Qualitative analysis: Group 4 Cations ($$\text{Zn}^{2+}, \text{Ni}^{2+}$$)

Formula / Core Reaction:

$$\text{Zn}^{2+} + \text{H}_2\text{S} \xrightarrow{\text{NH}_4\text{OH}} \text{ZnS} \downarrow \text{(Off-White / Grey)}$$

$$\text{Ni}^{2+} + 2\text{DMG} + 2\text{NH}_4\text{OH} \rightarrow \text{Ni(DMG)}_2 \downarrow \text{(Cherry Red)} + 2\text{NH}_4^+ + 2\text{H}_2\text{O}$$

Concept: High $$\text{S}^{2-}$$ concentration provided by $$\text{H}_2\text{S}$$ in an alkaline medium ($$\text{NH}_4\text{OH}$$) exceeds the higher $$K_{sp}$$ of Group 4 sulfides. Nickel is confirmed using Dimethylglyoxime (DMG), yielding a bright red chelating complex.

Notation / Key Observation:

• Group Reagent: $$\text{H}_2\text{S}$$ gas in presence of $$\text{NH}_4\text{OH}$$ and $$\text{NH}_4\text{Cl}$$;
• $$\text{ZnS}$$: Zinc sulfide (Off-white/dirty white precipitate);
• $$\text{Ni(DMG)}_2$$: Nickel dimethylglyoximate complex (Brilliant cherry-red precipitate).