Selenium Toner Concentrate

TonerShelf life: 24 mo

Physical Properties

Also known as: Kodak Rapid Selenium Toner, KRST, Selenium Toner

Selenium toner concentrate is the ready-to-use commercial formulation that most darkroom workers actually use for selenium toning — a solution of sodium selenosulfate (Na₂SeSO₃) in ammonium thiosulfate carrier, typically sold as a concentrate diluted 1:3 (for visible toning) to 1:20 (for archival protection) with a working-strength washing aid.[1] Kodak Rapid Selenium Toner was the industry standard for decades; since Kodak discontinued the product, identical formulations are distributed under various brand names (Bostick & Sullivan, Clayton, ADOX, others).

Photographic uses

  • Archival protection: The primary modern use. 1:20 dilution for 2–4 minutes converts trace silver species (which attack the image over decades) to stable silver selenide; print tone is almost unchanged at this dilution. Every archival-minded silver gelatin printer uses selenium toning as the permanence step.
  • Visible toning (reddish-purple shift): 1:3 to 1:10 dilution for 3–10 minutes produces a noticeable tone shift toward warmer reddish-purple. The shift is more pronounced on warm-tone papers (Ilford Warmtone, Foma Warmtone) than on neutral papers.
  • Split toning: Brief selenium toning after sepia toner produces classical split-tone effects — cool selenium shadows + warm sepia highlights.
  • Density boost: Selenium toning slightly intensifies silver print density; useful for slightly thin prints that would benefit from ~⅓ grade of contrast gain.

Practical notes

Supplied as a dark brown to amber viscous liquid in 500 mL or 1 gallon bottles. Strong ammonia smell from the thiosulfate/ammonium carrier. Working-strength dilutions have mild ammonia odor that fades as the bath is used.

Mix with hypo-clear bath, not plain water, to prevent pH issues during toning. Kodak Hypo Clearing Agent (sulfite + citrate) is the traditional choice; any sulfite-based wash aid works.

Stability: The concentrate keeps for years tightly sealed. Working dilutions keep for the duration of a single toning session; discard after use. The bath darkens progressively as silver accumulates in solution — not a sign of exhaustion, but of silver complex formation.

Related compounds

Sodium selenite is an alternative selenium source used in some DIY selenium toner formulations. Selenium dioxide is the raw selenium oxide used by workers preparing selenium toner from scratch. Ammonium thiosulfate is the base carrier solvent — essentially fixer chemistry adapted for selenium toning.

Safety Notes

Acute hazards

Selenium toner concentrate is significantly safer than raw selenium dioxide because the sodium selenosulfate form is less volatile and less skin-absorbable than the free oxide, but it is still a selenium compound and carries moderate hazards:

  • Harmful if swallowed (H302): medically significant at gram-scale ingestion.
  • Skin and eye irritant: concentrated solution causes stinging and persistent yellow-brown staining.
  • Ammonia vapor: the concentrate smells strongly of ammonia (from the thiosulfate carrier); working dilutions are milder.
  • Toxic to aquatic life: selenium discharge is a water-quality regulatory concern. Dispose of exhausted toner as hazardous waste.

Chronic hazards

  • Cumulative selenium exposure: frequent handling of concentrated toner over years can contribute to systemic selenium load. Glove compliance is the main mitigation.

Storage & handling

  • Personal protective equipment: Nitrile gloves (strict compliance), safety glasses, long-sleeved apron. No respirator needed for working dilutions; some workers use a dust mask when opening the concentrate because of ammonia vapor.
  • Ventilation: Normal darkroom ventilation is adequate for working dilutions; open the concentrate in well-ventilated space.
  • Storage: Tightly sealed in a cool dark cabinet. The brown bottle the concentrate ships in is the right storage container — don't transfer to secondary containers.

First aid

Consult the Sigma-Aldrich SDS for sodium selenite (sigald/s5261) for the closest authoritative first-aid reference, adjusting for the less-concentrated form used in commercial toners.[2]

  • Skin contact: Flush with water. Stain may persist; the underlying chemistry is not immediately dangerous at working dilution.
  • Eye contact: Flush with running water for at least 15 minutes. Seek medical attention if irritation persists.
  • Ingestion: Rinse mouth; seek medical attention for any substantial swallow.

References

  1. BOOK Anchell, Steve. The Darkroom Cookbook 4th ed. Focal Press, 2016. ISBN 9781138959170. ↩
  2. WEB Sigma-Aldrich (Merck KGaA). Sigma-Aldrich Safety Data Sheets Sigma-Aldrich. https://www.sigmaaldrich.com/US/en/search/safety-data-sheets ↩

Last reviewed 2026-04-22