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).