p-Aminophenol

Developing AgentC6H7NOCAS: 123-30-8Shelf life: 36 mo
p-Aminophenol
Image: NEUROtiker — Public domain

Physical Properties

  • Molecular Weight: 109.13 g/mol
  • Solubility (20°C): 15 g/L

Also known as: 4-Aminophenol, Activol, Certinal, PAP

p-Aminophenol (4-aminophenol; C₆H₇NO; CAS 123-30-8) is the active developing agent in Rodinal — the longest continuously-manufactured photographic developer in the world, introduced by Agfa in 1891 and still sold today under various brand names after Agfa's dissolution.[1] Rodinal uses p-aminophenol at high concentration in a potassium hydroxide / potassium sulfite base to produce an extremely vigorous developer designed for high-dilution (1:25 to 1:100) working solutions. The result is characteristic: very high acutance (apparent sharpness from edge-effect development), distinctly grainy rendering at high dilutions, and an unmistakable "Rodinal look" that has defined European B&W photography for 130+ years.

Photographic uses

  • Rodinal / R09 / Studional / Adonal: The classical p-aminophenol developer. Currently sold as R09 One Shot (Compard, Germany) and Adonal (Adox, Germany); chemically essentially identical to the original.[2]
  • Stand development: 1:100 Rodinal with 60–90 minute stand development produces extreme local-contrast edge enhancement on 35mm film.
  • DIY aminophenol developers: Several published formulas use p-aminophenol as the sole developing agent in custom compensating or high-contrast workflows.
  • Compounded developers: Historical developer formulas occasionally pair p-aminophenol with glycin for specialized fine-grain work.

Practical notes

Supplied as white to tan crystalline powder (darkens on air exposure). The commercial product is typically the hydrochloride salt (p-aminophenol·HCl) for better solubility and shelf stability than the free base. Rodinal itself is a concentrated liquid formula (p-aminophenol + KOH + potassium sulfite + potassium bromide), shelf-stable for many years in a tightly sealed brown bottle.

Very high working dilutions: typical Rodinal use is 1:25 to 1:200. This means a small bottle of concentrate lasts a very long time — one of Rodinal's economic appeals.

Related compounds

Metol is the N-methyl variant (4-methylaminophenol sulfate) with related but milder photographic chemistry. Hydroquinone is the structurally different dihydroxy counterpart used in MQ/PQ developers.

Safety Notes

Acute hazards

p-Aminophenol has a similar hazard profile to metol with somewhat higher acute toxicity:

  • Skin sensitizer: repeated contact builds allergic dermatitis. Same occupational pattern as metol sensitization.
  • Toxic if ingested: substantial swallow causes methemoglobinemia. Medically significant.
  • Skin and eye irritant.
  • Suspected mutagen (H341).

Storage & handling

  • Personal protective equipment: Nitrile gloves, safety glasses, dust mask for weighing. Sensitized individuals should substitute phenidone where the formula permits.
  • Storage: Tightly sealed in a cool dark cabinet. The free base is more prone to air oxidation than the hydrochloride salt.

First aid

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

  • Skin contact: Flush with soap and water. Seek medical attention for persistent reaction.
  • 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 Crawford, William. The Keepers of Light: A History and Working Guide to Early Photographic Processes 1st ed. Morgan & Morgan, 1979. ISBN 0-87100-158-6. ↩
  2. BOOK Anchell, Steve; Troop, Bill. The Film Developing Cookbook 2nd ed. Routledge, 2019. ISBN 9781138959187. ↩
  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