Ferric ion forms a prussian blue coloured ppt. of :

${\mathrm{Fe}}_4{\left[\mathrm{Fe}(\mathrm{CN}{)}_6\right]}_3$, a ferric ion-forming compound, precipitates as a Prussian blue substance.

This complex is formed as a result of the test used to assess whether nitrogen is present in the sample. In this procedure, newly made ferrous sulphate, FeSO₄ solution is added to a fraction of sodium fusion extract and warmed.