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Salt Analysis: Cations (Groups 0-III)

Part 3: Systematic Analysis of Basic Radicals.

By chemca Team • Updated Jan 2026

Cations are separated into groups based on the solubility product ($K_{sp}$) of their salts. We always check for Group 0 first, then proceed sequentially from Group I to VI.

1. Group 0 (Ammonium)

Cation: $NH_4^+$

Group Reagent: None (Detected by heating with NaOH).

Test: Heat Salt + NaOH solution.
Observation: Smell of Ammonia ($NH_3$). Turns red litmus blue.

Confirmatory Test (Nessler's Reagent): Pass gas through Nessler's reagent ($K_2HgI_4$).
Result: Brown precipitate (Iodide of Millon's base).

2. Group I (Silver Group)

Cation: $Pb^{2+}$ (Lead)

Group Reagent: Dilute $HCl$.

Precipitate: White ppt of $PbCl_2$.

Confirmatory Test ($KI$): Dissolve ppt in hot water and add $KI$.
Result: Golden Yellow ppt of $PbI_2$.

3. Group II (Copper Group)

Cations: $Cu^{2+}$ (Blue), $As^{3+}$ (Yellow), etc.

Group Reagent: $H_2S$ gas in presence of Dilute $HCl$.

Copper ($Cu^{2+}$): Black ppt ($CuS$). Dissolve in $HNO_3$. Add $NH_4OH$.
Result: Deep Blue solution $[Cu(NH_3)_4]^{2+}$.

Arsenic ($As^{3+}$): Yellow ppt ($As_2S_3$).

4. Group III (Iron Group)

Cations: $Fe^{3+}$ (Reddish Brown), $Al^{3+}$ (Gelatinous White).

Group Reagent: $NH_4OH$ in presence of $NH_4Cl$.

Iron ($Fe^{3+}$): Brown ppt ($Fe(OH)_3$). Dissolve in $HCl$. Add $K_4[Fe(CN)_6]$.
Result: Prussian Blue color.

Aluminium ($Al^{3+}$): White Gelatinous ppt ($Al(OH)_3$).
Result: Lake Test (Blue floating mass on colorless solution).

Role of NH4Cl: It suppresses the ionization of $NH_4OH$ (Common Ion Effect), ensuring only Group III hydroxides ($low K_{sp}$) precipitate, preventing Group IV, V from precipitating.

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