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 prior to ammonium hydroxide to regulate the concentration of \( \text{OH}^- \) ions. This is accomplished via the common ion effect:

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

The addition of \( \text{NH}_4\text{Cl} \) elevates the concentration of \( \text{NH}_4^+ \) ions, thereby shifting the equilibrium leftward and diminishing the concentration of \( \text{OH}^- \) ions.

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

By reducing the \( \text{OH}^- \) concentration, the formation of precipitates from higher group cations (e.g., Group IV and V cations) is prevented, ensuring the selective precipitation of only Group III cations.

Conclusion:

The addition of ammonium chloride reduces the concentration of \( \text{OH}^- \) ions through the common ion effect, aligning with Option (2).