CD-3

Developing AgentC12H21N3O2S·1.5H2SO4·H2OCAS: 25646-71-3
CD-3
Image: Def2010 — CC0

Physical Properties

  • Molecular Weight: 437.5 g/mol
  • Solubility (20°C): 100 g/L

Also known as: Color Developing Agent 3

CD-3 (4-(N-ethyl-N-2-methanesulfonamidoethyl)-2-methyl-1,4-phenylenediamine sesquisulfate monohydrate; CAS 25646-71-3) is the colour developing agent for E-6 colour reversal film, RA-4 colour paper, and some alternative-process colour formulas.[1] It is a substituted p-phenylenediamine that reduces silver halide in the normal developing-agent fashion; critically, its oxidized form couples with colour-couplers in the emulsion to form dye molecules — the defining chemistry of chromogenic colour photography. Without CD-3 (and its sibling CD-4), modern colour negative and reversal processes don't work.

Photographic uses

  • E-6 colour reversal first developer: CD-3 is the primary developing agent in the E-6 colour development step (after B&W first development and fogging). Produces cyan, magenta, and yellow dye images from the coupler system in each emulsion layer.
  • RA-4 colour paper developer: The colour paper-printing process uses CD-3 at working strength 4–6 g/L in a buffered phenidone-activated developer.
  • Specialty colour formulas: Some DIY colour development experiments use CD-3 as the "fast" colour developer for Ektachrome-family films.

Practical notes

Supplied as white to pale tan crystalline powder, usually as the sesquisulfate monohydrate hydrate salt for solubility. Extremely oxidation-sensitive — dissolves readily in water but the resulting solution darkens within hours as atmospheric oxygen attacks the molecule. Commercial E-6 developer kits include hydroxylamine sulfate or similar antioxidants to extend working-solution life; DIY colour work without antioxidant preservation gets very short developer life.

Solutions should be refrigerated and used within a few days. Fresh-mixed E-6 first developer with CD-3 produces best colour reproduction; aged solutions shift colour balance.

Regulatory status

Colour developing agents (CD-3, CD-4, p-phenylenediamine) are all classified as skin sensitizers, suspected mutagens, and (in most jurisdictions) suspected carcinogens.[2] Workers handling colour developers regularly are at meaningful risk of occupational sensitization; casual home use for a few rolls per year is a much smaller risk but still warrants glove compliance.

Related compounds

CD-4 is the C-41 colour developer sibling. p-Phenylenediamine is the parent compound from which CD-3 and CD-4 are derived. Hydroxylamine sulfate is the antioxidant paired with colour developers.

Safety Notes

Acute hazards

CD-3 shares the severe p-phenylenediamine-family hazard profile:

  • Toxic if swallowed (H301): substantial ingestion is a medical emergency.
  • Toxic in contact with skin (H311): systemic absorption through intact skin at gram-scale exposure.
  • Causes severe skin burns and eye damage (H314) in concentrated solution.
  • Strong skin sensitizer (H317): repeated exposure builds permanent allergic contact dermatitis in most workers.
  • Fatal if inhaled (H330): fine dust.
  • Suspected mutagen / possible carcinogen (H341/H350).
  • Very toxic to aquatic life (H410).

Chronic hazards

  • Permanent sensitization on repeated exposure: once sensitized, workers can no longer handle any colour developer — they must switch to B&W or abandon darkroom work.
  • Possible carcinogenicity: animal evidence; human epidemiological data is limited but cautionary.

Storage & handling

  • Personal protective equipment: Nitrile gloves (strict compliance, replace frequently), safety goggles, respirator (P100 filter) for dust, long-sleeved chemical-resistant clothing.
  • Ventilation: Fume hood for weighing dry powder. Local exhaust for working-solution handling.
  • Storage: Tightly sealed amber container, cool dark cabinet, locked.

First aid

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

  • Skin contact: Remove contaminated clothing. Flush with copious water for at least 20 minutes. Seek medical attention.
  • Eye contact: Flush with running water for at least 30 minutes; seek medical attention immediately.
  • Inhalation: Move to fresh air. Seek emergency medical attention.
  • 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. STANDARD European Chemicals Agency (ECHA). REACH Regulation (EC) No 1907/2006, Annex XVII – Restrictions on manufacture, placing on the market and use European Union. https://echa.europa.eu/substances-restricted-under-reach ↩
  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