Question

A student was given the compound $H_{2}N-C_{6}H_{4}-SO_{3}H$ for elemental analysis. While performing Lassaigne’s test for N, what colour will he get and why?

Answer 1

Lassaigne's test or sodium fusion test is done to identify the presence of "halogens", "phosphorous", "nitrogen", and "sulphur" in organic compounds.

When Lassaigne's extract is treated with the organic compound, it will produce a Prussian blue precipitate if it has nitrogen in it.

When the student tests the compound $H_{2} N-C_{6}H_{4}SO_{3}H$ for elemental analysis by the Lassaigne's test for N, it does not produce a blue precipitate but in turn produces a blood-red precipitate.

This is because of the presence of both nitrogen and sulphur in the compound and NaCN is not formed and free cyanide ions are also absent. The red colour is due to the "formation" of sodium thiocyanate.

Answer 2

Lassaigne's test or sodium fusion test is done to identify the presence of"halogens", "phosphorous", "nitrogen", and "sulphur" in organic compounds.

When Lassaigne's extract is treated with the organic compound, it will produce a Prussian blue precipitate if it has nitrogen in it.

When the student tests the compound H_{2} N-C_{6}H_{4}SO_{3}HH2​N−C6​H4​SO3​H for elemental analysis by the Lassaigne's test for N, it does not produce a blue precipitate but in turn produces a blood-red precipitate.

This is because of the presence of both nitrogen and sulphur in the compound and NaCN is not formed and free cyanide ions are also absent. The red colour is due to the "formation" of sodium thiocyanate.