Cobalt(II) Nitrate
Properties
| State | Solid (crystalline, deliquescent) |
| Color | Deep red to magenta (hexahydrate); pale red (anhydrous) |
| Solubility | Highly soluble in water (1030 g/L at 20°C) |
| Melting Point | 55°C (hexahydrate); 100°C (decomposes, anhydrous) |
| Boiling Point | Decomposes before boiling |
About Cobalt(II) Nitrate
Cobalt(II) nitrate is the deliquescent magenta hexahydrate that catalyst chemists reach for when they need a clean cobalt source. The reason is straightforward: the nitrate counterion decomposes thermally to N2 and O2 in the temperature window 100–300 °C, leaving behind pure cobalt oxide with no chloride poisoning, no sulfur residues, and no carbon coke. That's why Co(NO3)2·6H2O is the standard precursor for Fischer-Tropsch catalyst preparation (cobalt impregnated on alumina or silica, calcined to Co3O4, then reduced to metallic Co⁰ for CO+H2 chemistry to liquid hydrocarbons), and for cobalt-zeolite catalysts used in selective NH3-SCR of NOx emissions in lean-burn diesel exhaust. The hexahydrate has [Co(H2O)6]²⁺ octahedra in the solid state and stays in that octahedral coordination in dilute aqueous solution — same magenta color as the chloride hexahydrate, same d-d transitions around 510 nm. Rinmann's green test exploits its qualitative-analysis behavior: dab a chip of suspected zinc compound with cobalt nitrate solution, ignite in a flame, and zinc oxide reacts with cobalt oxide at high temperature to form the spinel CoZnO2 (Rinmann's green), confirming zinc presence with a distinctive green color. The same chemistry made cobalt nitrate the historical reagent of choice for testing aluminum and magnesium minerals — Thénard's blue (CoAl2O4) for aluminum, pink CoMgO2 for magnesium. As a strong oxidizer, it ignites paper and organic solvents on contact when concentrated.
Where you'll encounter it
If you've ever prepared a supported cobalt catalyst by incipient-wetness impregnation, you weighed out Co(NO3)2·6H2O (the hexahydrate is the only form that comes off the shelf — the anhydrous decomposes too readily in air), dissolved it in just enough water to fill the pore volume of your alumina support, then calcined at 400 °C to drive off NO2 and water. The slow weight loss as nitrate decomposes is a clean marker on a TGA trace, which is one reason it's still preferred over the acetate or chloride for catalyst-prep procedures.
Common Uses
- Precursor for Fischer-Tropsch cobalt-on-alumina catalysts in coal-to-liquids and gas-to-liquids plants
- Wet-impregnation cobalt source for NH3-SCR catalysts treating diesel-engine NOx emissions
- Pickling-bath additive in nickel-cobalt electroplating formulations targeting gold-color decorative finishes
- Rinmann's green qualitative test reagent identifying zinc by forming CoZnO2 spinel after ignition
- Thénard's blue (CoAl2O4) preparation for ceramic underglaze and stained-glass coloration
- Sympathetic-ink demonstrations for K-12 chemistry showing reversible Oh/Td color change on heating
- Cathodic dye source for cobalt-pink ceramic glazes fired below 1100 °C to prevent black-spinel formation
- Calibration standard in ICP-OES at 228.6 nm cobalt emission line for trace metals analysis
Safety Information
GHS H272 (oxidizer, may intensify fire), H302 (harmful if swallowed), H317 (skin sensitization), H334 (respiratory sensitization with occupational asthma), H341 (suspected mutagen), H350i (carcinogenic by inhalation, IARC Group 2B), H360F (reproductive toxicity), H410 (very toxic to aquatic life). OSHA PEL for cobalt is 0.1 mg Co/m³ as an 8-hour TWA; ACGIH TLV is 0.02 mg/m³. The strong oxidizer behavior is a concrete fire risk: contact between solid Co(NO3)2 and reducing organics like glycerol or paper can ignite spontaneously, and the molten salt above 55 °C is extremely reactive. Store separated from organics in DOT Class 5.1 oxidizer locker. Cobalt sensitization is permanent — once you react to any cobalt salt, exposure to cement dust, hard-metal grinding, or even orthopedic implant wear-debris triggers contact dermatitis.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.