Concentrated aqueous sodium hydroxide can separate a mixture of $\mathrm{A}{\mathrm{l}}^{3+}$ and $\mathrm{F}{\mathrm{e}}^{3+}$

$\mathrm{A}{\mathrm{l}}^{3+}$ forms white gelatinous precipitate of $\mathrm{A}\mathrm{l}{\left(\mathrm{O}\mathrm{H}\right)}_3$ , which is soluble in excess of NaOH.

$\mathrm{A}\mathrm{l}{\left(\mathrm{O}\mathrm{H}\right)}_3+\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}\to \mathrm{N}\mathrm{a}\mathrm{A}\mathrm{l}{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}$

$\mathrm{F}{\mathrm{e}}^{3+}$ forms reddish brown precipitate of $\mathrm{F}\mathrm{e}{\left(\mathrm{O}\mathrm{H}\right)}_3$ , which is insoluble in excess of NaOH.