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 use of Thioglycolic acid in limit tests. Thioglycolic acid is a chelating agent used in analytical chemistry. In the limit test for iron, it acts as a reducing agent, converting ferric ions (Fe\(^{3+}\)) into ferrous ions (Fe\(^{2+}\)).

 Step 2: Role in the limit test for iron. - In this test, Thioglycolic acid reacts with Fe\(^{3+}\), reducing it to Fe\(^{2+}\), which then forms a pink to purple-colored complex with o-phenanthroline in an acidic medium. - The intensity of this color is compared against a standard solution to determine the permissible limit of iron impurities. 

Step 3: Why other options are incorrect. 
- (A) Limit test for arsenic: Uses Gutzeit or Marsh’s test, involving hydrogen sulfide or silver diethyldithiocarbamate. 
- (B) Limit test for sulphate: Uses barium chloride, not thioglycolic acid. 
- (D) Limit test for chloride: Uses silver nitrate, forming a white precipitate of silver chloride (AgCl).

Answer 2

The question asks in which pharmacopoeial limit test thioglycolic acid is used as a reagent. Since each limit test relies on a distinct chemical reaction to reveal the impurity being checked, matching thioglycolic acid's chemistry to each test shows where it belongs.

1. Limit test for arsenic: This test relies on generating arsine gas, using zinc and an acid or stannous chloride and potassium iodide, which then reacts with mercuric chloride paper or produces a colored complex. Thioglycolic acid plays no part in this reaction.
2. Limit test for sulphate: This test detects sulphate through turbidity formed with barium chloride in an acidic medium. It does not involve thioglycolic acid at any step.
3. Limit test for iron: Thioglycolic acid acts as a reducing agent in this test, converting ferric ions (\(Fe^{3+}\)) to ferrous ions (\(Fe^{2+}\)). The ferrous ions then form a pink to purple colored complex with a reagent such as ammonium citrate and ammonia, and the intensity of this color, compared against a standard, indicates the amount of iron present. This is the specific role thioglycolic acid plays.
4. Limit test for chloride: This test detects chloride through the white precipitate formed when silver nitrate reacts with chloride ions. Thioglycolic acid is not part of this reaction either.

Only the iron limit test depends on thioglycolic acid's reducing action to generate the colored complex used for comparison.

Therefore, the correct answer is Limit test for iron.