Freshly prepared solution of sodium nitroprusside is added to the sodium extract. Appearance of a deep violet colour indicates the presence of

Solution :
$$N{a_2}S + \mathop {N{a_2}\left[ {Fe{{\left( {CN} \right)}_5}\left( {NO} \right)} \right]}\limits_{{\text{Sod}}{\text{. nitroprusside}}} \to $$       \[\underset{\begin{smallmatrix} \text{Sod}\text{. thionitroprusside} \\ \text{(Violet)} \end{smallmatrix}}{\mathop{N{{a}_{4}}\left[ Fe{{\left( CN \right)}_{5}}\left( NOS \right) \right]}}\,\]