The use of `NH_4Cl` in the detection of third group radicals is to

Options

A. decrease the solubility of the hydroxides of the group III cations
B. counter the activity of any interfering anions
C. prevent the precipitation of group IV cations as hydroxides
D. ensure complete precipitation of the third group cations.

Explanation

To solve the question regarding the use of `NH4Cl` in the detection of third group radicals, we can break down the explanation into clear steps:

### Step-by-Step Solution:

1. **Understanding the Context**:
- The question pertains to the detection of third group radicals in qualitative analysis. Third group radicals typically include ions such as Fe³⁺, Cr³⁺, and Al³⁺.

2. **Role of Ammonium Hydroxide (NH4OH)**:
- Ammonium hydroxide is used to provide hydroxide ions (OH⁻) in the solution. This is crucial for the precipitation of hydroxides of the third group radicals.

3. **Role of Ammonium Chloride (NH4Cl)**:
- NH4Cl acts as an electrolyte and dissociates into ammonium ions (NH4⁺) and chloride ions (Cl⁻) in solution.

4. **Common Ion Effect**:
- The presence of NH4⁺ ions from NH4Cl leads to a decrease in the dissociation of NH4OH. This is due to the common ion effect, which reduces the concentration of hydroxide ions (OH⁻) in the solution.

5. **Precipitation of Third Group Hydroxides**:
- The lower concentration of hydroxide ions favors the precipitation of third group hydroxides (like Fe(OH)₃, Cr(OH)₃, and Al(OH)₃) because these hydroxides have low solubility product (Ksp) values.

6. **Separation from Fourth Group Radicals**:
- Since fourth group radicals (like Zn²⁺, Mn²⁺) have higher Ksp values, they do not precipitate under the same conditions of low hydroxide ion concentration. Thus, the use of NH4Cl helps in selectively precipitating only the third group radicals.

7. **Conclusion**:
- The primary function of NH4Cl in this context is to suppress the dissociation of NH4OH, allowing for the selective precipitation of third group radicals while preventing the precipitation of fourth group radicals.

### Final Answer:
The use of `NH4Cl` in the detection of third group radicals is to suppress the dissociation of ammonium hydroxide, thereby allowing for the selective precipitation of third group hydroxides.

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