JEE Main · 2021 · 17 Mar 2021 · Shift 2 · Practical and Analytical Chemistry

Kjeldahl's method digests the compound with hot concentrated sulphuric acid so that its nitrogen is converted quantitatively into ammonium sulphate, which is then liberated as ammonia by alkali and titrated. That conversion is complete only when the nitrogen is held in an amino or amido linkage. Nitrogen in a nitro group, in an azo or diazonium linkage, or in an aromatic ring is not reduced to ammonia under these conditions, so nitrobenzene, benzenediazonium chloride and pyridine all fall outside the scope of the method. The estimation succeeds for benzylamine, whose nitrogen sits in a primary aliphatic amino group.