Kjeldahl's Method is widely used but has significant limitations.

• Inability: It cannot be used for organic compounds where nitrogen is part of a Nitro group ($-N{O}_2$), Azo group ($-N=N-$), or present in a ring (like Pyridine).
• Reason: In these compounds, nitrogen does not quantitatively convert into ammonium sulfate when heated with concentrated sulfuric acid. Instead, most of the nitrogen escapes as $N_2$ gas during the digestion process, leading to inaccurate (too low) results.