*Qualitative Analysis of Cations (Oxidizing & Reducing Flame)
Principle
When borax (Na2B4O7·10H2O) is heated, it forms a transparent glassy bead via the following reaction:
Na2B4O7·10H2O (heat) → Na2B4O7 (heat) → 2NaBO2 + B2O3
(Boric Anhydride)
(Boric Anhydride)
The Boric Anhydride (B2O3) is non-volatile and reacts with basic metal oxides to form colored metaborates. The color depends on the oxidation state of the metal, which varies between the oxidizing and reducing flames.
Procedure
1. Preparation: Make a small loop (2–3 mm) at the end of a clean platinum wire.
2. Heating: Heat the loop in a Bunsen flame, dip into borax powder, and heat again until it swells and shrinks.
3. Bead Formation: A transparent, colorless, glass-like bead forms.
4. Testing: Touch the hot bead to a tiny speck of the unknown salt/oxide. Heat again.
5. Observation: Observe the color in both Oxidizing (outer) and Reducing (inner) flames, both when hot and cold.
Platinum wire loop with borax bead
Typical coloured borax beads of different metals
Colour Chart – Oxidizing & Reducing Flame
| Cation | Oxidizing Flame (Hot → Cold) |
Reducing Flame (Hot → Cold) |
Characteristic Metaborate |
|---|---|---|---|
| Copper (Cu²⁺) | Green → Blue Blue (cold) |
Brown-red (metallic bead) Opaque red |
Cu(BO2)2 → Cu (reduced) |
| Iron (Fe²⁺/Fe³⁺) | Yellow → Pale yellow Colourless (cold) |
Bottle green Green |
Fe(BO2)3 → Fe(BO2)2 |
| Cobalt (Co²⁺) | Deep blue Deep blue (cold) |
Deep blue Deep blue |
Co(BO2)2 (intense blue) |
| Chromium (Cr³⁺) | Yellow-green → Emerald green Green (cold) |
Green Green |
Cr(BO2)3 |
| Manganese (Mn²⁺) | Violet Pink (cold) |
Colourless → Pink Pink |
Mn(BO2)2 |
| Nickel (Ni²⁺) | Violet Brown (cold) |
Grey (metallic Ni) Opaque |
Ni(BO2)2 → Ni |
| Vanadium (V⁵⁺) | Yellow Colourless |
Green Green |
Varies with oxidation state |
Key Reactions
Copper:
Oxidizing: CuO + B2O3 → Cu(BO2)2 (Blue Metaborate)
Reducing: 2Cu(BOv)2 + C → 2Cu (Red Opaque) + 2B2O3 + CO2
Reducing: 2Cu(BOv)2 + C → 2Cu (Red Opaque) + 2B2O3 + CO2
Cobalt:
CoO + B2O3 → Co(BO2)2 (Intense Blue in both flames)
Important Notes & Limitations
- Platinum wire is essential (does not react with borax).
- Only for cations that form coloured compounds with borate.
- Interference: Mixture of Cu + Fe may mask colours.
- Phosphate, silicate, fluoride interfere (form insoluble compounds).
- Modern labs often use spot tests, but borax bead test is still in syllabi (Class 11–12, NEET, JEE).
Quick Identification Summary (Most Common)
| Observation | Cation Confirmed |
|---|---|
| Blue bead in both flames | Cobalt |
| Green → Blue in oxidizing, red opaque in reducing | Copper |
| Yellow → colourless in oxidizing, green in reducing | Iron |
| Emerald green in oxidizing | Chromium |
| Pink/violet bead | Manganese |
*Qualitative analysis is the process of identification or determination of specific substances or elements from a given sample by going through the physical and chemical reactions of the sample.