${\mathrm{Fe}}^{3+}$ cation gives a prussian blue precipitate on addition of potassium ferrocyanide solution due to the formation of

Ferric ion forms a prussian blue precipitate due to the formation of $F{e}_4{\left[Fe{\left(CN\right)}_6\right]}_3$ . In this method, to a portion of sodium fusion extract, freshly prepared ferrous sulphate $FeS{O}_4$ solution is added and warmed. Then about $2$ to $3$ drops of $FeC{l}_3$ solution are added and acidified with conc. $HCl$ . The appearance of a prussian blue colour indicates the presence of nitrogen.

$FeS{O}_4+2NaOH\to Fe{\left(OH\right)}_2+N{a}_{2}S{O}_4$

$6NaCN+Fe{\left(OH\right)}_2\to \underset{\text{Sodium}\text{ferrocyanide}}{N{a}_4\left[Fe{\left(CN\right)}_6\right]}+2NaOH$

$3N{a}_4\left[Fe{\left(CN\right)}_6\right]+4FeC{l}_3\to \underset{\begin{array}{l}\text{Ferric ferrocyanide}\\ \left(\text{prussian}\text{blue}\right)\end{array}}{F{e}_4{\left[Fe{\left(CN\right)}_6\right]}_3}+12NaCl$