JEE Main · 2022 · 28 Jun 2022 · Shift 1 · Practical and Analytical Chemistry

In Lassaigne's test for sulphur, the organic compound is fused with sodium metal to convert sulphur to sodium sulphide ($\mathrm{Na}{}_2\mathrm{S}$). When this extract is treated with sodium nitroprusside ($\mathrm{Na}{}_2[\mathrm{Fe}(\mathrm{CN}){}_5\mathrm{NO}]$), a purple/violet coloration appears due to the formation of $\mathrm{Na}{}_4[\mathrm{Fe}(\mathrm{CN}){}_5\mathrm{NOS}]$. The reaction is: $\mathrm{Na}{}_2\mathrm{S}+\mathrm{Na}{}_2[\mathrm{Fe}(\mathrm{CN}){}_5\mathrm{NO}]\to \mathrm{Na}{}_4[\mathrm{Fe}(\mathrm{CN}){}_5\mathrm{NOS}]$ (purple complex).