The sodium fusion extract is made alkaline by adding NaOH. To this mixture, freshly prepared FeSO solution is added and boiled for sometimes and then cooled. A few drops of FeCl are added and Prussian blue (bluish green) color forms due to formation of ferric ferrocyanide along with NaCl. This shows the presence of nitrogen in the organic compound.<blockquote><chem>6CN- +Fe^2+ -> [Fe(CN)6]^4-
<blockquote><chem>{3[Fe(CN)6]}^{4-} +4Fe^3+ ->[\ce{xH2O}]Fe4[Fe(CN)6]3\cdot xH2O