Question

Ninhydrin test is used for the detection of?

• A. Bile salts
• B. Amino acids
• C. Nucleic acid
• D. Lipids

Hint:

Ninhydrin gives Ruhemann's purple with the free alpha-amino group of amino acids.

Answer 1

The Correct Option is B

Step 1: The ninhydrin test is a colour reaction used to detect a specific class of biomolecule. We need to match it to the right group.

Step 2: Ninhydrin reacts with the free alpha-amino group of alpha-amino acids. It oxidatively deaminates the amino acid and the product forms a purple-blue complex (Ruhemann's purple); proline and hydroxyproline give a yellow colour.

Step 3: Therefore the test detects amino acids (option b). It is widely used to visualise amino acids on chromatograms and in forensic fingerprint detection.

Step 4: Why the others are wrong. Bile salts are tested by the Hay's sulphur or Pettenkofer reaction, nucleic acids by reactions such as the diphenylamine or orcinol test, and lipids by Sudan staining. None of these uses ninhydrin. Hence the answer is Amino acids.