How do we differentiate between `Fe^(3+)` and `Cr^(3+)` ions in group III ?

Options

A. By adding excess of `NH_(4)OH` solution
B. By increasing `NH_(4)^(+)` ion concentration
C. By decreasing `OH^(-)` ion concentration
D. Both (b) and (c) are correct

Explanation

To differentiate between `Fe^(3+)` and `Cr^(3+)` ions in group III, we can follow these steps:

### Step-by-step Solution:

1. **Preparation of the Solution**:
- Start by preparing a solution containing the ions `Fe^(3+)` and `Cr^(3+)`. This is typically done by dissolving the salts of these ions in water.

2. **Addition of Ammonium Chloride (NH4Cl)**:
- Add ammonium chloride (NH4Cl) to the solution. This salt helps to maintain a controlled concentration of ammonium ions in the solution.

3. **Addition of Ammonium Hydroxide (NH4OH)**:
- Gradually add ammonium hydroxide (NH4OH) to the solution. The NH4OH will dissociate to provide hydroxide ions (OH^-) which will react with the metal ions.

4. **Precipitation of Hydroxides**:
- As you add NH4OH, both `Fe^(3+)` and `Cr^(3+)` will precipitate as their respective hydroxides:
- `Fe^(3+)` will precipitate as `Fe(OH)3`.
- `Cr^(3+)` will precipitate as `Cr(OH)3`.

5. **Observation of Precipitation**:
- Observe the order of precipitation. Due to the differences in solubility products (Ksp) of the hydroxides:
- `Fe(OH)3` has a lower Ksp (1.1 x 10^-36) compared to `Cr(OH)3` (3.0 x 10^-29). This means that `Fe(OH)3` will precipitate first when the concentration of OH^- ions is low.

6. **Increasing OH^- Concentration**:
- After the initial precipitation, continue to add NH4OH to increase the concentration of OH^- ions. This will eventually lead to the precipitation of `Cr(OH)3` after `Fe(OH)3` has already formed.

7. **Final Differentiation**:
- The presence of `Fe(OH)3` before `Cr(OH)3` indicates the presence of `Fe^(3+)` ions. If you observe the precipitation of `Cr(OH)3` after `Fe(OH)3`, you can confirm the presence of both ions.

### Conclusion:
By controlling the concentration of hydroxide ions and observing the order of precipitation, we can differentiate between `Fe^(3+)` and `Cr^(3+)` ions in group III.