Question

In Kjeldahl method for estimation of milk protein, which among the following is used as a digestion mixture?

• A. Copper sulphate and sodium tartarate
• B. Copper sulphate and potassium chromate
• C. Copper sulphate and potassium iodide
• D. Copper sulphate and potassium sulphate

Hint:

In the Kjeldahl digestion mixture, potassium sulphate functions as a boiling point elevator, while copper sulphate acts as a catalyst to speed up the acid digestion of organic nitrogen.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
The Kjeldahl method is the international reference standard for determining the nitrogen and protein content of milk and dairy products.
The process consists of three main stages: digestion, distillation, and titration.
The digestion stage involves heating the sample with concentrated sulfuric acid to oxidize organic matter and convert organic nitrogen into ammonium sulfate.

Step 2: Detailed Explanation:
During the digestion stage, a specific digestion mixture is added to concentrated sulfuric acid (\(\text{H}_2\text{SO}_4\)) to optimize the reaction:
Potassium sulphate (\(\text{K}_2\text{SO}_4\)): This salt is added to raise the boiling point of the concentrated sulfuric acid from its native \(337^{\circ}\text{C}\) to a range between \(370^{\circ}\text{C}\) and \(400^{\circ}\text{C}\).
The higher temperature increases the rate of organic matter decomposition.
Copper sulphate (\(\text{CuSO}_4\)): This salt acts as a catalyst to accelerate the chemical oxidation of organic carbon and the subsequent conversion of nitrogen to ammonium sulfate.
Other salts, such as sodium tartarate, potassium chromate, or potassium iodide, do not raise the acid boiling point effectively or can cause interfering side reactions, making them unsuitable for this standardized digestion process.

Step 3: Final Answer:
Consequently, a mixture of copper sulphate and potassium sulphate is used as the digestion mixture in the Kjeldahl method.