Question

In Lassaigne’s test for sulphur, the purple colour formed after the addition of sodium nitroprusside is due to:

• A. $N{a}_4\left[Fe\right(CN{)}_{5}NOS]$
• B. $N{a}_2\left[Fe\right(CN{)}_{5}NOS]$
• C. $N{a}_3\left[Fe\right(CN{)}_{5}S]$
• D. $N{a}_3\left[Fe\right(CN{)}_6]$

Answer 1

The correct option is A $N{a}_4\left[Fe\right(CN{)}_{5}NOS]$

The purple colour is formed due to formation of $N{a}_4\left[Fe\right(CN{)}_{5}NOS]$ as per the reaction:
$S^{2-}+\left[Fe\right(CN{)}_{5}NO{]}^{2-}\to \left[Fe\right(CN{)}_{5}NOS{]}^{4-}$