Salt Analysis: Preliminary Tests
Part 1: Physical Examination, Dry Heating, and Flame Tests.
Qualitative analysis involves the identification of cations (basic radicals) and anions (acid radicals) in an inorganic salt. The process begins with preliminary tests which give valuable clues about the ions present.
1. Physical Examination
Color: The color of the salt often indicates the cation.
- Blue: $Cu^{2+}$ (Hydrated)
- Light Green: $Fe^{2+}$
- Green: $Ni^{2+}$
- Pink/Violet: $Co^{2+}, Mn^{2+}$
- Yellow/Brown: $Fe^{3+}$
- White: Absence of transition metals ($Zn^{2+}, Ca^{2+}, Mg^{2+}, Pb^{2+}, Al^{3+}$)
2. Dry Heating Test
Action of Heat
A small amount of salt is heated in a dry test tube. Observations of gas evolved or residue color change are noted.
| Observation | Inference |
|---|---|
| Colorless, odorless gas ($CO_2$) | Carbonate ($CO_3^{2-}$) |
| Rotten egg smell ($H_2S$) | Sulphide ($S^{2-}$) |
| Reddish-brown fumes ($NO_2$) | Nitrate ($NO_3^-$) |
| Vinegar smell | Acetate ($CH_3COO^-$) |
| Yellow when hot, White when cold | $Zn^{2+}$ (Zinc Oxide) |
| Brown when hot, Yellow when cold | $Pb^{2+}$ (Lead Oxide) |
3. Flame Test
Color of the Flame
A paste of the salt with Conc. HCl is introduced into a non-luminous Bunsen flame using a platinum wire.
| Flame Color | Inference |
|---|---|
| Golden Yellow | Sodium ($Na^+$) |
| Brick Red | Calcium ($Ca^{2+}$) |
| Crimson Red | Strontium ($Sr^{2+}$) |
| Grassy Green | Barium ($Ba^{2+}$) |
| Bluish Green | Copper ($Cu^{2+}$) |
4. Borax Bead Test
For Colored Salts
Borax ($Na_2B_4O_7 \cdot 10H_2O$) swells on heating to form a transparent glassy bead of sodium metaborate and boric anhydride. This bead reacts with transition metal salts to form colored metaborates.
- Blue: Copper ($Cu$), Cobalt ($Co$)
- Green: Chromium ($Cr$)
- Yellow/Brown: Iron ($Fe$)
- Violet: Manganese ($Mn$)
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