Reagent used to test `Ni^(2+)` ion is (a)_____.

Answer

To determine the reagent used to test for the presence of Ni²⁺ ions, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Ion**: We are testing for the nickel ion, which is represented as Ni²⁺.

2. **Select the Appropriate Reagent**: The reagent used to test for Ni²⁺ ions is dimethylglyoxime (DMG).

3. **Understand the Reaction**: When dimethylglyoxime is added to a solution containing Ni²⁺ ions in an ammoniacal solution (which contains ammonia, NH₄OH), a specific reaction occurs.

4. **Reaction Details**: The nickel ions react with dimethylglyoxime to form a red precipitate known as nickel dimethylglyoxime complex. The overall reaction can be summarized as:
\[ \text{Ni}^{2+} + 2 \text{DMG} + 2 \text{NH}_4\text{OH} \rightarrow \text{Ni(DMG)}_2 \downarrow + 2 \text{NH}_4\text{Cl} + 2 \text{H}_2\text{O} \]
Here, Ni(DMG)₂ is the red precipitate that confirms the presence of nickel ions.

5. **Conclusion**: The formation of a red precipitate indicates the presence of Ni²⁺ ions in the solution.

### Final Answer:
The reagent used to test for Ni²⁺ ions is **Dimethylglyoxime (DMG)**.

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