Hydroxylamine Sulfate

Preservative(NH2OH)2·H2SO4CAS: 10039-54-0
Hydroxylamine Sulfate
Image: Benjah-bmm27 — Public domain

Physical Properties

  • Molecular Weight: 164.14 g/mol
  • Solubility (20°C): 640 g/L

Also known as: HAS, Hydroxylammonium Sulfate

Hydroxylamine sulfate ((NH₂OH)₂·H₂SO₄; CAS 10039-54-0) is a colour-process preservative — the powerful antioxidant that protects the colour developing agents CD-3 and CD-4 from aerial oxidation in C-41 (colour negative) and E-6 (colour reversal) developer solutions.[1] It has no role in B&W photography. The hydroxylamine moiety (NH₂OH) is itself a strong reducing agent that reacts preferentially with dissolved O₂, sparing the much more expensive and specific colour developer molecules.

Because hydroxylamine chemistry is considerably more hazardous than sulfite-based preservation, this page is authored with more depth than most Tier-2 entries.

Photographic uses

  • C-41 developer preservation: 2–4 g/L hydroxylamine sulfate in Kodak Flexicolor and compatible C-41 developer concentrates, protecting CD-4 from oxidation during working-solution tray or machine use.
  • E-6 first developer: Similar role in E-6 colour reversal first developer.
  • Replenisher formulas: High-volume colour processors use hydroxylamine sulfate in their replenisher mixes to extend developer tank life by dozens of rolls.
  • Specialty colour formulas: DIY colour developer formulations for home-scale C-41/E-6 processing rely on hydroxylamine sulfate as essentially the only commercially-available preservative that can handle the colour-developer oxidation kinetics.

Practical notes

Supplied as off-white or pale yellow crystalline powder — the free-base hydroxylamine is an unstable liquid/solid that can decompose explosively, so the sulfate salt is the safe commercial form. Solutions are stable in cold darkness for weeks; exposure to transition-metal ions (iron, copper) in hard water accelerates decomposition. Use distilled water for colour developer preparation, never tap.

Critical incompatibility: hydroxylamine salts can decompose exothermically in contact with metal oxides, strong bases, or organic reducing agents. The darkroom consequence is that metal scoops, iron counter surfaces, or brass weighing trays should not touch dry hydroxylamine sulfate — use plastic or glass utensils exclusively.

Regulatory status

Hydroxylamine sulfate is classified as a Category 2 suspected carcinogen (H351) under the EU CLP Regulation and is on various jurisdictional mutagen watch-lists.[2] B&W photographers have no need to handle it; the material is specialty colour-process chemistry only.

Related compounds

Colour developers (CD-3, CD-4) are themselves the chemicals that hydroxylamine sulfate protects. Sodium sulfite is the corresponding preservative for B&W developers, where hydroxylamine's aggressive chemistry is not needed. Hydroxylamine hydrochloride ((NH₂OH)·HCl) is the chloride analog sometimes substituted in DIY colour formulas; equivalent reducing power, different solubility profile.

Safety Notes

Acute hazards

Hydroxylamine sulfate is the most dangerous chemical in this batch:

  • Corrosive / severe irritant: causes serious eye damage and skin burns on direct contact with concentrated solution. Even dilute working solutions cause notable skin irritation on extended contact.
  • Moderately toxic if ingested or absorbed through skin: substantial exposure causes methemoglobinemia (impaired oxygen transport in blood), with symptoms of weakness, dizziness, and bluish skin coloration. Medically serious, requiring prompt treatment.
  • Skin sensitizer: repeated exposure can develop allergic contact dermatitis in some workers.

Chronic hazards

  • Suspected carcinogen (EU CLP Cat 2, H351): animal evidence of carcinogenicity; human evidence is limited but cautionary. Long-term skin contact should be minimized.
  • Mutagen concerns: some evidence of chromosomal effects in laboratory studies.

Fire & reactivity

  • Unstable when heated: the free-base hydroxylamine decomposes explosively; the sulfate salt is much safer but will decompose exothermically if heated above ~100 °C, potentially vigorously. Do NOT attempt to dry on a hot plate or in an oven above 50 °C.
  • Reactive with metal oxides, strong bases, organic reducers: can ignite or decompose. Keep away from iron oxide (rust), copper surfaces, potassium permanganate, strong oxidizers.
  • Water-stable in solution at typical darkroom concentrations, but acidification to pH < 2 or strong alkalinity (pH > 10) accelerates decomposition significantly.

Storage & handling

  • Personal protective equipment: Nitrile or butyl gloves (check compatibility chart for your specific glove), safety goggles (not just safety glasses), long-sleeved apron, acid-gas respirator cartridge when working with bulk dry powder. Face shield is appropriate for opening bulk containers.
  • Ventilation: Dedicated local exhaust or fume hood for dry powder handling and concentrated solution preparation.
  • Storage: Tightly sealed plastic or glass container (never metal) in a cool dark cabinet. Segregate from metal oxides, acids, and organic reducing agents. Label clearly — the similar-sounding hydroxylamine hydrochloride is a distinct compound.
  • Training: This is not a casual-handling chemical. Work with it only if you have a specific colour-process workflow that requires it; B&W photographers should have no reason to stock it.

First aid

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

  • Skin contact: Remove contaminated clothing; flush affected skin with running water for at least 15 minutes; seek medical attention for any persistent redness or burn.
  • Eye contact: Flush eyes with running water for at least 30 minutes, lifting the lids; seek medical attention immediately.
  • Inhalation: Move to fresh air; seek medical attention if breathing difficulty, dizziness, or cyanosis (bluish skin) develops — these are signs of methemoglobinemia.
  • Ingestion: Do NOT induce vomiting. Rinse mouth with water. Seek medical attention immediately; hydroxylamine ingestion is a medical emergency.

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