Sodium fusion extract of thiourea is prepared using excess sodium, freshly prepared $FeS{O}_4$ is now added to this Lassaigne’s extract and acidified with conc. $H_{2}S{O}_4$.  the colour of the product formed is

$N{H}_2-\stackrel{S}{\stackrel{\left|\right|}{C}}-N{H}_2$ contains both ‘N’ is ‘S’

Its Lassaigne’s extract must contain NaSCN but since we used execss NaSCN decomposer.

$2NaSCN\to N{a}_{2}S+NaCN$

Thus

$SC{N}^{-}+F{e}^{3+}\to \underset{\mathrm{Re}d}{{\left[Fe\left(SCN\right)\right]}^{2+}}$ is not

possible

$\underset{excess}{NaCN}\stackrel{FeS{O}_4}{\to }N{a}_4\left[Fe{\left(CN\right)}_6\right]$

$F{e}^{3+}+{\left[Fe{\left(CN\right)}_6\right]}^{4-}\to F{e}_4\left[Fe{\left(CN\right)}_6\right]$.... is formed