Presence of Nitrogen in Acidified Lassaigne Solution

To understand why the blood red coloration indicates the presence of nitrogen in acidified Lassaigne solution, we need to examine the chemical reactions involved.

1. Formation of Sodium Fusion Extract:
Lassaigne's test is used to detect the presence of nitrogen and sulfur in an organic compound. The compound is first heated with sodium metal, which forms a fusion extract containing sodium derivatives of the elements present in the compound.

2. Acidification of Sodium Fusion Extract:
The fusion extract is dissolved in water and then acidified with dilute HNO3. This step helps to convert the sodium derivatives into their respective acids.

3. Reaction with FeCl3:
After acidification, FeCl3 solution is added to the acidified Lassaigne solution. The addition of FeCl3 is crucial in detecting the presence of nitrogen.

Explanation of the Blood Red Coloration:

When FeCl3 is added to the acidified Lassaigne solution, a blood red coloration is observed. This coloration is indicative of the presence of nitrogen compounds, specifically nitro groups (NO2-).

The blood red coloration is due to the formation of a complex between FeCl3 and the nitro group. The FeCl3 oxidizes the nitro group to form a stable complex called ferric nitroso-ortho-sulfonate. This complex has a characteristic blood red color.

FeCl3 + NO2- → [Fe(III)-NO2]2- (blood red complex)

Since the nitro group is derived from nitrogen, the blood red coloration confirms the presence of nitrogen in the acidified Lassaigne solution.

Significance of Acidification:

Acidification of the Lassaigne solution is important in this test because it converts the sodium derivatives of nitrogen compounds into their corresponding acids. This step allows for the reaction between FeCl3 and the nitro group to occur, resulting in the blood red coloration.

Conclusion:

The blood red coloration produced upon adding FeCl3 to acidified Lassaigne solution indicates the presence of nitrogen compounds, specifically nitro groups. Acidification of the solution is necessary to convert the sodium derivatives into their respective acids, enabling the reaction with FeCl3 to occur. This test is widely used in organic chemistry to detect the presence of nitrogen in organic compounds.