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Carbon and nitrogen on Fusion with sodium metal give sodium cyanide Na CN.
It is soluble in water. Na CN converted into sodium ferrocyanide by adding of ferrous sulphate. Ferric ions generated (during this process) react with ferrocyanide to form prussian blue precipitate of ferric ferro-cyanide.
    Na + C + N → NaCN

    6NaCN + FeSO4   Na4[Fe(CN)6] +      Na2SO4  
                                 Sodium ferrocyanide

    Na4[Fe(CN)6] + Fe3+ Fe4[Fe(CN)6]3                                                                                                                 Ferric ferrocyanide