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

Options

A. Decreases concentration of `OH^(Θ)` ions
B. Prevent interference by phosphate ions
C. increases concentration of `Cl^(Θ)` ions
D. Increases concentration of `NH_(4)^(o+)` ions

Explanation

### Step-by-Step Solution:

1. **Understanding the Context**: In qualitative analysis, the iron group consists of certain metal ions (like Fe³⁺, Al³⁺, and Cr³⁺) that are precipitated as hydroxides. The solubility of these hydroxides is quite low, which is important for their separation from other ions.

2. **Role of Ammonium Chloride (NH4Cl)**: Ammonium chloride is added to the solution before ammonium hydroxide (NH4OH) is introduced. This is crucial for controlling the precipitation process.

3. **Common Ion Effect**: The addition of NH4Cl introduces NH4⁺ ions into the solution. The presence of these common ions (NH4⁺) affects the solubility of the hydroxides due to the common ion effect. This effect reduces the solubility of the hydroxides of iron, chromium, and aluminum.

4. **Decreasing Hydroxide Ion Concentration**: By adding NH4Cl first, the concentration of hydroxide ions (OH⁻) in the solution is effectively decreased. This is important because lower concentrations of OH⁻ ions help ensure that only the desired metal ions (Fe³⁺, Al³⁺, and Cr³⁺) precipitate as their hydroxides.

5. **Conclusion**: The primary reason for adding ammonium chloride before ammonium hydroxide is to decrease the concentration of hydroxide ions, which helps in selectively precipitating the hydroxides of iron, chromium, and aluminum without interference from other ions.

### Final Answer:
The correct reason for adding ammonium chloride before ammonium hydroxide in the iron group precipitate is to **decrease the concentration of hydroxide ions**.