The following can be used to regulate the concentration of `OH^(-)` ions for the scheme of basic radical analysis (`III` group)

Options

A. `NH_(4)OH`
B. `NH_(4)Cl`
C. `(NH_(4))_(2)SO_(4)`
D. `(NH_(4))_(2)CO_(3)`

Explanation

To solve the question regarding the regulation of the concentration of `OH^(-)` ions for the scheme of basic radical analysis (III group), we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Context**:
- We are dealing with the basic radical analysis of group III, which involves the precipitation of certain metal hydroxides.

2. **Recognize the Role of `OH^(-)` Ions**:
- In this analysis, the concentration of `OH^(-)` ions must be controlled to prevent the precipitation of certain metal cations as hydroxides.

3. **Identify Common Reagents**:
- The primary reagent used in this analysis is ammonium hydroxide (NH4OH). However, to regulate the concentration of `OH^(-)` ions, another reagent is needed.

4. **Introduce Ammonium Chloride (NH4Cl)**:
- Ammonium chloride (NH4Cl) is introduced alongside NH4OH. This is crucial because it provides a source of NH4^+ ions.

5. **Explain the Common Ion Effect**:
- The presence of NH4^+ ions from NH4Cl leads to a common ion effect. This effect shifts the equilibrium of the dissociation of NH4OH:
\[ \text{NH4OH} \rightleftharpoons \text{NH4}^+ + \text{OH}^- \]
- With an increase in NH4^+ concentration, the equilibrium shifts to the left, decreasing the concentration of `OH^(-)` ions.

6. **Conclusion**:
- Therefore, NH4Cl is the correct choice for regulating the concentration of `OH^(-)` ions in the analysis, while NH4OH alone would not effectively regulate the `OH^(-)` concentration.

### Final Answer:
The reagent that can be used to regulate the concentration of `OH^(-)` ions for the scheme of basic radical analysis (III group) is **NH4Cl**.

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