Question

In Kjeldahl’s method for estimation of nitrogen, CuSO\(_4\) acts as:

• A. Reducing agent
• B. Catalytic agent
• C. Hydrolysis agent
• D. Oxidising agent

Answer 1

The Correct Option is B

The question requests the specific function of copper sulfate (CuSO₄) in the Kjeldahl method, a technique for quantifying nitrogen in organic compounds.

Concept Used:

The Kjeldahl method consists of three primary stages:

1. Digestion: The nitrogen-containing organic compound is heated with concentrated sulfuric acid (\( \text{H}_2\text{SO}_4 \)), converting the nitrogen into ammonium sulfate (\( (\text{NH}_4)_2\text{SO}_4 \)). This step, typically slow and conducted at high temperatures, oxidizes the organic material.
2. Distillation: An excess strong base (e.g., NaOH) is added to the acidic mixture, releasing ammonia gas (\( \text{NH}_3 \)). The ammonia is distilled and collected in a known excess of a standard acid (such as \( \text{H}_2\text{SO}_4 \) or HCl).
3. Titration: The unreacted standard acid is titrated with a standard base to determine the amount of ammonia produced, thereby calculating the nitrogen percentage in the original sample.

Additives are introduced to the digestion flask with the sample and concentrated \( \text{H}_2\text{SO}_4 \) to enhance the digestion process.

Step-by-Step Solution:

Step 1: Examine the digestion phase of the Kjeldahl method.

The digestion reaction involves the oxidation of the organic compound by hot, concentrated sulfuric acid. The general reaction is:

\[\text{Organic Compound (C, H, N)} + \text{H}_2\text{SO}_4 \xrightarrow{\text{heat}} (\text{NH}_4)_2\text{SO}_4 + \text{CO}_2 + \text{H}_2\text{O}\]

This digestion is characteristically slow.

Step 2: Evaluate the role of additives in the digestion process.

To accelerate the decomposition and digestion of the organic sample, two types of substances are commonly added:

• Boiling Point Elevators: Potassium sulfate (\( \text{K}_2\text{SO}_4 \)) is added to increase the boiling point of sulfuric acid (from approximately 337 °C to 370-400 °C). This enables digestion at a higher temperature, significantly increasing the reaction rate.
• Catalysts: A metallic salt is added to catalyze the oxidation of organic matter, further speeding up the digestion rate.

Step 3: Determine the specific role of CuSO₄.

Copper(II) sulfate (\( \text{CuSO}_4 \)) is a frequently used catalyst in the Kjeldahl method. It enhances the reaction rate between sulfuric acid and the organic compound by facilitating the oxidation process, leading to a quicker and more efficient conversion of organic nitrogen to ammonium sulfate. Other catalysts, such as selenium or mercury(II) oxide, can also be employed, but CuSO₄ is a popular selection.

Final Result:

In the digestion stage of the Kjeldahl method for nitrogen determination, copper sulfate (CuSO₄) functions as a catalyst, accelerating the reaction rate.