Boric Acid

OtherH3BO3CAS: 10043-35-3Shelf life: 60 mo
Boric Acid
Image: ChemSim — Public domain

Physical Properties

  • Molecular Weight: 61.83 g/mol
  • Solubility (20°C): 47 g/L

Also known as: Orthoboric Acid, Boracic Acid, H3BO3

Boric acid (H₃BO₃; CAS 10043-35-3) is a weak acid used as a pH buffer in developer and fixer formulas. Paired with borax, it forms a borate buffer system that holds solution pH within a narrow range (around 8.0–9.2) throughout a development cycle — critical for fine-grain and high-acutance developers where shifts in pH materially change contrast and grain.[1]

Photographic uses

  • Buffer in fine-grain developers: Kodak D-76, Microdol-X, and several Paterson formulas rely on a borate buffer to stabilize the mildly alkaline pH needed for metol / hydroquinone action.
  • Hardener / acidifier in fixers: Some acid-hardening fixer formulas use boric acid alongside potassium alum to maintain the acidic pH at which alum hardening is most effective.
  • Carryover buffer in stop baths: Occasionally used in place of acetic acid where odor or sensitivity to acetic fumes is a concern.

Practical notes

Supplied as white crystalline powder; dissolves readily in warm water (5% solubility at 20 °C, about 27% at 100 °C). Working solutions are stable for weeks. Store dry powder in a sealed container in a cool dry cabinet.

Related compounds

Borax (sodium tetraborate) is the alkaline partner in borate buffers; the ratio of borax-to-boric-acid sets the pH.

Safety Notes

Acute hazards

  • H319 eye irritant: dust and working solutions irritate eyes on contact. Flush with water for 15 minutes.
  • H360FD reproductive toxicant (EU CLP Category 1B): boric acid is classified as a reproductive toxicant for both fertility and developmental effects. The hazard is primarily chronic; incidental darkroom exposure at working concentrations is low-risk, but pregnant individuals should use PPE and avoid dust inhalation.

Storage & handling

  • Personal protective equipment: Nitrile gloves and safety glasses when handling dry powder; dust mask (N95 or better) if mixing more than ~100 g.
  • Storage: Sealed container in a dry cabinet, away from incompatibles (anhydrides, acetic anhydride).
  • California Proposition 65: boric acid is on the Prop 65 list for developmental toxicity — California retailers must provide a warning label.[2]

References

  1. BOOK Haist, Grant. Modern Photographic Processing, Volume 1 1st ed. John Wiley & Sons, 1979. ISBN 0-471-02228-0. ↩
  2. STANDARD California Office of Environmental Health Hazard Assessment (OEHHA). California Proposition 65 List of Chemicals State of California. https://oehha.ca.gov/proposition-65/proposition-65-list ↩
  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