A reagent which gives brilliant red precipitate with Nickel ions in basic medium is

We need to identify the reagent that gives a brilliant red precipitate with nickel ions ($$Ni^{2+}$$) in basic medium.

Dimethylglyoxime (DMG) is a classic analytical reagent used for the qualitative and quantitative detection of nickel ions.

When dimethylglyoxime reacts with $$Ni^{2+}$$ ions in a basic (ammoniacal) medium, it forms a brilliant red (scarlet) precipitate known as nickel dimethylglyoximate:

$$Ni^{2+} + 2 \text{DMG} \xrightarrow{\text{NH}_4\text{OH}} \underset{\text{(brilliant red ppt)}}{\text{Ni(DMG)}_2}$$

The complex has a square planar geometry, and the red colour arises from the chelation of nickel with the two dimethylglyoxime molecules through the nitrogen atoms, with intramolecular hydrogen bonding between the hydroxyl groups.

Let us check the other options:

• Sodium nitroprusside is used to test for sulfide ions ($$S^{2-}$$), giving a violet colour.
• Neutral FeCl$$_3$$ is used to test for phenols and enols.
• Meta-dinitrobenzene is not a standard reagent for nickel detection.

Therefore, the correct answer is Option 4: Dimethyl glyoxime.