Benzotriazole

RestrainerC6H5N3CAS: 95-14-7
Benzotriazole
Image: Leyo — Public domain

Physical Properties

  • Molecular Weight: 119.12 g/mol
  • Solubility (20°C): 20 g/L

Also known as: BZT, Kodak Anti-Fog No. 1, 1,2,3-Benzotriazole

Benzotriazole (1,2,3-benzotriazole, C₆H₅N₃; CAS 95-14-7), sometimes called BZT, is an organic heterocyclic restrainer much more potent than the alkali-halide salts — a few tenths of a gram per litre replaces the 1–3 g/L of potassium bromide in a paper developer.[1] Its defining property is cleaner highlight rendering: where bromide tends to fog gradually as a working developer ages, benzotriazole suppresses fog without the speed-loss or warm-tone-shift characteristic of bromide. This makes it the preferred restrainer in cold-tone developer formulas and in developers for elderly or partially-fogged emulsions.

Photographic uses

  • Cold-tone paper developer: 0.2–0.5 g/L benzotriazole replaces or complements bromide in formulas designed for neutral or slightly cool black tones.[2]
  • Anti-fog for aged film: A few milligrams per litre added to a developer reduces base fog from film stored for years, recovering usable images that bromide alone cannot.
  • Replenished high-volume developer preservation: Commercial photo labs add benzotriazole to developer replenisher to maintain low fog across hundreds of rolls processed through the same working tank.
  • Metol-hydroquinone clean-tone formulas: Kodak Selectol-Soft and related warm-to-cold paper developers include benzotriazole in the cold-tone direction.

Practical notes

Supplied as fine white crystalline powder. Sparingly soluble in cold water (~2 g/100 mL at 20 °C). For developer mixing, dissolve benzotriazole in a small volume of hot water or in ethanol first, then add to the main working volume. A 0.1% stock solution (1 g/L) is a convenient dispensing form.

Shelf-stable dry for years in a tightly sealed container. Solutions keep for several months in amber glass.

Regulatory status

Benzotriazole is classified as a Category 2 suspected carcinogen (H351) under the EU CLP system.[3] The classification reflects animal evidence; human evidence is limited. Darkroom-scale exposure to working-strength developer poses no meaningful chronic risk, but handling bulk dry powder or concentrated stock solutions warrants conservative PPE.

Related compounds

Potassium bromide is the inorganic-halide restrainer with similar function but different tone/contrast behaviour. Tolyltriazole is a methylated variant of benzotriazole sometimes used in DIY formulas; essentially interchangeable at slightly lower working concentrations. 6-nitrobenzimidazole is a specialty organic restrainer for extremely aged film stock.

Safety Notes

Acute hazards

Benzotriazole has moderate hazards with a specific chronic concern:

  • Skin and eye irritant in concentrated solution. Direct contact with dry powder produces mild irritation; solution splashes cause transient stinging.
  • Harmful if ingested: substantial swallow causes nausea and gastrointestinal distress. Medically significant at large doses.
  • Suspected carcinogen: EU Cat 2 classification. Minimize cumulative skin contact.

Chronic hazards

  • Suspected carcinogenicity: animal studies show positive response; human epidemiological evidence is limited but cautionary. Long-term occupational exposure is the concern, not single-incident darkroom use.
  • Environmental persistence: benzotriazole does not biodegrade readily and appears as a contaminant in municipal wastewater. Dilute darkroom quantities contribute negligibly to the load, but high-volume commercial users should treat spent developer as chemical waste.

Storage & handling

  • Personal protective equipment: Nitrile gloves (replace if punctured), safety glasses, dust mask when weighing dry powder. A face shield is appropriate for opening bulk containers.
  • Ventilation: Normal darkroom ventilation is adequate for working-strength solutions; use local exhaust for dry powder handling.
  • Storage: Tightly closed container in a cool dry cabinet.

First aid

Consult the full Sigma-Aldrich Safety Data Sheet for authoritative first-aid instructions.[4] In brief:

  • Skin contact: Wash with soap and water. Seek medical attention for persistent reaction.
  • Eye contact: Flush with running water for at least 15 minutes; seek medical attention.
  • Inhalation: Move to fresh air; seek medical attention if cough or irritation persists.
  • Ingestion: Rinse mouth; do NOT induce vomiting; 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. 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 ↩
  4. 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