Chlorhydroquinone

Developing AgentC6H3ClO2H2CAS: 615-67-8Shelf life: 36 mo
Chlorhydroquinone
Image: Def2010 — CC0

Physical Properties

Also known as: Chloroquinol, 2-Chlorohydroquinone, CHQ

Chlorhydroquinone (2-chloro-1,4-benzenediol; C₆H₅ClO₂; CAS 615-67-8) is a chlorine-substituted hydroquinone developing agent that produces warmer image tones than plain hydroquinone and slightly finer grain. Used primarily in warm-tone paper developers and in specialty high-contrast formulas where the chlorine substitution shifts development kinetics favourably.[1] Current use is niche; few contemporary published formulas call for it specifically, but it remains valuable for printers chasing particular tonal signatures on older warm-tone papers.

Photographic uses

  • Warm-tone paper developer component: 1–3 g/L chlorhydroquinone as an additive to MQ paper developer formulas shifts working tone from neutral to warm-brown without needing a toning step.[2]
  • High-contrast lithographic developer: Some lithographic formulas use chlorhydroquinone as the sole reducer (where classical formulas use hydroquinone) for slightly different contrast shoulder response.
  • Fine-grain film developer (rare): A few published formulas use chlorhydroquinone in place of hydroquinone for marginally finer grain in MQ pairs.

Practical notes

Supplied as off-white to tan crystalline powder. Less stable than hydroquinone on air exposure — darkens more quickly, oxidizes more readily in solution. Solutions have shorter working life than MQ stocks; mix small quantities.

Chlorhydroquinone is more difficult to source than hydroquinone — most photo-chemistry suppliers stock it only on special order. For most warm-tone work, a chlorhydroquinone-free formula (glycin, pyrocatechol) is more convenient.

Related compounds

Hydroquinone is the non-chlorinated parent compound. Chloro- and bromo-substituted hydroquinone derivatives exist for specialty applications.

Safety Notes

Acute hazards

Chlorhydroquinone shares the hazard profile of hydroquinone with slightly increased toxicity from the chlorine substitution:

  • Skin and eye irritant.
  • Toxic if ingested: moderate toxicity, substantial swallow causes systemic reaction.
  • Skin sensitizer: repeated contact risk.
  • Suspected mutagen / possible carcinogen (similar profile to hydroquinone).

Storage & handling

  • Personal protective equipment: Nitrile gloves, safety glasses, dust mask for weighing, long-sleeved apron.
  • 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 water for 15 minutes.
  • Eye contact: Flush with running water for at least 15 minutes.
  • Ingestion: Do NOT induce vomiting. Rinse mouth. Seek medical attention.

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