid against each option:

1. Limit test for arsenic: The limit test for arsenic (Gutzeit method) relies on generating arsine gas by reaction with zinc and acid, which then reacts with mercuric chloride paper to give a yellow to brown stain; thioglycolic acid plays no role in this test.
2. Limit test for sulphate: Sulphate is limit-tested by precipitating it as barium sulphate with barium chloride and comparing the resulting turbidity to a standard; this test does not involve thioglycolic acid.
3. Limit test for iron: In the limit test for iron, thioglycolic acid is added to the test solution in an ammoniacal medium, where it reduces any ferric iron present and forms a pink to purple coloured complex with iron, whose intensity is compared against a standard iron solution treated the same way. This is precisely the reagent's recognised pharmacopoeial use.
4. Limit test for chloride: Chloride is limit-tested using silver nitrate, which reacts with chloride ion to give a white turbidity of silver chloride; thioglycolic acid is not involved in this determination.

Thioglycolic acid's characteristic colour-forming reaction with iron under ammoniacal conditions is what makes it the reagent of choice specifically in the iron limit test.

Therefore, the correct answer is Limit test for iron.