Sodium thiosulfate pentahydrate
CAS 10102-17-7 · EC 600-156-5
Also known as hypo, sodium hyposulphite (historic)
Handle chemistry safely
Wear gloves and eye protection, work with good ventilation, and read the supplier’s safety data sheet before use. Hazard information here is a summary, not a replacement for it. Dispose of all photographic chemistry correctly, in line with local laws and your local authority’s guidance. Never mix waste solutions, and check the supplier’s safety data sheet before disposing of anything.
Hazards
There is no official EU classification. Shown is the classification most companies have notified to ECHA: 96.39% of notifications, 4 distinct classifications notified.
No hazard statements in this classification.
Photographic uses
- fixing agent (F-5, F-6, F-24)
- Farmer's reducer (R-4a)
- hypo-alum sepia toner (T-1a)
- alt-process fixing
Used in: Kodak F-5 acid hardening fixer, Kodak F-24 non-hardening fixer, Kodak R-4a Farmer's reducer, Kodak T-1a hypo-alum sepia toner
Sources
- ECHA CHEM substance record: Thiosulfuric acid (H2S2O3), sodium salt, hydrate (1:2:5) (100.120.977)
- ECHA CHEM – C&L Inventory (harmonised CLP Annex VI and notified classifications) and legal obligations (chem.echa.europa.eu APIs: /api-cnl-inventory/harmonized|industry, /api-obligation-substance/v1)
Checked October 2026.