Reaction sequence: Na + C + N → NaCN (in fusion) NaCN + FeSO4 → Fe(CN)2 + Na2SO4 (initial precipitation) Fe(CN)2 + 4NaCN → Na4[Fe(CN)6] (sodium ferrocyanide — excess CN- converts Fe2+ to complex) 3Na4[Fe(CN)6] + 4FeCl3 → Fe4[Fe(CN)6]3 + 12NaCl (Prussian blue precipitate)

The green color of FeSO4 + NaCN initially gives a dirty green precipitate of Fe(OH)2 upon boiling with NaOH. Adding FeCl3 and acidifying with dilute H2SO4 converts it to bright Prussian blue.

Why boil with FeSO4 before adding FeCl3? To first form Na4[Fe(CN)6] (ferrocyanide complex). If FeCl3 is added directly to NaCN solution, it may oxidize CN- to CNO- → no Prussian blue.

False positive: If the organic compound itself contains Fe (organometallic), the test may give misleading color. Rare in JEE context.