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

Options

A. decrease concentration of `OH^(-)` ions
B. prevent interference by phosphate ions
C. increase concentration of `Cl^(-)` ions
D. increase concentration of `NH_(4)^(+)` ions

Answer

To answer the question regarding why ammonium chloride is added before ammonium hydroxide in the precipitation of the iron group in qualitative analysis, we can break down the reasoning step by step.

### Step-by-Step Solution:

1. **Understanding the Iron Group in Qualitative Analysis**:
- The iron group consists of cations such as Fe³⁺, Al³⁺, and Cr³⁺, which are typically precipitated as their hydroxides during qualitative analysis.

2. **Role of Ammonium Chloride (NH₄Cl)**:
- Ammonium chloride is added to the solution before ammonium hydroxide. The purpose of adding NH₄Cl is to provide a source of NH₄⁺ ions.

3. **Common Ion Effect**:
- The presence of NH₄⁺ ions from ammonium chloride leads to the common ion effect. The common ion effect states that the solubility of a salt is reduced when a common ion is added to the solution.

4. **Precipitation of Hydroxides**:
- When ammonium hydroxide (NH₄OH) is added, it dissociates to produce OH⁻ ions. However, the presence of NH₄⁺ ions from NH₄Cl decreases the concentration of free OH⁻ ions in the solution due to the common ion effect.

5. **Selective Precipitation**:
- By reducing the concentration of OH⁻ ions, we ensure that only the cations of the third group (like Fe³⁺) will precipitate as their hydroxides, while other cations that require a higher concentration of OH⁻ for precipitation will remain in solution.

6. **Conclusion**:
- Therefore, the addition of ammonium chloride before ammonium hydroxide is crucial for selectively precipitating the iron group cations while controlling the concentration of hydroxide ions in the solution.

### Final Answer:
Ammonium chloride is added before ammonium hydroxide to decrease the concentration of OH⁻ ions in the solution, thereby facilitating the selective precipitation of iron group cations.