Formalin

HardenerCH2O (aq)CAS: 50-00-0Shelf life: 24 mo
Formalin
Image: Wereon — Public domain

Physical Properties

Also known as: Formaldehyde Solution (37%), HCHO Solution

Formalin is the 37% aqueous solution of formaldehyde — the form actually stocked in darkrooms and biological labs, stabilized with 10–15% methanol to prevent polymerization to paraformaldehyde. Photochemically identical to formaldehyde for all practical purposes; see the formaldehyde page for full use, chemistry, and hazard discussion.[1]

Photographic uses

Same as formaldehyde. Formalin is what's specified in published formulas that call for "formaldehyde solution" or simply "formalin" — the two names are interchangeable in darkroom context.

Working dilutions: typical pre-hardener is 1–3% formalin in water (equivalent to 0.4–1.1% formaldehyde). Stabilizer formulas typically use 0.5% formalin.

Practical notes

Supplied as a colorless liquid with a characteristic pungent odour. The odour is itself an exposure indicator — if you smell formalin in your workspace, ventilation is inadequate. Below the olfactory threshold (~1 ppm) the smell is undetectable but health risk remains.

Storage stability: a closed container keeps for years. Opened containers gradually lose potency as formaldehyde escapes to air; paraformaldehyde precipitate on the container walls indicates formaldehyde loss and is an inhalation hazard when the container is next opened.

See formaldehyde for full chemistry, photographic uses, regulatory status, and related compounds.

Safety Notes

Same hazard profile as formaldehyde — see that page for full acute hazards, chronic hazards, fire & reactivity, storage/handling, and first aid.

Critical reminders:

  • Formaldehyde is an IARC Group 1 human carcinogen. Workspace exposure limits are strict (OSHA 0.75 ppm 8-hour TWA).[2]
  • Fume hood required for any substantial handling.
  • Use glyoxal as a substitute where the formula permits — virtually all contemporary darkroom hardener applications can use glyoxal with comparable results and dramatically lower health risk.[3]

References

  1. BOOK Haist, Grant. Modern Photographic Processing, Volume 2 1st ed. John Wiley & Sons, 1979. ISBN 0-471-04635-X. ↩
  2. STANDARD U.S. Occupational Safety and Health Administration. OSHA Permissible Exposure Limits – Table Z-1 (29 CFR 1910.1000) U.S. Department of Labor. https://www.osha.gov/annotated-pels/table-z-1 ↩
  3. 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