An aqueous solution of an unknown salt is made slightly alkaline by...

The addition of dimethylglyoxime (DMG) in an ammoniacal medium is a specific test for the $\text{Ni}^{2+}$ ion in qualitative analysis.

When DMG is added to a slightly basic solution containing $\text{Ni}^{2+}$ ions, it forms a brilliant rosy red precipitate of bis(dimethylglyoximato)nickel(II).

$\text{Ni}^{2+} + 2\text{DMG} \xrightarrow{\text{NH}_4\text{OH}} [\text{Ni}(\text{DMG})_2]\downarrow$ (red precipitate)

Hence, the cation present is $\text{Ni}^{2+}$.

Answer: $\text{Ni}^{2+}$