Pyrocatechol

Developing AgentC6H4(OH)2CAS: 120-80-9Shelf life: 24 mo
Pyrocatechol
Image: Bryan Derksen — Public domain

Physical Properties

  • Molecular Weight: 110.11 g/mol
  • Solubility (20°C): 450 g/L

Also known as: Catechol, 1,2-Benzenediol, Pyrocatechin

Pyrocatechol (1,2-dihydroxybenzene, also called catechol; C₆H₄(OH)₂; CAS 120-80-9) is the lower-toxicity staining developer alternative to pyrogallol, and the active ingredient in Sandy King's Pyrocat-HD formula.[1] Chemically it is the 1,2-isomer of hydroquinone (which is 1,4-dihydroxybenzene) — same empirical formula, different atomic arrangement, dramatically different photographic behaviour. Pyrocatechol develops silver halide with a more controlled staining response than pyrogallol, produces cleaner highlights, and carries a meaningfully lower hazard profile.

Photographic uses

  • Pyrocat-HD (Sandy King, 2003): The modern successor to classical pyro developers. Two-stock concentrate (A: pyrocatechol + sulfite + phenidone; B: potassium carbonate) mixed at working time at 1:1:100 dilution. Produces proportional staining with long tonal range and less cumulative toxicity than ABC Pyro. Current favourite of many large-format fine-art workers.[2]
  • Pyrocat-MC: Metol-Catechol variant with finer grain on smaller formats.
  • Staining intensification: Similar to pyrogallol but more predictable staining kinetics — useful for densitometry-calibrated workflow.
  • Industrial specialty developers: Some high-contrast and photogravure formulas use pyrocatechol for its particular chemistry of proportional tanning.

Practical notes

Supplied as white to off-white crystalline powder, often with a faint phenolic odour. Much more stable on exposure to air than pyrogallol — a tightly sealed container keeps for years without visible oxidation. Solutions in working-strength developer oxidize at a manageable rate (hours, not minutes), making Pyrocat-HD more forgiving to mix and use than classical pyro.

Solutions are less intensely staining than pyrogallol — trays and skin darken over time but less dramatically.

Related compounds

Pyrogallol is the 1,2,3-trihydroxybenzene sibling — the higher-toxicity classical staining developer. Hydroquinone is the 1,4-isomer used in MQ/PQ developers — same empirical formula, no staining behaviour. Tert-butyl pyrocatechol is a derivative used in some specialty formulas.

Safety Notes

Acute hazards

Pyrocatechol is meaningfully less hazardous than pyrogallol but still a reactive developing agent:

  • Toxic if ingested: moderately toxic systemic reaction if swallowed; medically significant at gram-scale doses.
  • Skin irritant and sensitizer: repeated contact builds contact dermatitis in some workers.
  • Dust inhalation irritant.

Chronic hazards

  • Limited evidence of genotoxicity; EU CLP classification H341 (suspected mutagen) in some product grades.
  • Minimize cumulative skin contact.

Storage & handling

  • Personal protective equipment: Nitrile gloves, safety glasses, dust mask for weighing. Less strict than pyrogallol but gloves are still essential for regular use.
  • Storage: Tightly sealed amber container in a cool dark cabinet.

First aid

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

  • Skin contact: Flush with soap and water.
  • Eye contact: Flush with running water for at least 15 minutes.
  • Ingestion: Do NOT induce vomiting. Rinse mouth. Seek medical attention.

References

  1. BOOK Anchell, Steve; Troop, Bill. The Film Developing Cookbook 2nd ed. Routledge, 2019. ISBN 9781138959187. ↩
  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