Problem 60 - A Complete Resource Book in Chemistry for JEE Main

Question

Mark the incorrect statement in Kjeldahl's method of estimation of nitrogen (a) copper sulphate or mercury acts as a catalyst (b) nitrogen is quantitatively decomposed to give ammonium sulphate (c) nitrogen gas is collected over caustic potash solution (d) potassium sulphate is used as boiling point elevator of \(\mathrm{H}_{2} \mathrm{SO}_{4}\)

Step-by-Step Solution

Answer

Statement (c) is incorrect.

Step 1: Understanding Kjeldahl's Method

The Kjeldahl method is used for estimating nitrogen content in organic compounds. The organic material is digested with concentrated sulfuric acid, which decomposes it and converts nitrogen into ammonium sulfate.

Step 2: Role of Catalysts

In Kjeldahl's method, copper sulfate or mercury is used as a catalyst to speed up the digestion process, ensuring complete conversion of nitrogen to ammonium sulfate. Therefore, statement (a) is correct.

Step 3: Formation of Ammonium Sulfate

During digestion, nitrogen from the organic compound is converted to ammonium sulfate. Thus, statement (b) is correct.

Step 4: Collection of Nitrogen Gas

Kjeldahl's method does not involve the collection of nitrogen gas over caustic potash. Instead, the distilled ammonia is trapped in an acid solution. This makes statement (c) incorrect.

Step 5: Use of Potassium Sulfate

Potassium sulfate is added to increase the boiling point of sulfuric acid, allowing the digestion to proceed at a higher temperature. This ensures more efficient conversion, making statement (d) correct.

Key Concepts

Nitrogen Estimation

Kjeldahl's method is a classical technique used to determine the nitrogen content in organic compounds. It is particularly useful in food and feed analysis, where knowing the protein content is crucial. This method involves digesting the organic material in concentrated sulfuric acid, breaking down the structure and releasing nitrogen. The nitrogen is then converted into ammonium ions, which are further transformed into ammonium sulfate.

• Kjeldahl's method is widely applied in agriculture, food, and environmental science studies.
• The method is known for its precision and accuracy in quantifying nitrogen content.
• Understanding nitrogen estimation is essential for calculating protein levels as nitrogen makes up a significant part of protein molecules.

Through the Kjeldahl method, we gain accurate insights into nitrogen levels, directly influencing assessments of nutritional values in various materials.

Catalysts in Chemistry

The use of catalysts in chemistry is crucial for accelerating reactions that would otherwise be too slow or inefficient. In Kjeldahl's method, catalysts such as copper sulfate or mercury play a significant role in ensuring the complete conversion of nitrogen into ammonium sulfate. Catalysts work by lowering the activation energy needed for a reaction to proceed. This means that the reactions can occur at a faster rate and under milder conditions, saving both time and energy. In the Kjeldahl method:

• Catalysts enhance the digestion of organic material in sulfuric acid.
• Their presence ensures the efficient breakdown of nitrogen components.
• They help in achieving complete and accurate conversion, which is vital for precise nitrogen estimation.

Without catalysts, the process would be significantly less efficient, highlighting their importance in chemical processes.

Ammonium Sulfate Formation

Ammonium sulfate formation is a key step in the Kjeldahl method. During the digestion process, organic nitrogen is transformed into ammonium ions by interacting with sulfuric acid. These ammonium ions then bind with sulfate ions to form ammonium sulfate, a white crystalline solid.

• Ammonium sulfate acts as the main indicator for nitrogen presence in the sample.
• The entire process is critical for converting all nitrogen into a measurable form.
• This allows for subsequent titration to accurately determine the nitrogen content.

The formation of ammonium sulfate is a cornerstone of the Kjeldahl method, without which accurate nitrogen estimation would not be possible.

Boiling Point Elevation with Potassium Sulfate

Potassium sulfate is often used in the Kjeldahl method to increase the boiling point of the sulfuric acid being used for digestion. This technique is known as boiling point elevation. By adding potassium sulfate, the solution can reach higher temperatures without boiling, allowing for a more thorough and efficient reaction.

• This elevated boiling point aids in the complete digestion of the organic sample.
• Higher temperatures facilitate the breakdown of tough organic bonds, ensuring all nitrogen is released.
• The use of potassium sulfate thus leads to a more effective and quicker digestion process.

Overall, potassium sulfate helps optimize the conditions under which organic samples are digested, making it a crucial component in nitrogen estimation through Kjeldahl's method.