Addition of ${\mathrm{FeCl}}_3$ to ${\mathrm{K}}_4\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]$ in dilute and cold solution gives a solution, prussian blue solution ${\mathrm{Fe}}_4{\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]}_3$.

The reaction is as follows:

$4{\mathrm{FeCl}}_3 + 3{\mathrm{K}}_4\left[\mathrm{Fe}\right(\mathrm{CN}{)}_6] \to {\mathrm{Fe}}_4 [\mathrm{Fe}(\mathrm{CN}{)}_6 {]}_3 + 12\mathrm{KCl}$