Chrome Alum

HardenerKCr(SO4)2·12H2OCAS: 7788-99-0Shelf life: 12 mo

Physical Properties

  • Molecular Weight: 499.4 g/mol
  • Solubility (20°C): 200 g/L

Also known as: Chromium Potassium Sulfate, Chromalum

Chrome alum (potassium chromium(III) sulfate dodecahydrate, KCr(SO₄)₂·12H₂O; CAS 7788-99-0) is the strongest common photographic hardener — a mineral crosslinker that produces more durable gelatin hardening than potassium alum through chromium(III) coordination with gelatin amino groups.[1] Critically, chrome alum is a chromium(III) compound, not the hexavalent chromium that makes potassium dichromate so hazardous — the two oxidation states have dramatically different toxicity profiles. Chrome-alum solutions are characteristic deep violet, shifting to green as they oxidize in use.

Photographic uses

  • Chrome-alum hardening fixer: Classical formula combines chrome alum with sodium thiosulfate and sodium bisulfite for maximum gelatin hardening in tropical processing or high-volume commercial work.
  • Pre-hardening bath: A dilute chrome-alum solution before development protects gelatin from reticulation during processing at elevated temperatures.
  • Post-fix hardening: Some photographic workflows use chrome alum as a separate hardening step after a non-hardening fixer, allowing the hardener bath to be replaced independently.[2]

Practical notes

Supplied as violet-to-deep-purple crystalline granules. Solutions in cold water are stable for weeks; warming the solution or extended use causes gradual oxidation (partially to Cr(VI), visibly greening). Discard greenish solutions — they may contain chromium(VI) species and have lost hardening activity.

Critical warning: although chrome alum is predominantly Cr(III), partial oxidation under certain storage or use conditions can produce small amounts of hexavalent chromium. Avoid strong oxidizers, avoid very alkaline conditions, and discard aged solutions promptly.

Regulatory status

Chromium(III) compounds are regulated less strictly than Cr(VI) but are still classified as skin and respiratory sensitizers under EU CLP.[3] Workers developing chromium sensitization from Cr(III) exposure are effectively sensitized to all chromium compounds — contact with potassium dichromate or ammonium dichromate will then trigger severe reactions.

Related compounds

Potassium alum is the gentler aluminum hardener. Formaldehyde is the aldehyde crosslinker used where mineral hardeners are inadequate. Sodium chromium sulfate is a less common Cr(III) hardener substitute.

Safety Notes

Acute hazards

  • Skin sensitizer (H317): repeated contact builds allergic dermatitis; once sensitized, the worker is allergic to all chromium compounds. This is the dominant chronic concern.
  • Skin and eye irritant in concentrated solution.
  • Respiratory irritant as dust.
  • Possible reproductive toxin (H361) per EU CLP.

Chronic hazards

  • Permanent chromium sensitization on repeated exposure. Strict glove compliance essential for any regular use.
  • Possible conversion to Cr(VI): trace hexavalent species may form in aged or oxidatively-stressed solutions, inheriting the much more serious Cr(VI) toxicity.

Storage & handling

  • Personal protective equipment: Nitrile gloves (strict compliance), safety glasses, dust mask for weighing, long-sleeved clothing.
  • Storage: Tightly sealed in a cool dry cabinet.

First aid

Consult the full Sigma-Aldrich Safety Data Sheet for authoritative first-aid instructions.[4]

  • Skin contact: Flush with water; seek medical attention for persistent reaction (early sensitization sign).
  • Eye contact: Flush with running water for at least 15 minutes.
  • Ingestion: Rinse mouth; seek medical attention for substantial swallow.

References

  1. BOOK Haist, Grant. Modern Photographic Processing, Volume 2 1st ed. John Wiley & Sons, 1979. ISBN 0-471-04635-X. ↩
  2. BOOK Anchell, Steve. The Darkroom Cookbook 4th ed. Focal Press, 2016. ISBN 9781138959170. ↩
  3. STANDARD European Chemicals Agency (ECHA). REACH Regulation (EC) No 1907/2006, Annex XVII – Restrictions on manufacture, placing on the market and use European Union. https://echa.europa.eu/substances-restricted-under-reach ↩
  4. WEB Sigma-Aldrich (Merck KGaA). Sigma-Aldrich Safety Data Sheets Sigma-Aldrich. https://www.sigmaaldrich.com/US/en/search/safety-data-sheets ↩

Reference databases

Last reviewed 2026-04-22