How do we differentiate between ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Cr}}^{3+}$in group III rd ?

Increased ${\mathrm{NH}}_{4+}$ ions prevent the release of $\mathrm{OH}-$ ions, resulting in the solubility of the $\mathrm{Fe}{\left(\mathrm{OH}\right)}_3$ product. Precipitation varies in color.

Thus, By increasing ${\mathrm{NH}}_4^{+}$ ion concentration and by decreasing ${\mathrm{OH}}^{-}$ ion concentration differentiate between ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Cr}}^{3+}$in group III rd.