Question

In limit test for iron, _______

• A. Thioglycolic acid
• B. Ferric ammonium sulphate
• C. Citric acid
• D. Potassium chloride

Hint:

In the limit test for iron, citric acid prevents precipitation of iron by forming a soluble complex.

Answer 1

The Correct Option is C

Concept:
The limit test for iron is used to detect and control small amounts of iron impurity in pharmaceutical substances.

Step 1: Problem in iron limit test.
In alkaline medium, iron may precipitate as ferric hydroxide due to the presence of ammonia. \[ Fe^{3+} + 3OH^- \rightarrow Fe(OH)_3 \]

Step 2: Role of citric acid.
Citric acid forms a soluble complex with iron. This prevents precipitation of iron by ammonia.

Step 3: Importance of preventing precipitation.
If iron precipitates, the test result becomes inaccurate. Citric acid keeps iron in solution so that the test reaction can proceed properly.

Step 4: Checking other options.
Thioglycolic acid helps in colour formation with iron but does not mainly prevent precipitation by ammonia.
Ferric ammonium sulphate is used as an iron standard.
Potassium chloride is not used for this purpose.

Step 5: Final answer.
Therefore, citric acid is used to prevent precipitation of iron by ammonia. \[ \boxed{\text{Citric acid}} \]