Simultaneous Detection Of Nitrogen And Sulphur

When an organic compound contains both nitrogen and sulphur, the outcome of Lassaigne's test depends on the amount of sodium used. If sodium is insufficient for complete fusion, nitrogen and sulphur combine to form sodium thiocyanate, NaSCN, rather than separate sodium cyanide and sodium sulphide. On adding ferric ions to this extract, the thiocyanate ion reacts to give ferric thiocyanate, Fe(SCN)3, which produces a characteristic blood-red colouration. This is why a deficiency of sodium gives a red colour instead of the usual Prussian blue. The Prussian blue option is incorrect here because that forms only when excess sodium decomposes the thiocyanate into separate cyanide and sulphide ions, giving the blue ferric ferrocyanide. The violet nitroprusside complex indicates free sulphide ion, which is not the species formed when thiocyanate dominates. Silver thiocyanate as a white precipitate is not the observation associated with the ferric ion test described. This sodium-dependence is an important NCERT subtlety often tested in JEE Advanced. Plausibility check: with limited sodium, the combined SCN- ion logically yields the blood-red ferric thiocyanate rather than the products of complete fusion.