Chromium(VI) Oxide
Properties
| State | Solid (dark red to orange crystals; hygroscopic) |
| Color | Dark red to orange |
| Solubility | Very soluble in water (630 g/L at 25°C, forming chromic acid); soluble in sulfuric acid |
| Melting Point | 197°C |
| Boiling Point | 250°C (decomposes) |
About Chromium(VI) Oxide
CrO3 is a dark red, hygroscopic solid built from corner-sharing CrO4 tetrahedra arranged in infinite chains — the structural analog of an anhydride waiting for water. Drop a crystal onto a wet surface and it goes into solution in seconds as orange chromic acid; drop it onto an organic solvent like methanol and it can flash-ignite, because the same +6 oxidation state that makes it useful in the lab also makes it one of the strongest oxidizers a chemist routinely handles. Dissolved in dilute H2SO4 with a co-solvent of acetone, it becomes Jones reagent, the workhorse of late-20th-century alcohol oxidation — primary alcohols go all the way to carboxylic acids and secondary alcohols stop cleanly at ketones, with the orange Cr(VI) reducing to a green Cr(III) precipitate that you can see disappear into the layer at the bottom of the flask. Mix CrO3 with pyridine in the right ratio and you get Collins reagent (CrO3·2py); add HCl and you get pyridinium chlorochromate (PCC), which is mild enough to stop primary oxidations cleanly at the aldehyde. Industrially, CrO3 dissolved in concentrated H2SO4 is the active species in hard-chrome plating baths that lay down 100–500 µm of wear-resistant chromium on hydraulic cylinder rods and aircraft landing gear — a use that REACH Annex XIV authorization has been chipping away at since 2017, with ongoing transitions to trivalent baths and HVOF tungsten-carbide coatings.
Where you'll encounter it
If you've ever opened a bottle of CrO3 and found it has clumped into a single hygroscopic mass, that's normal — keep it in a tightly sealed jar in a desiccator with drierite. The other thing you'll notice fast: the crystals stain everything they touch a permanent rust-orange, including your fume-hood floor and your skin. That's not just dye; it's Cr(VI) in your stratum corneum, and it's a Group 1 carcinogen, so a spill on bare skin is a real exposure event, not a cosmetic problem. In any modern lab, the procedure is to switch to PCC, Dess-Martin, or TEMPO/NaOCl rather than open a CrO3 bottle at all.
Common Uses
- Jones reagent (CrO3/H2SO4/acetone) for oxidizing primary alcohols to carboxylic acids and secondaries to ketones
- PCC (pyridinium chlorochromate) precursor for mild oxidation of primary alcohols to aldehydes
- Hard-chrome plating bath additive for wear-resistant coatings on hydraulic rods and engine cylinders
- Decorative chrome electroplating for automotive trim and plumbing fixtures (legacy use under REACH authorization)
- Collins reagent (CrO3·2pyridine) for selective oxidation of acid-sensitive substrates
- Anodizing pigmentation and conversion-coating treatments on aluminum aerospace alloys
- Chromic-acid cleaning solution for restoring sintered-glass crucibles in legacy lab procedures
- Bench oxidant in Sarett oxidation and Cornforth modification for steroid synthesis
Safety Information
EXTREMELY HAZARDOUS. IARC Group 1 human carcinogen. GHS H271 (oxidizer, may cause fire), H300 (fatal if swallowed), H310 (fatal in contact with skin), H330 (fatal if inhaled), H340 (mutagenic), H350 (carcinogenic), H361 (reproductive toxicity), H372 (target organ damage from prolonged exposure), H410 (very toxic to aquatic life). OSHA PEL is 5 µg Cr(VI)/m³ as an 8-hour TWA, action level 2.5 µg/m³. ACGIH TLV for water-soluble Cr(VI) is 0.2 µg/m³. REACH Annex XIV listing — every commercial use in the EU requires authorization. Causes lung cancer, nasal septum perforation, allergic contact dermatitis with chromium-cement-style cross-reactivity, and skin ulcerations called 'chrome holes.' Strong oxidizer — contact with paper, wood, alcohol, or any reducing organic ignites combustion. Store in glass under desiccation away from organics; transfer in a fume hood with full PPE.
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.