(a) Nitrogen
The test is done by boiling a portion of the alkaline solution from the solution
fusion with iron(II) sulfate and then acidifying. Sodium cyanide reacts with
iron(II) sulfate to produce ferrocyanide, which combines with iron(III) salts,
inevitably formed by air oxidation in the alkaline solution, to give insoluble
Prussian Blue, NaFe[Fe(CN)6]. Iron(II) and iron(III) hydroxide precipitate along
with the blue pigment but dissolve on acidification.