Why NH4Cl is added in precipitating III group hydroxides before the addition of NH4OH?
To prevent precipitation of IV group hydroxides (especially Mn) along with III group hydroxides. NH4Cl decreases dissociation of NH4OH and thus limited OH ions are present in solution to precipitate III group cations only NH4OH ⇌ NH4 (aq) + OH (aq) to a small extent NH4Cl (aq) ⇌ NH4 (aq) + Cl (aq) to a large extent Due to common ion affect the degree of dissociation of NH4OH decreases leaving only a small no. of OH̅ ions.
