Charcoal Cavity & Cobalt Nitrate Tests
Advanced Dry Tests for Cation Identification.
By chemca Team • Updated Jan 2026
While flame tests identify Group V cations, the Charcoal Cavity and Cobalt Nitrate tests are essential for detecting cations like Zinc, Aluminium, and Magnesium that do not impart distinct flame colors.
1. Charcoal Cavity Test
Principle
The salt is mixed with anhydrous Sodium Carbonate ($Na_2CO_3$) and heated in a charcoal cavity. The carbonate converts metal salts into metal carbonates, which decompose to metal oxides.
$$ MSO_4 + Na_2CO_3 \rightarrow MCO_3 + Na_2SO_4 $$
$$ MCO_3 \xrightarrow{\Delta} MO + CO_2 $$
The carbon in charcoal may further reduce the oxide to a metallic bead.
Observations
| Observation | Inference |
|---|---|
| White residue (glows on heating) | $Ba^{2+}, Sr^{2+}, Ca^{2+}, Mg^{2+}, Al^{3+}, Zn^{2+}$ |
| Yellow residue (hot) $\rightarrow$ White (cold) | $Zn^{2+}$ (Zinc Oxide) |
| Brown residue (hot) $\rightarrow$ Yellow (cold) | $Pb^{2+}$ (Lead Oxide) |
| Grey Metallic Bead (marks paper) | $Pb^{2+}$ (Lead) |
| Reddish scales/bead | $Cu^{2+}$ (Copper) |
2. Cobalt Nitrate Test
Ash Test
This test is performed only if the residue in the Charcoal Cavity test is WHITE. A drop of Cobalt Nitrate solution is added to the residue and heated strongly.
Principle: Cobalt nitrate decomposes to form Cobalt Oxide ($CoO$). This reacts with the metal oxide (from the white residue) to form a colored mixed oxide (spinel).
$2Co(NO_3)_2 \xrightarrow{\Delta} 2CoO + 4NO_2 + O_2$
$2Co(NO_3)_2 \xrightarrow{\Delta} 2CoO + 4NO_2 + O_2$
Observations (Color of Ash)
| Color of Ash | Composition | Inference |
|---|---|---|
| Green | $ZnO \cdot CoO$ (Rinmann's Green) |
Zinc ($Zn^{2+}$) |
| Blue | $Al_2O_3 \cdot CoO$ (Thenard's Blue) |
Aluminium ($Al^{3+}$) |
| Pink | $MgO \cdot CoO$ | Magnesium ($Mg^{2+}$) |
| Dirty Blue | $P_2O_5 \cdot CoO$ | Phosphate ($PO_4^{3-}$) |
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