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). Occupational exposure limits for cobalt are very low, and the advisory values are lower still — they are set by respiratory sensitization, not acute toxicity. 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. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.