The group reagent for the precipitation of group III cations ($\mathrm{Fe}{}^{3+}$, $\mathrm{Al}{}^{3+}$, $\mathrm{Cr}{}^{3+}$) as hydroxides is

Group III hydroxides have very small solubility products and are precipitated by $\mathrm{NH}{}_4\mathrm{OH}$ in the presence of $\mathrm{NH}{}_4\mathrm{Cl}$. Excess $\mathrm{NaOH}$ would redissolve amphoteric $\mathrm{Al}(\mathrm{OH}){}_3$ and $\mathrm{Cr}(\mathrm{OH}){}_3$, and $\mathrm{H}{}_2\mathrm{S}$ in ammoniacal medium is the group IV reagent.