Question

Nitrogen detection in an organic compound is carried out by Lassaigne's test. The blue colour formed corresponds to which of the following formulae ?

• A. $Fe_3[Fe(CN)_6]_2$
• B. $Fe_4[Fe(CN)_6]_3$
• C. $Fe_4[Fe(CN)_6]_2$
• D. $Fe_3[Fe(CN)_6]_3$

Answer 1

The Correct Option is B

Lassaigne's Test is an important method in qualitative organic analysis used to detect the presence of elements such as nitrogen, sulfur, and halogens in organic compounds. In this procedure, an organic compound is fused with sodium metal to create sodium salts of these elements, which upon further treatment, help in the detection of the specific elements. 

To detect nitrogen, the compound is first fused with sodium to produce sodium cyanide (\(NaCN\)):

C+N \rightarrow NaCN

This resultant sodium cyanide is then treated with ferrous sulfate (\(FeSO_4\)) in an acidic medium. Through a series of reactions, this leads to the formation of ferric ferrocyanide, commonly known as Prussian blue.

The reaction can be illustrated as follows:

• Ferrous sulfate reacts with sodium cyanide to form sodium ferrocyanide:
• In the presence of ferric ions, it converts to a blue complex ferric ferrocyanide, which is Prussian blue:

The blue color observed in Lassaigne's test for nitrogen is primarily due to the formation of \(Fe_4[Fe(CN)_6]_3\), known as Prussian blue.

Therefore, the correct answer is \(Fe_4[Fe(CN)_6]_3\). This complex is responsible for the characteristic blue color indicating the presence of nitrogen in the original organic compound.

In summary, the test reveals nitrogen through a specific sequence of reactions culminating in the formation of Prussian blue, as demonstrated above.