Qualitative Analysis - Borax Bead Test

Qualitative Analysis of Cations (Borax Bead Test)

Principle

When borax (Na₂B₄O₇·10H₂O) is heated, it loses water of crystallization and swells up into a porous mass, which on further heating melts into a transparent glass-like bead:

Na₂B₄O₇·10H₂O (Δ) → Na₂B₄O₇ (Δ) → 2NaBO₂ + B₂O₃
(Boric Anhydride)

The Boric Anhydride (B₂O₃) reacts with basic metal oxides to form colored metaborates. The color depends on the oxidation state of the metal in 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 fuses.

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 in the flame.

5. Observation: Observe the bead color in both Oxidizing (outer) and Reducing (inner) flames while hot and cold.

Colour Chart – Oxidizing & Reducing Flame

Cation	Oxidizing Flame (Hot → Cold)	Reducing Flame (Hot → Cold)	Characteristic Metaborate
Copper (Cu²⁺)	Green → Blue	Colorless → Red Opaque	Cu(BO2)2 → Cu
Iron (Fe²⁺/Fe³⁺)	Yellow → Pale yellow	Bottle green → Green	Fe(BO2)3 → Fe(BO2)2
Cobalt (Co²⁺)	Deep blue → Deep blue	Deep blue → Deep blue	Co(BO2)2
Chromium (Cr³⁺)	Yellow-green → Emerald green	Green → Green	Cr(BO2)3
Manganese (Mn²⁺)	Violet → Pink	Colorless → Colorless	Mn(BO2)2
Nickel (Ni²⁺)	Violet → Brown	Grey → Opaque grey	Ni(BO2)2 → Ni

Key Reactions

Copper:

Oxidizing Flame: CuO + B₂O₃ → Cu(BO₂)₂ (Blue Metaborate)
Reducing Flame: 2Cu(BO₂)₂ + 2NaBO₂ + C → 2Cu + Na₂B₄O₇ + CO

Cobalt:

Both Flames: CoO + B₂O₃ → Co(BO₂)₂ (Intense Blue Metaborate)

Important Notes & Limitations

• Platinum wire is essential because it does not react with borax or oxygen.
• Limited to cations that form colored borate compounds (transition metals).
• Interference: Mixed salts (e.g., Cu + Fe) can mask each other's colors.

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