First, when the sodium fusion extract reacts with iron(II) sulfate, a chemical reaction happens that forms a compound called sodium ferrocyanide. 6NaCN+FeSO4→Na4[Fe(CN)6]+Na2SO4 Next, when you add concentrated H2SO4, the iron(II) changes to iron(III). This iron(III) reacts with sodium ferrocyanide to make a blue colored compound called Prussian blue. 3Na4[Fe(CN)6]+4Fe3+
xH2O
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Fe4[Fe(CN)6]3â‹…xH2O The blue color (Prussian blue) shows that nitrogen is present in the organic compound.