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 asks to identify the specific role of copper sulfate (CuSO₄) in the Kjeldahl method, which is a technique used for the quantitative estimation of nitrogen in organic compounds.

Concept Used:

The Kjeldahl method involves three main steps:

1. Digestion: The organic compound containing nitrogen is heated with concentrated sulfuric acid (\( \text{H}_2\text{SO}_4 \)). The nitrogen in the compound is quantitatively converted into ammonium sulfate (\( (\text{NH}_4)_2\text{SO}_4 \)). This step is often slow and is carried out at high temperatures.
2. Distillation: The resulting acidic solution is treated with an excess of a strong base (like NaOH) to liberate ammonia gas (\( \text{NH}_3 \)). The ammonia is then distilled and absorbed in a known excess volume of a standard acid (e.g., \( \text{H}_2\text{SO}_4 \) or HCl).
3. Titration: The amount of unreacted acid is determined by titrating it against a standard base. This allows for the calculation of the amount of ammonia produced, and subsequently, the percentage of nitrogen in the original sample.

To facilitate the digestion process, certain substances are added to the digestion flask along with the sample and concentrated \( \text{H}_2\text{SO}_4 \).

Step-by-Step Solution:

Step 1: Analyze the digestion step of the Kjeldahl method.

The digestion reaction involves the oxidation of the organic compound by hot, concentrated sulfuric acid. The reaction can be represented generally as:

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

This digestion process is typically very slow.

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

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

• A salt to elevate the boiling point: Potassium sulfate (\( \text{K}_2\text{SO}_4 \)) is added to raise the boiling point of the sulfuric acid (from ~337 °C to ~370-400 °C). This allows the digestion to proceed at a higher temperature, which significantly increases the reaction rate.
• A catalyst: A metallic salt is added to catalyze the oxidation of the organic matter. This further accelerates the rate of digestion.

Step 3: Identify the specific function of CuSO₄.

Copper(II) sulfate (\( \text{CuSO}_4 \)) is one of the most commonly used catalysts in the Kjeldahl method. Its function is to increase the rate of the reaction between the sulfuric acid and the organic compound. It does so by facilitating the oxidation process, allowing the conversion of organic nitrogen to ammonium sulfate to occur more quickly and efficiently. Other catalysts like selenium or mercury(II) oxide can also be used, but CuSO₄ is a popular choice.

Final Result:

In the digestion step of Kjeldahl’s method for nitrogen estimation, copper sulfate (CuSO₄) acts as a catalyst to increase the speed of the reaction.