A sodium fusion extract of thiourea is prepared using excess of sodium. This Lassaigne's extract is made alkaline and freshly prepared ${\mathrm{FeSO}}_4$is added followed by conc. ${\mathrm{H}}_2{\mathrm{SO}}_4$. The colour observed is

Thiourea is ${\mathrm{H}}_2\mathrm{N}-\stackrel{\underset{\left|\right|}{\mathrm{S}}}{\mathrm{C}}-{\mathrm{NH}}_2$;

It is an organic compound containing nitrogen and sulphur. In sodium fusion extract, NaSCN is formed

I) Expected reaction and colour;

$$\begin{gathered} \mathrm{conc}. {\mathrm{H}}_2{\mathrm{SO}}_4 \mathrm{oxidizes} {\mathrm{Fe}}^{+2}\left[{\mathrm{FeSO}}_4\right] \mathrm{to} {\mathrm{Fe}}^{+3}. \\ {\mathrm{Fe}}^{+3}+{\mathrm{SCN}}^{-}\stackrel{ }{\to }{\left[\mathrm{Fe}\left(\mathrm{SCN}\right)\right]}^{+2} \end{gathered}$$

II) Actual reaction and colour

While preparing the extract, excess sodium is used which decomposes NaSCN.

$$\begin{gathered} \therefore \mathrm{NaSCN}+2\mathrm{Na}\stackrel{ }{\to }{\mathrm{Na}}_2\mathrm{S}+\mathrm{NaCN} \\ {\mathrm{Fe}}^{+2}\left({\mathrm{FeSO}}_4\right)+6{\mathrm{CN}}^{-}\left(\mathrm{NaCN}\right)\stackrel{ }{\to }{\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]}^{-4} \\ \mathrm{conc}.{\mathrm{H}}_2{\mathrm{SO}}_4 \mathrm{oxidizes} {\mathrm{Fe}}^{+2} \mathrm{to} {\mathrm{Fe}}^{+3} \\ {\mathrm{Fe}}^{+3}+{\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]}^{-4}\stackrel{ }{\to }\underset{(\mathrm{Prussian} \mathrm{blue})}{\underset{\mathrm{Ferri} \mathrm{ferrocyanide}}{{\mathrm{Fe}}_4{\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]}_3}} \end{gathered}$$