Chromic Acid
Properties
| State | Exists in aqueous solution; CrO3 solid dissolves to form it |
| Color | Dark red to orange |
| Solubility | Highly soluble in water |
| Melting Point | 197°C (as CrO3) |
| Boiling Point | 250°C (decomposes) |
About Chromic Acid
Chromic acid is what you get when CrO3 dissolves in water — a deep orange-red solution where chromium is locked in its +6 oxidation state and looking for electrons. In aqueous solution it exists as an equilibrium between H2CrO4, the chromate dianion CrO4²⁻, and the dichromate Cr2O7²⁻ that dominates below pH ~6. That orange-to-yellow shift across the chromate/dichromate equilibrium is one of the cleanest acid-base color demonstrations in undergraduate inorganic chem. The Cr(VI) center is a tetrahedral d⁰ ion, and the LMCT (ligand-to-metal charge transfer) bands from oxide lone pairs into empty Cr d-orbitals are what give the species its intense color. Chemically, chromic acid was the workhorse oxidant of 20th-century organic synthesis: Jones reagent (CrO3 in dilute H2SO4/acetone) takes primary alcohols cleanly to carboxylic acids and secondaries to ketones, and the orange-to-green color change as Cr(VI) reduces to Cr(III) is the visible TLC stain that tells you the reaction worked. The same chemistry underpinned hard-chrome plating baths and the old chromic-cleaning solution (sulfuric acid saturated with K2Cr2O7) once used to scour glassware. IARC and OSHA have largely shut all of that down — Cr(VI) is a Group 1 carcinogen — and synthetic chemistry has moved to milder oxidants like Dess-Martin, TPAP, and Swern.
Where you'll encounter it
If you've ever pulled an undergrad lab notebook off a shelf and seen a yellow-stained ground-glass joint, that was chromic-acid cleaning solution at work. Most teaching labs replaced it with Nochromix or Alconox decades ago — the disposal paperwork for a hexavalent chromium waste stream is its own justification. In organic chemistry, you'll still see Jones oxidation cited in older procedures, but if you're at the bench today and need to oxidize an alcohol, you'll reach for PCC, Dess-Martin periodinane, or TEMPO/bleach long before you'd touch a CrO3 bottle.
Common Uses
- Jones oxidation of primary alcohols to carboxylic acids and secondaries to ketones in synthesis
- Hard-chrome and decorative chrome electroplating baths (legacy use, being phased out under REACH)
- Chromated copper arsenate (CCA) wood preservation prior to 2003 EPA restrictions
- TLC visualization stain for oxidizable functional groups (Cr(VI) orange to Cr(III) green)
- Qualitative test for primary and secondary alcohols and aldehydes in undergraduate labs
- Glassware cleaning solution in older lab procedures (now replaced by Nochromix)
- Surface passivation of zinc and aluminum prior to chromate-conversion-coating restrictions
- Analytical reagent for chromium-based redox titrations against Mohr's salt
Safety Information
IARC Group 1 human carcinogen — Cr(VI) crosses cell membranes via the sulfate/phosphate transport system and generates reactive Cr(V)/Cr(IV) intermediates that damage DNA. Airborne limits for Cr(VI) are quoted in micrograms per cubic meter — among the lowest set for any workplace chemical — and lower still for the water-soluble forms. GHS H350 (carcinogenicity), H340 (germ cell mutagenicity), H361 (reproductive toxicity), H314 (severe skin burns), H271 (oxidizer that may cause fire). REACH Annex XIV authorization required in the EU. Chronic exposure causes nasal septum perforation and lung cancer. Handle in a fume hood with nitrile-over-butyl gloves; segregate from organic solvents.
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.