Sodium Phosphate Tribasic

AcceleratorNa3PO4CAS: 7601-54-9
Sodium Phosphate Tribasic
Image: Edgar181 — Public domain

Physical Properties

  • Molecular Weight: 163.94 g/mol
  • Solubility (20°C): 120 g/L

Also known as: Trisodium Phosphate, TSP

Sodium phosphate tribasic (Na₃PO₄; CAS 7601-54-9), commonly abbreviated TSP, is a strong alkali accelerator with good buffering capacity and secondary hard-water sequestering action. Solutions produce pH around 12, putting it close to sodium hydroxide in alkali strength but with meaningfully gentler handling properties and better pH stability through a working session.[1] It appeared in several older Kodak and industrial developer formulas but has fallen out of fashion in contemporary home-darkroom work; its current primary darkroom role is as a pre-wash and equipment cleaning aid.

Photographic uses

  • Older Kodak developer formulas: D-61a, D-70, and related mid-century formulas used TSP as the activator for a high-contrast, pH-stable working solution.
  • Darkroom equipment cleaner: A 1–2% TSP solution scrubs stubborn silver and developer stains from trays, tanks, and bottles — the same household TSP paint-preparation cleaner sold at hardware stores.
  • Hard-water sequestering: TSP's phosphate ion binds calcium and magnesium, preventing mineral scum in developing baths mixed from tap water in hard-water regions.[2]
  • Pre-wash for fibre-based paper: A dilute TSP pre-wash ahead of development helps emulsion swell uniformly and reduces drying marks on fibre paper.

Practical notes

Supplied as white crystalline granules (usually the dodecahydrate Na₃PO₄·12H₂O, sometimes the anhydrous form). The dodecahydrate is the typical photographic and cleaning-product form; convert to anhydrous by multiplying by 0.43 if a formula calls for the anhydrous weight.

Environmental note: phosphate discharge into watershed is an eutrophication concern at commercial scale. Home-darkroom quantities are negligible versus household cleaning-product phosphate loads, but many jurisdictions have restricted phosphate-based cleaners for this reason.

Related compounds

Sodium hydroxide is the stronger hydroxide alkali; sodium carbonate is the more common moderate alkali. Sodium phosphate dibasic (Na₂HPO₄) is the less-alkaline phosphate salt used in buffered photographic solutions.

Safety Notes

Acute hazards

Sodium phosphate tribasic is corrosive in concentrated solution:

  • Severe skin burns: concentrated solution causes chemical burns on extended contact.
  • Serious eye damage: splashes require extended flushing and medical attention.
  • Irritating to mucous membranes: dust inhalation causes upper-respiratory irritation.
  • Harmful if ingested: a substantial swallow causes gastrointestinal irritation and, at larger doses, systemic phosphate overload.

Storage & handling

  • Personal protective equipment: Nitrile or butyl gloves; safety goggles for concentrated-solution work; dust mask when handling dry powder.
  • Storage: Tightly closed in a dry cabinet; the dodecahydrate is mildly hygroscopic.

First aid

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

  • Skin contact: Flush with water for 15 minutes; seek medical attention for burn-like reaction.
  • Eye contact: Flush with running water for at least 15 minutes; seek medical attention.
  • Inhalation: Move to fresh air.
  • Ingestion: Rinse mouth; do NOT induce vomiting; seek medical attention.

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. 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