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

Step 1: Understand the group classification in qualitative analysis

Group III consists of metal cations like $\ce{Fe^{3+}}$, $\ce{Al^{3+}}$, and $\ce{Cr^{3+}}$. To identify these ions, they are precipitated as their hydroxides using ammonium hydroxide ($\ce{NH4OH}$), which is a weak base, in the presence of solid ammonium chloride ($\ce{NH4Cl}$).

Step 2: Note the ionization behavior of the reagents

Ammonium hydroxide ionizes partially: $\ce{NH4OH <=> NH4+ + OH-}$

When we add ammonium chloride ($\ce{NH4Cl}$), a strong electrolyte, it completely dissociates: $\ce{NH4Cl -> NH4+ + Cl-}$

Step 3: Analyze the common ion effect

The large supply of ammonium ($\ce{NH4+}$) ions from $\ce{NH4Cl}$ forces the equilibrium of the weak base $\ce{NH4OH}$ to shift to the left, obeying Le Chatelier's Principle.

This dramatically suppresses the ionization of $\ce{NH4OH}$, thereby decreasing the concentration of $\ce{OH-}$ ions in the solution to a very low selected level.

Step 4: Understand the desired outcome

Since the solubility products ($K_{sp}$) of Group III hydroxides ($\ce{Fe(OH)3}$, $\ce{Al(OH)3}$, $\ce{Cr(OH)3}$) are extremely low, even this depressed $\ce{OH-}$ concentration is geometrically sufficient to exceed their ionic product, causing them to precipitate. Meanwhile, the hydroxides of higher groups like Group IV ($\ce{Mn^{2+}}$, $\ce{Zn^{2+}}$, $\ce{Ni^{2+}}$, $\ce{Co^{2+}}$) have larger $K_{sp}$ values, and the reduced number of $\ce{OH-}$ ions fails to reach their precipitation threshold. This allows a clear separation.

Step 5: Conclusion

The primary chemical purpose of adding ammonium chloride is to decrease the concentration of hydroxide ($\ce{OH-}$) ions.

Key Points to Remember:

• Addition of a strong electrolyte offering a common ion to a weak electrolyte suppresses its ionization. This is the Common Ion Effect.
• In Group III analysis, $\ce{NH4Cl}$ suppresses $\ce{NH4OH}$ to prevent the unwanted precipitation of subsequent group cations.