In the systematic precipitation of Group III basic radicals (), the group reagent is added in the pr — Qualitative and Quantitative Analysis Chemistry Question
Question
In the systematic precipitation of Group III basic radicals ($Fe^{3+}, Al^{3+}, Cr^{3+}$), the group reagent is $NH_4OH$ added in the presence of solid $NH_4Cl$. What is the primary chemical function of adding $NH_4Cl$?
💡 Solution & Explanation
$NH_4OH$ is a weak base that ionizes to give $NH_4^+$ and $OH^-$. The addition of $NH_4Cl$ (a strong electrolyte) provides a high concentration of $NH_4^+$ ions. Due to the common ion effect, the ionization equilibrium of $NH_4OH$ is shifted to the left, suppressing it. The resulting low concentration of $OH^-$ is sufficient to exceed the very low $K_{sp}$ of Group III hydroxides ($Fe(OH)_3, Al(OH)_3, Cr(OH)_3$) but insufficient to precipitate the hydroxides of higher groups (like Mg, Ca, Zn) which have higher $K_{sp}$ values.