Question

The formula of the purple colour formed in Laissaigne’s test for sulphur using sodium nitroprusside is

• A. NaFe[Fe(CN)₆]
• B. Na[Cr(NH₃)₂(NCS)₄]
• C. Na₂[Fe(CN)₅(NO)]
• D. Na₄[Fe(CN)₅(NOS)]

Answer 1

The Correct Option is A

In Laissaigne’s test, which is used to detect the presence of elements such as sulfur in organic compounds, sodium nitroprusside is used to detect sulfur.

1. When sulfur is present in the organic compound, it reacts with sodium to form sodium sulfide (Na₂S) during the fusion process.
2. The sodium sulfide formed reacts with sodium nitroprusside (Na₂[Fe(CN)₅NO]) to give a purple color. The reaction is as follows: \text{Na}_2\text{S} + \text{Na}_2[\text{Fe}(\text{CN})_5\text{NO}] \rightarrow \text{Na}_4[\text{Fe}(\text{CN})_5\text{NOS}]

The purple color in Laissaigne’s test for sulfur arises due to the formation of the complex ion Na₄[Fe(CN)₅(NOS)]. This is a common test for sulfur in organic qualitative analysis by observing the purple color complex.

However, it seems there might be a mistake in the provided solution as the known product of this reaction should be Na₄[Fe(CN)₅(NOS)], which gives the purple color indicating the presence of sulfur. This corresponds to one option: Na₄[Fe(CN)₅(NOS)] in typical solutions.

Hence, the correct formula for the purple color formed in Laissaigne’s test for sulfur using sodium nitroprusside should be Na₄[Fe(CN)₅(NOS)], not NaFe[Fe(CN)₆] as per typical preparatory materials.