Sodium Metaborate

AcceleratorNaBO2CAS: 7775-19-1
Sodium Metaborate
Image: Claudio Pistilli — CC BY-SA 4.0

Physical Properties

  • Molecular Weight: 65.8 g/mol
  • Solubility (20°C): 260 g/L

Also known as: Kodalk, Balanced Alkali

Sodium metaborate (NaBO₂; CAS 7775-19-1), marketed by Kodak as Kodalk Balanced Alkali, is a borate alkali that sits between borax and sodium carbonate in activity: it produces working pH around 9.7, activating developing agents well without the contrast-boosting vigor of carbonate-based activators.[1] Its defining property is superior buffering capacity — sodium metaborate holds pH steady through an entire development cycle even as oxidation products accumulate, which makes it the activator of choice for replenished developer systems and high-volume processing.

Photographic uses

  • Kodak Xtol: The primary alkali in the canonical hydroquinone-free commercial developer.
  • Kodak Ektamatic / Kodak D-76 variants: Kodalk-based variants of standard formulas give slightly different working contrast and better pH stability across a working session.
  • Replenished developer systems: Commercial darkrooms running replenisher-based workflows favour Kodalk over carbonate because the pH stays within tolerance for many hundreds of rolls or prints of continuous use.[2]
  • Fine-grain compensating developers: Some MQ-compensating formulas substitute sodium metaborate for borax when a slightly more active working pH is desired.

Practical notes

Supplied as a free-flowing white powder (the tetrahydrate, Na₂B₂O₄·4H₂O is the commonly-stocked commercial form; some suppliers sell the dihydrate). Less hygroscopic than borax; relatively easy to store and weigh accurately. Solutions are stable indefinitely in closed containers.

The Kodak "Kodalk" product is sodium metaborate plus small amounts of other buffering salts; generic sodium metaborate from a chemistry supplier is close enough for all photographic purposes.

Regulatory status

Like borax, sodium metaborate carries the EU reproductive toxin Category 1B classification because of its boron content.[3] Occupational exposure at the levels used in darkroom work is far below any regulatory concern, but anyone pregnant or trying to conceive may prefer a non-borate alkali (carbonate or metaborate alternatives).

Related compounds

Borax is the milder borate (pH ~9.2 vs Kodalk's 9.7), used in D-76 and similar. Sodium carbonate is the stronger non-borate alternative at pH ~10.5.

Safety Notes

Acute hazards

Sodium metaborate is a mild alkali with the same boron-related chronic concern as borax:

  • Mild skin and eye irritant in concentrated solution. Working-strength exposure produces no skin reaction.
  • Harmful if ingested in quantity: a substantial swallow (a tablespoon or more) would be medically significant.
  • Reproductive toxicity concern: EU Category 1B for reproductive toxicity. See borax for further context.

Storage & handling

  • Personal protective equipment: Gloves and safety glasses for bulk handling; dust mask for weighing powder.
  • Storage: Closed container in a dry cabinet.

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; seek medical attention if irritation persists.
  • Inhalation: Move to fresh air.
  • Ingestion: Rinse mouth; seek medical attention for substantial swallow.

References

  1. BOOK Haist, Grant. Modern Photographic Processing, Volume 2 1st ed. John Wiley & Sons, 1979. ISBN 0-471-04635-X. ↩
  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 2026-04-21