Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A): For determination of total nitrogen in soil, the sample is usually digested with concentrated \(\text{H}_2\text{SO}_4\) in the presence of catalyst mixture consisting of \(\text{K}_2\text{SO}_4\) and \(\text{CuSO}_4\) in the ratio of 10:1.
Reason (R): The \(\text{K}_2\text{SO}_4\) is used to raise the boiling point of \(\text{H}_2\text{SO}_4\) that promote the oxidation of organic matter and conversion of organic N to \(\text{NH}_4^+\)-N, while the \(\text{CuSO}_4\) acts as a catalyst and hastens the breaking down of organic matter and conversion of N to \((\text{NH}_4)_2\text{SO_4\).
Choose the correct answer from the options given below:

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

In Kjeldahl digestion:
- \(\text{K}_2\text{SO}_4\) acts as a boiling point elevator.
- \(\text{CuSO}_4\) (or Selenium powder) acts as a catalyst to accelerate the digestion process.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The Kjeldahl digestion method is the standard technique used to determine total nitrogen in soils and plants. It involves acid digestion to convert organic nitrogen into ammonium sulphate.

Step 2: Detailed Explanation:
- Digestion Process: Soil is heated with concentrated sulphuric acid (\(\text{H}_2\text{SO}_4\)), which oxidizes organic matter and converts organic nitrogen into ammonium ions: \[ \text{Organic Nitrogen} \xrightarrow{\text{H}_2\text{SO}_4} (\text{NH}_4)_2\text{SO}_4 \] - Role of \(\text{K}_2\text{SO}_4\): Pure concentrated sulphuric acid boils at around \(338^\circ\text{C}\). Adding potassium sulphate (\(\text{K}_2\text{SO}_4\)) increases the salt concentration, raising the boiling point of the mixture to approximately \(370^\circ\text{C}\). This higher temperature accelerates the oxidation of organic matter.
- Role of \(\text{CuSO}_4\): Copper sulphate acts as a catalyst to speed up the decomposition of organic molecules. A standard \(10:1\) ratio of \(\text{K}_2\text{SO}_4\) to \(\text{CuSO}_4\) is used in the catalyst mixture, which validates the Assertion.
- Because the Reason accurately explains the chemical roles of both salts in the digestion mixture, it serves as the correct explanation for the Assertion.

Step 3: Final Answer:
Both (A) and (R) are true, and (R) is the correct explanation of (A).