Question

Statement I: Nitrogen in pyridine cannot be estimated by Kjeldahl's method
Statement II: Nitrogen in pyridine changes to ammonium sulphate when heated with conc. H\(_2\)SO\(_4\) in Kjeldahl's method.
Read the above given statements and choose the correct answer from the given options.

• A. Statement I is true but Statement II is false
• B. Both Statement I and Statement II are false
• C. Both Statement I and Statement II are true
• D. Statement I is false but Statement II is true

Hint:

Remember the key limitations of the Kjeldahl method for nitrogen estimation. It fails for compounds containing:

• Nitrogen in a ring (e.g., Pyridine, Quinoline).
• Nitro groups (-NO\(_2\)).
• Azo groups (-N=N-).

This is a frequently tested exception in qualitative and quantitative analysis.

Answer 1

The Correct Option is A

Step 1: Analyzing Statement I.
Kjeldahl's method is a classical method for the quantitative determination of nitrogen in organic compounds. However, it has limitations. The method is not applicable to compounds where the nitrogen atom is part of a heterocyclic ring (like pyridine), or in nitro (-NO\(_2\)) and azo (-N=N-) groups. This is because the nitrogen in these compounds is very stable and is not completely converted to ammonium sulfate under the conditions of the digestion step. Therefore, the statement that nitrogen in pyridine cannot be estimated by Kjeldahl's method is true.
Step 2: Analyzing Statement II.
This statement claims that the nitrogen in pyridine *does* change to ammonium sulfate when heated with concentrated H\(_2\)SO\(_4\). This contradicts the reason why Kjeldahl's method fails for pyridine. The nitrogen atom in the pyridine ring is part of an aromatic system, which makes it very resistant to the acid digestion. It does not get converted to ammonium sulfate. Therefore, Statement II is false.
Step 3: Conclusion.
Statement I is true because Kjeldahl's method has limitations and does not work for heterocyclic compounds like pyridine. Statement II is false because the reason the method fails is precisely that the nitrogen is *not* converted to ammonium sulfate.
Step 4: Final Answer.
Statement I is true but Statement II is false.