Borax

AcceleratorNa2B4O7·10H2OCAS: 1303-96-4
Borax
Image: Master Uegly — Public domain

Physical Properties

  • Molecular Weight: 381.37 g/mol
  • Solubility (20°C): 25 g/L
  • Solubility (50°C): 150 g/L

Also known as: Sodium Tetraborate Decahydrate, Boracic

Borax (sodium tetraborate decahydrate, Na₂B₄O₇·10H₂O; CAS 1303-96-4) is a mild alkali buffer that produces a working pH of roughly 9.2 — low enough to be gentle on emulsions but high enough to activate phenolic developing agents. It is the alkali in D-76 / ID-11, the reference film developer, and appears in most fine-grain and compensating developer formulas where controlled, steady development at moderate pH is the design goal.[1]

Photographic uses

  • D-76 / ID-11 film developer: 2 g borax per litre in the standard MQ formula.
  • Fine-grain compensating developers: Borax's buffering action keeps pH stable through the development cycle, producing consistent contrast across a roll.
  • Low-contrast paper formulas: A borax paper developer gives softer working than a sodium carbonate or sodium hydroxide equivalent.

Practical notes

Supplied as the decahydrate — the white crystalline form with 10 waters of hydration. Solubility is modest (~5% at 20 °C). Most formulas assume decahydrate; if substituting anhydrous borax, multiply the weight by 0.53.[2]

Dry borax is shelf-stable indefinitely. Solutions keep for months in a closed container. The compound is widely sold as a laundry booster ("20 Mule Team Borax") which is pure enough for photographic use — a meaningful cost saving versus chemistry-supplier borax at equivalent purity.

Regulatory status

Boron compounds are classified as reproductive toxins (Repro Category 1B) under the EU CLP Regulation and are on the REACH Candidate List of Substances of Very High Concern.[3] Working-strength darkroom quantities pose no meaningful reproductive-health risk, but pregnant workers or those trying to conceive may reasonably prefer sodium metaborate (which has a similar classification but is used in smaller quantities) or non-borate alkalis entirely.

Related compounds

Sodium metaborate (Kodalk) is a related borate with a higher working pH (~9.7) and superior buffering — Kodak's preferred borate when borax's mild pH is too low. Sodium carbonate is the more vigorous non-borate alternative; sodium bicarbonate the milder non-borate alternative.

Safety Notes

Acute hazards

Borax has low acute toxicity and is sold as a household cleaning product, but the EU classification has become more conservative with accumulating evidence of reproductive-health effects:

  • Mild skin and eye irritant in concentrated solution. Working-strength (typically 0.2% in D-76) causes no skin reaction.
  • Harmful if ingested in quantity — a single spoonful of dry borax is a medically significant exposure. Keep away from food and children.
  • Reproductive toxicity concern: Regulatory evidence supports Category 1B classification for boron compounds. Chronic occupational exposure (not single-incident darkroom use) is the concern.

Storage & handling

  • Personal protective equipment: Gloves and safety glasses when weighing dry powder. A dust mask is appropriate for larger quantities.
  • Storage: Dry container in a cool dry cabinet. Stable indefinitely.

First aid

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

  • Skin contact: Rinse with water.
  • Eye contact: Flush with running water for at least 10 minutes.
  • Inhalation: Move to fresh air.
  • Ingestion: Rinse mouth; seek medical attention for any substantial swallow.

References

  1. BOOK Anchell, Steve; Troop, Bill. The Film Developing Cookbook 2nd ed. Routledge, 2019. ISBN 9781138959187. ↩
  2. BOOK Haist, Grant. Modern Photographic Processing, Volume 2 1st ed. John Wiley & Sons, 1979. ISBN 0-471-04635-X. ↩
  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 2026-04-21