In the precipitation of the iron group (III) in qualitative analysis, ammonium chloride is added before adding ammonium hydroxide to :

In the qualitative analysis of basic radicals (cations), Group III includes cations like iron (${\mathrm{Fe}}^{3+}$), aluminum (${\mathrm{Al}}^{3+}$), and chromium (${\mathrm{Cr}}^{3+}$). These cations are precipitated as their respective hydroxides by adding a weak base, ammonium hydroxide (${\mathrm{NH}}_4\mathrm{OH}$).

Ammonium hydroxide dissociation in water is represented by the equilibrium:
${\mathrm{NH}}_4\mathrm{OH}\rightleftharpoons {{\mathrm{NH}}_4}^{+}+{\mathrm{OH}}^{-}$
This dissociation is relatively weak, but it still yields a significant concentration of hydroxide ions (${\mathrm{OH}}^{-}$).

To selectively precipitate only the Group III cations without precipitating cations of higher groups (like Group IV or V, such as zinc, manganese, nickel, cobalt, etc., which have higher solubility products for their hydroxides), the concentration of ${\mathrm{OH}}^{-}$ ions in the solution must be controlled and kept low.

To achieve this, a strong electrolyte containing a common ion, ammonium chloride (${\mathrm{NH}}_4\mathrm{Cl}$), is added before adding ammonium hydroxide. Ammonium chloride dissociates completely:
${\mathrm{NH}}_4\mathrm{Cl}\to {{\mathrm{NH}}_4}^{+}+{\mathrm{Cl}}^{-}$
The large concentration of ammonium ions (${{\mathrm{NH}}_4}^{+}$) from the dissociation of ${\mathrm{NH}}_4\mathrm{Cl}$ shifts the equilibrium of ${\mathrm{NH}}_4\mathrm{OH}$ ionization backward to the left, in accordance with the Common Ion Effect.

This backward shift significantly suppresses the ionization of ammonium hydroxide, thereby decreasing the concentration of hydroxide (${\mathrm{OH}}^{-}$) ions. The lowered hydroxide ion concentration is sufficient to exceed the low solubility product ($K_{\mathrm{sp}}$) values of Group III metal hydroxides, but is too low to exceed the higher solubility product values of subsequent group hydroxides, preventing their unwanted premature precipitation.

Therefore, ammonium chloride is added to decrease the concentration of hydroxide (${\mathrm{OH}}^{-}$) ions.