Cobalt(II) Oxide
Properties
| State | Solid |
| Color | Olive green to dark gray |
| Solubility | Insoluble in water; soluble in acids and alkalis |
| Melting Point | 1933 °C |
About Cobalt(II) Oxide
CoO is the rock-salt-structured cobalt monoxide that gives 'cobalt blue' its name — when 0.1–0.5 wt% CoO is dissolved into a silicate glass melt, the Co²⁺ ions enter tetrahedral interstitial sites and produce the intense, photochemically stable blue that has colored stained-glass cathedral windows for nine centuries and Chinese export porcelain since the Tang dynasty. The Co²⁺ ion is high-spin d⁷ (three unpaired electrons), and at the bulk crystal level CoO is one of the cleanest examples of a 3d-transition-metal antiferromagnet: below the Néel temperature of 291 K (~18 °C) it orders into an antiferromagnetic state with collinear spins along the [117] direction, with strong magnetoelastic coupling that distorts the cubic cell to monoclinic on cooling. That magnetism, combined with strong electron-electron correlations, made CoO a textbook example for developing the Mott-Hubbard model of insulating transition-metal oxides — the band-theory picture predicts CoO should be a metal because of its half-filled d-band, but the actual electron correlations open a charge-transfer gap of ~2.6 eV that DFT can't get right without a Hubbard-U correction. Industrially, CoO is a mid-step in cobalt-metal production from cobaltite/pentlandite ores: roast the sulfide concentrate to CoO + SO2, reduce CoO with carbon or hydrogen at 700–900 °C to Co⁰. It's also a CoO/MoO3/Al2O3 hydrodesulfurization (HDS) catalyst component that strips ppm-level sulfur out of petroleum diesel before it reaches your fuel pump.
Where you'll encounter it
If you've worked in a ceramics studio, the blue you scoop out of a bin labeled 'cobalt oxide' for underglaze decoration is technically Co3O4 dust that converts to CoO in the glaze melt above 900 °C — the cobalt content needs to be limited to ~0.5 wt% or you get black instead of blue, because CoO's tinting strength is enormous. In a refinery, every batch of catalyst that goes into a HDS unit was once a slurry of CoO, MoO3, and γ-alumina that was sulfided with H2S/H2 in situ to form the active CoMoS phase that hydrogenates C-S bonds in dibenzothiophene at 350 °C and 50 bar.
Common Uses
- Cobalt-blue colorant for stained glass, fine porcelain, and ceramic underglaze fired between 900 and 1300 °C
- Hydrodesulfurization catalyst component (CoMoS phase on γ-Al2O3) for stripping ppm sulfur from diesel fuel
- Smalt pigment ground from cobalt-doped potassium glass, the dominant blue in 16th-18th century European oil painting
- Mid-step intermediate in cobalt-metal production from sulfide-ore roasting and carbothermic reduction
- Antiferromagnetic reference material for neutron-scattering experiments below the 291 K Néel temperature
- p-type semiconductor with 2.6 eV charge-transfer gap for transparent-conducting-oxide research
- Component in Li-ion cathode synthesis as a spinel-precursor before lithiation to LiCoO2
- Catalyst for steam reforming of methane and ethanol decomposition in hydrogen production research
Safety Information
GHS H332 (harmful if inhaled), H317 (skin sensitization), H334 (respiratory sensitization), H341 (suspected germ-cell mutagen), H350i (carcinogenic by inhalation, IARC Group 2B for cobalt and cobalt compounds), H360F (reproductive toxicity), H410 (very toxic to aquatic life with long-lasting effects). OSHA PEL is 0.1 mg Co/m³ as an 8-hour TWA. ACGIH TLV is 0.02 mg/m³ — the more restrictive limit reflecting respiratory-sensitization concerns. Hard-metal lung disease (giant-cell interstitial pneumonia) is the chronic-exposure endpoint historically documented in tungsten-carbide grinding workers. Skin sensitization, once established, cross-reacts with all cobalt species — including the wear debris from cobalt-chromium orthopedic implants. Handle in a fume hood with N95 respirator minimum for any operation generating dust; full-face P100 for grinding or calcination.
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.