Qualitative Analysis: Group 2 Cation ($$\text{Cu}^{2+}$$)

Formula / Core Reaction:

$$\text{Cu}^{2+} + \text{H}_2\text{S} \xrightarrow{\text{dil. } \text{HCl}} \text{CuS} \downarrow \text{(Black)}$$

$$2\text{Cu}^{2+} + \text{K}_4[\text{Fe}(\text{CN})_6] \rightarrow \text{Cu}_2[\text{Fe}(\text{CN})_6] \downarrow + 4\text{K}^+$$

Concept: Precipitated as insoluble sulfides in dilute acidic medium ($$\text{HCl}$$) via common ion effect on $$\text{H}_2\text{S}$$ ionization. Confirmation with potassium ferrocyanide yields a chocolate-brown precipitate.

Notation / Key Observation:

• Group Reagent: $$\text{H}_2\text{S}$$ gas in presence of dilute $$\text{HCl}$$;
• $$\text{CuS}$$: Copper sulfide (Black precipitate);
• $$\text{Cu}_2[\text{Fe}(\text{CN})_6]$$: Copper ferrocyanide (Chocolate brown precipitate).