Question

Detection of sulphur in sodium extract (Lassaigne test) is not done by which of the following reagents?
(a).\[Pb{{(OAC)}_{2}}\]
(b).sodium nitroprusside
(c).Both\[Pb{{(OAC)}_{2}}\] and sodium nitroprusside
(d).Silver nitrate

Answer 1

Hint: If sulphur is present in a mixture; it can form its sulphide with sodium while preparing sodium extract. This can be helpful for the detection of sulphur.

Complete answer:
The Lassaigne’s Test, also known as sodium fusion test, is a test used for elemental analysis for qualitative determination of presence of elements such as nitrogen, sulphur and halogens.
To confirm the presence of sulphur in a mixture, its sodium fusion extract is prepared.
-When sodium fusion extract is reacted with lead acetate, the reaction is as follows
\[N{{a}_{2}}S+{{(OAc)}_{2}}Pb\to PbS+2AcONa\]
If sulphur is present, when sodium extract is made, it forms sodium sulphide, which when reacted with lead acetate we get a black precipitate of lead sulphide. The formation of black precipitate confirms the presence of sulphur.
-When sodium fusion extract is reacted with sodium nitroprusside, the reaction is as follows
\[N{{a}_{2}}S+N{{a}_{2}}[Fe{{(CN)}_{5}}(NO)]\to \underset{violet colour}{\mathop{N{{a}_{4}}[Fe{{(CN)}_{5}}(NOS)]}}\,\]
If sulphur is present, the sodium sulphide of the extract will react with sodium nitroprusside and form a complex which is violet in colour. This violet coloured solution confirms the presence of sulphur in the mixture.
Hence, sulphur can be detected by using lead acetate as well as sodium nitroprusside and

The correct option is (c).

Additional Information:
In order to confirm the presence of sulphur in a mixture, first we have to prepare sodium fusion extract using the mixture. To prepare such an extract, we need to take a piece of sodium metal in an ignition tube, and melt it by direct heating. To this melted sodium, add the pinch of mixture to it. Now again heat the mixture, till the ignition tube turns red hot. This is when the sodium and the mixture would have fused. Now break the tube into a china dish containing water and then reduce the volume of water by boiling it for a while. This solution is sodium fusion extract.

Note: Silver nitrate will not be helpful for the detection of sulphur. But with sodium carbonate we can confirm the presence of sulphur due to the formation of sodium sulphate.