To solve the problem of avoiding the precipitation of hydroxides of Ni²⁺, Co²⁺, Zn²⁺, and Mn²⁺ along with those of Fe³⁺, Al³⁺, and Cr³⁺, we can follow these steps:
### Step 1: Understand the Groups of Cations
- **Group 3 Cations**: Fe³⁺, Al³⁺, Cr³⁺
- **Group 4 Cations**: Ni²⁺, Co²⁺, Zn²⁺, Mn²⁺
### Step 2: Identify the Hydroxide Precipitation
- Hydroxides of Group 3 cations (Fe³⁺, Al³⁺, Cr³⁺) have lower solubility products compared to those of Group 4 cations (Ni²⁺, Co²⁺, Zn²⁺, Mn²⁺). This means that under certain conditions, Group 3 hydroxides will precipitate before Group 4 hydroxides.
### Step 3: Use of Ammonium Chloride (NH₄Cl)
- To prevent the precipitation of hydroxides of Group 4 cations while allowing the Group 3 cations to remain in solution, we can add **excess NH₄Cl** to the solution.
### Step 4: Common Ion Effect
- When NH₄Cl is added, it dissociates into NH₄⁺ and Cl⁻ ions. The NH₄⁺ ion will affect the dissociation of NH₄OH (ammonium hydroxide) in the solution.
- The presence of NH₄⁺ (common ion) reduces the degree of dissociation of NH₄OH, which means that the concentration of OH⁻ ions in the solution is limited.
### Step 5: Control of Hydroxide Ion Concentration
- By controlling the concentration of OH⁻ ions through the common ion effect, we can ensure that the concentration of OH⁻ is just sufficient to reach the solubility product constant of the Group 3 hydroxides, preventing their precipitation while keeping the Group 4 cations in solution.
### Conclusion
- Therefore, to avoid the precipitation of hydroxides of Ni²⁺, Co²⁺, Zn²⁺, and Mn²⁺ along with those of Fe³⁺, Al³⁺, and Cr³⁺, the solution should be treated with **excess NH₄Cl**.
### Final Answer
The solution should be treated with excess NH₄Cl.
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