Sodium Selenite

TonerNa2SeO3CAS: 10102-18-8Shelf life: 24 mo
Sodium Selenite
Image: Innerstream — Public domain

Physical Properties

Also known as: Selenium Toner (source), Na2SeO3

Sodium selenite (Na₂SeO₃; CAS 10102-18-8) is the direct-toning form of selenium — an alternative to selenium toner concentrate for workers who prefer mixing their own toner from defined ingredients.[1] Dissolved in ammonium thiosulfate at carefully-controlled concentrations, sodium selenite produces the same silver-to-silver-selenide conversion as commercial selenium toners but with more precise control over selenium concentration and pH.

Photographic uses

  • DIY selenium toner formulation: 1–5 g/L sodium selenite in ammonium thiosulfate (or hypo clearing agent) produces a working selenium toner. Working strength varies with intended purpose: low concentration for archival only, higher for visible tone shift.
  • Research-grade toning calibration: Densitometric workflows benefit from the known sodium-selenite concentration versus the proprietary commercial concentrates.
  • Specialty archival treatments: Some conservation protocols for historical silver gelatin prints specify sodium selenite as the defined selenium source for reproducible results.

Practical notes

Supplied as white to pale yellow crystalline powder. Soluble in water (~85 g/100 mL at 20 °C). Solutions are mildly alkaline (pH ~8.5) and stable in closed containers for months.

Formulation precision matters: selenium toning is concentration-sensitive, and 10% over-concentration can produce a much more aggressive tone shift than intended. Weigh sodium selenite on a precision scale (0.01 g resolution) for reproducible results.

Related compounds

Selenium toner concentrate is the ready-to-use commercial formulation containing a related selenium species. Selenium dioxide is the raw selenium oxide. Sodium sulfide is the primary sulfur-based alternative toner.

Safety Notes

Acute hazards

Sodium selenite is toxic — more hazardous than the commercial toner concentrate because it is the neat salt rather than a dilution:

  • Toxic if swallowed (H301): substantial swallow is a medical emergency.
  • Acute dermal toxicity: penetrates intact skin with systemic absorption — the systemic toxicity, not direct irritation, is the primary skin-contact concern.
  • Very toxic to aquatic life (H410).
  • Organ damage (H373) from repeated exposure.

Chronic hazards

  • Selenosis: cumulative selenium toxicity over extended exposure (see selenium dioxide for details).

Storage & handling

  • Personal protective equipment: Nitrile or butyl gloves (strict compliance), safety goggles, dust mask for weighing, long-sleeved clothing.
  • Ventilation: Use local exhaust when weighing dry powder or mixing concentrated stocks.
  • Storage: Tightly sealed in a cool dry cabinet separated from acids and reducing agents.

First aid

Consult the full Sigma-Aldrich Safety Data Sheet for authoritative first-aid instructions.[2]

  • Skin contact: Remove contaminated clothing. Flush with copious water for at least 20 minutes. Seek medical attention.
  • Eye contact: Flush eyes with running water for at least 20 minutes. Seek medical attention immediately.
  • Inhalation: Move to fresh air; seek medical attention for any symptoms.
  • Ingestion: Do NOT induce vomiting. Rinse mouth. Seek medical attention immediately.

References

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