Salt Analysis: Cations (Groups IV-VI)
Part 4: Systematic Analysis of Zinc, Calcium, Barium, Magnesium, etc.
By chemca Team • Updated Jan 2026
After ensuring the absence of Groups I-III, the filtrate is tested for Groups IV, V, and VI.
1. Group IV (Zinc Group)
Cations: $Zn^{2+}, Mn^{2+}, Ni^{2+}, Co^{2+}$.
Group Reagent: $H_2S$ gas in presence of $NH_4OH$ (Alkaline medium).
Zinc ($Zn^{2+}$): Dull White ppt ($ZnS$). Dissolve in HCl. Add NaOH.
Result: White ppt soluble in excess NaOH.
Result: White ppt soluble in excess NaOH.
Manganese ($Mn^{2+}$): Buff/Skin color ppt ($MnS$).
Principle: In alkaline medium ($NH_4OH$), the concentration of $S^{2-}$ ions increases (Common ion effect removed), allowing sulphides with higher $K_{sp}$ (Group IV) to precipitate.
2. Group V (Calcium Group)
Cations: $Ba^{2+}, Sr^{2+}, Ca^{2+}$.
Group Reagent: $(NH_4)_2CO_3$ in presence of $NH_4Cl$ and $NH_4OH$.
Precipitate: White Carbonates ($BaCO_3, SrCO_3, CaCO_3$).
Separation (Flame Test):
- Barium ($Ba^{2+}$): Grassy Green Flame. Gives Yellow ppt with $K_2CrO_4$.
- Strontium ($Sr^{2+}$): Crimson Red Flame. Gives White ppt with $(NH_4)_2SO_4$.
- Calcium ($Ca^{2+}$): Brick Red Flame. Gives White ppt with Ammonium Oxalate.
3. Group VI (Magnesium)
Cation: $Mg^{2+}$.
Reagent: Disodium Hydrogen Phosphate ($Na_2HPO_4$) + $NH_4OH$.
Test: To the filtrate of Group V, add reagents and scratch the sides of test tube.
Result: White crystalline precipitate of Magnesium Ammonium Phosphate ($MgNH_4PO_4$).
Result: White crystalline precipitate of Magnesium Ammonium Phosphate ($MgNH_4PO_4$).
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