Chemistry Q137 – AP-EAMCET 2026

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Which of the following will be suitable for estimation of nitrogen by Kjeldahl's method?

Clue 1 of 3: Principle of Kjeldahl Method

The Kjeldahl method is used for the quantitative estimation of nitrogen in organic compounds by converting nitrogen to ammonium sulfate upon digestion with concentrated sulfuric acid.

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Step 1: Understand the Kjeldahl Method's Scope✦ Active

The Kjeldahl method is a standard analytical technique for determining the nitrogen content in organic substances. It involves digesting the sample with concentrated sulfuric acid, which converts the nitrogen present into ammonium sulfate. The amount of ammonia produced is then quantified.

Crucially, this method is effective for nitrogen present in amino groups ( ${-NH}_{2}$ ), amides ( ${-CONH}_{2}$ ), and other similar forms where nitrogen is readily converted to ammonium ions under the strong acidic conditions. However, it fails for certain types of nitrogen-containing compounds.

💡 Teacher's Secret Hint

Remember that the goal of digestion is to convert all nitrogen into ammonium ions. If a nitrogen functional group is resistant to this conversion, the method will not yield accurate results.

Step 2: Identify Unsuitable Nitrogen Forms○ Expand

The Kjeldahl method is generally not suitable for compounds containing nitrogen in:

1. Nitro groups ( ${-NO}_{2}$ ): Nitrogen in this form is not quantitatively converted to ammonium sulfate.

2. Azo groups ( $-N=N-$ ): Similar to nitro groups, azo nitrogen is not reliably converted.

3. Heterocyclic rings (e.g., pyridine, quinoline): Nitrogen present in aromatic heterocyclic rings is often stable and does not convert to ammonium sulfate under typical Kjeldahl conditions.

💡 Teacher's Secret Hint

Some modifications to the Kjeldahl method exist to include nitro and azo compounds (e.g., reduction with salicylic acid before digestion), but for the standard method, these are typically excluded.

Step 3: Evaluate Each Option○ Expand

Let's examine each given option:

1. **Benzene diazonium chloride** ( $C_{6} H_{5} N_{2} Cl$ ): Contains nitrogen in an azo group ( $-N=N-$ ). Therefore, it is **not suitable**.

2. **Nitrobenzene** ( $C_{6} H_{5} {NO}_{2}$ ): Contains nitrogen in a nitro group ( ${-NO}_{2}$ ). Therefore, it is **not suitable**.

3. **Amino benzene** (Aniline, $C_{6} H_{5} {NH}_{2}$ ): Contains nitrogen in an amino group ( ${-NH}_{2}$ ). This form of nitrogen is readily converted to ammonium sulfate during Kjeldahl digestion. Therefore, it is **suitable**.

4. **Pyridine** ( $C_{5} H_{5} N$ ): Contains nitrogen within an aromatic heterocyclic ring. This nitrogen is generally **not suitable** for direct estimation by the Kjeldahl method without modification.

💡 Teacher's Secret Hint

Visualizing the chemical structure of each compound can help quickly identify the type of nitrogen functional group present.

Step 4: Conclusion○ Expand

Based on the applicability criteria of the Kjeldahl method, only Amino benzene contains nitrogen in a form (amino group) that is suitable for estimation by this method.

💡 Teacher's Secret Hint

Always double-check the specific functional groups and their known reactivity under the conditions of the analytical method.

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Chemistry Question 137 – AP-EAMCET 2026

Clue 1 of 3: Principle of Kjeldahl Method

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