Qualitative Analysis : Zero Group Cation ($$\text{NH}_4^+$$)

Formula / Core Reaction:

$$\text{NH}_4^+ + \text{OH}^- \xrightarrow{\Delta} \text{NH}_3 \uparrow + \text{H}_2\text{O}$$

$$2\text{K}_2[\text{HgI}_4] + \text{NH}_3 + 3\text{KOH} \rightarrow \text{HgO} \cdot \text{Hg(NH}_2)\text{I} \downarrow \text{(Brown)} + 7\text{KI} + 2\text{H}_2\text{O}$$

Concept: Heating ammonium salts with strong bases ($$\text{NaOH}$$) evolves ammonia gas. Ammonia reacts with Nessler's reagent ($$\text{K}_2[\text{HgI}_4]$$ in alkaline solution) to produce a characteristic brown precipitate of iodide of Millon's base.

Notation / Key Observation:

• $$\text{K}_2[\text{HgI}_4]$$: Nessler's reagent;
• $$\text{HgO} \cdot \text{Hg(NH}_2)\text{I}$$: Iodide of Millon's base (Brown precipitate / coloration).