This is a general qualitative test for detecting sulfur and nitrogen, in most organic molecules. A proper process enables to synthesize Lassaigne’s stock solution that will be used for the various samples under examination. In detail the detected elements are:
If included in the sample, sulfur is present as sodium sulfide. A method for detecting it consists in pouring ten drops of Lassaigne’s stock solution into a well of a white ceramic plate, and introducing a tiny crystal of sodium nitroprusside in the same well without stirring. Coming into contact with a sulfide, the ion of the original reddish complex exchanges the nitrosyl group with sulfide, producing a new soluble complex of very deep magenta-purple colour. Adding some iron(II) sulfate with a spatula will enable to detect nitrogen. A gelatinous precipitate will form immediately and, if some nitrogen is present as cyanide, student can also see the formation of a soluble and colourless, very stable complex, that is sodium ferrocyanide. Then the solution will be acidified with sulfuring acid until pH becomes acid. If there is no cyanide, the gelatinous precipitate will completely be dissolved and the solution becomes colourless. On the contrary, if there is some cyanide, the iron(III) complex is decomposed by the strong acid to free iron(II) ion, whereas the ferrocyanide is kept intact. These two ions form an insoluble chemical compound called Prussian Blue.
THEORETICAL – EXPERIMENTAL HANDBOOK