During Lassaigne's test for the detection of sulphur in an organic...

During Lassaigne's test for sulphur, the sodium fusion extract contains sodium sulphide ($\mathrm{Na}_2\mathrm{S}$).

When sodium nitroprusside, $\mathrm{Na}_2\left[\mathrm{Fe}(\mathrm{CN})_5 \mathrm{NO}\right]$, is added to this extract, it reacts with the sulphide ions to form a deep purple coloured complex.

The chemical reaction is given by:
$\mathrm{Na}_2\mathrm{S} + \mathrm{Na}_2\left[\mathrm{Fe}(\mathrm{CN})_5 \mathrm{NO}\right] \rightarrow \mathrm{Na}_4\left[\mathrm{Fe}(\mathrm{CN})_5 \mathrm{NOS}\right]$

Thus, the correct formula of the purple complex is $\mathrm{Na}_4\left[\mathrm{Fe}(\mathrm{CN})_5 \mathrm{NOS}\right]$.

Answer: \mathrm{Na}_4\left[\mathrm{Fe}(\mathrm{CN})_5 \mathrm{NOS}\right]