Qualitative analysis: Group 3 Cations ($$\text{Al}^{3+}, \text{Fe}^{3+}$$)

Formula / Core Reaction:

$$\text{Al}^{3+} + 3\text{NH}_4\text{OH} \xrightarrow{\text{NH}_4\text{Cl}} \text{Al(OH)}_3 \downarrow \text{(Gelatinous White)}$$

$$\text{Fe}^{3+} + 3\text{NH}_4\text{OH} \xrightarrow{\text{NH}_4\text{Cl}} \text{Fe(OH)}_3 \downarrow \text{(Reddish-Brown)}$$

Concept: Precipitated as hydroxides using $$\text{NH}_4\text{OH}$$ in presence of $$\text{NH}_4\text{Cl}$$. $$\text{NH}_4\text{Cl}$$ suppresses $$[\text{OH}^-]$$ via common ion effect, selectively precipitating Group 3 cations with low solubility products ($$K_{sp}$$).

Notation / Key Observation:

• Group Reagent: $$\text{NH}_4\text{OH}$$ in presence of excess $$\text{NH}_4\text{Cl$$;
• $$\text{Al(OH)}_3$$: Gelatinous white precipitate;
• $$\text{Fe(OH)}_3$$: Reddish-brown precipitate.