Amidol

Developing AgentC6H8N2O·2HClCAS: 137-09-7Shelf life: 24 mo
Amidol
Image: Innerstream — Public domain

Physical Properties

  • Molecular Weight: 197.06 g/mol
  • Solubility (20°C): 20 g/L

Also known as: 2,4-Diaminophenol Dihydrochloride, Acrol, Dianol

Amidol (2,4-diaminophenol dihydrochloride, C₆H₈N₂O·2HCl; CAS 137-09-7) is a uniquely acidic developing agent — it develops silver in a solution at pH 6–7 without the alkali accelerator that every other photographic developer requires.[1] This property has made amidol the connoisseur's choice for exhibition-quality fine-art prints since the 1890s. Amidol-developed silver gelatin prints have a characteristic neutral-to-cold image tone with exceptional shadow detail separation and a luminous quality that many printers consider unmatched by MQ or PQ formulas.

Photographic uses

  • Exhibition paper developer: Neat amidol at 8–10 g/L plus sodium sulfite and trace potassium bromide produces cold-neutral tones on silver gelatin fibre-based paper. The formula is simple but the results are highly sensitive to working conditions.[2]
  • Fine-grain film developer (rare): A few historical film formulas use amidol alone for very fine grain at the cost of severely limited shelf life.
  • Low-pH specialized workflows: The acidic working pH makes amidol useful where alkaline developers would attack specific emulsion chemistry (early collodion, some warm-tone chloro-bromide papers).

Practical notes

Supplied as pale yellow to tan crystalline powder. Shelf life is amidol's defining practical limitation: a fresh-mixed working developer loses activity significantly within 2–4 hours as the developing agent auto-oxidizes in the open tray. Most amidol users mix only the quantity needed for one session and discard immediately after use. Dry amidol keeps for years in a tightly sealed amber container; solutions keep weeks as refrigerated concentrated stocks with a small amount of sulfite preservative, but the convenience benefits of long-shelf-life alternatives are absent.

Amidol solutions are intensely staining — darker brown than even pyrogallol. The tray discolors to a deep red-brown within minutes of use; use dedicated trays.

Related compounds

Metol is the closest classical alternative for neutral-tone paper work. Pyrocatechol is a different path to fine-grain development via staining chemistry. Glycin offers warm-tone fine-grain as a contrast to amidol's cool tones.

Safety Notes

Acute hazards

Amidol is a skin sensitizer with a hazard profile similar to metol:

  • Skin sensitization: repeated contact builds cumulative allergic dermatitis. Workers using amidol regularly should wear gloves strictly.
  • Skin and eye irritant in concentrated solution.
  • Toxic if ingested: substantial swallow causes methemoglobinemia (impaired oxygen transport); medically significant.
  • Staining: prints, trays, skin, clothing all stain brown on contact. Staining fades from skin over days; from clothing it does not fade.

Storage & handling

  • Personal protective equipment: Nitrile gloves (strict compliance), safety glasses, long-sleeved clothing. A dust mask when weighing powder.
  • Storage: Dry amber container in a cool dark cabinet.

First aid

Consult the full Sigma-Aldrich Safety Data Sheet for authoritative first-aid instructions.[3] In brief:

  • Skin contact: Wash with soap and water. Seek medical attention for persistent reaction (early sensitization sign).
  • Eye contact: Flush with running water for at least 15 minutes.
  • Ingestion: Do NOT induce vomiting. Rinse mouth. Seek medical attention immediately.

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. 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