Question

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

• A. Prevent interference by phosphate ions
• B. Decrease concentration of \( \text{OH}^- \) ions
• C. Increase concentration of \( \text{Cl}^- \) ions
• D. Increase concentration of \( \text{NH}_4^+ \) ions

Answer 1

The Correct Option is B

Role of Ammonium Chloride in Precipitation of Group III Cations:

In qualitative analysis, group III cations (such as \( \text{Fe}^{3+}, \text{Cr}^{3+}, \text{Al}^{3+} \)) are precipitated as hydroxides by adding ammonium hydroxide \( \text{NH}_4\text{OH} \).

Effect of Adding Ammonium Chloride:

Ammonium chloride \( \text{NH}_4\text{Cl} \) is added before ammonium hydroxide to control the concentration of \( \text{OH}^- \) ions. This is achieved through the common ion effect:

\[ \text{NH}_4\text{OH} \leftrightarrow \text{NH}_4^+ + \text{OH}^- \]

Adding \( \text{NH}_4\text{Cl} \) increases the concentration of \( \text{NH}_4^+ \) ions, which shifts the equilibrium to the left, decreasing the concentration of \( \text{OH}^- \) ions.

Importance of Controlled \( \text{OH}^- \) Concentration:

By reducing \( \text{OH}^- \) concentration, we avoid the formation of precipitates from cations of higher groups (such as Group IV and V cations), ensuring selective precipitation of Group III cations only.

Conclusion:

The addition of ammonium chloride decreases the concentration of \( \text{OH}^- \) ions through the common ion effect, which corresponds to Option (2).