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: Understanding the Concept:
Kjeldahl’s method is the standard quantitative analytical technique used to estimate nitrogen in organic compounds, but it has specific chemical limitations based on the structural environment of the nitrogen atom.
Step 2: Key Formula or Approach:
Evaluate the core principle of Kjeldahl digestion: the organic nitrogen MUST be capable of converting quantitatively into ammonium sulfate. Assess if pyridine meets this condition.
Step 3: Detailed Explanation:
1. Statement I: Kjeldahl's method fails for compounds containing nitrogen situated in a stable aromatic ring (like pyridine, quinoline), nitrogen present in nitro groups ($-NO_{2}$), or nitrogen in azo groups ($-N=N-$). (Statement I is True)

2. Statement II: The fundamental reason Kjeldahl's method fails for pyridine is precisely because its ring nitrogen is exceptionally stable and tightly bound. When heated with conc. $H_2SO_4$, it does not quantitatively undergo decomposition and thus does not change to ammonium sulphate under standard digestion conditions. (Statement II is False)
Step 4: Final Answer:
Statement I is correct, and Statement II is incorrect.