In ‘rapid’ fixers, Ammonium Thiosulfate is often used instead. Other chemicals can be in the blend as well. Check the label to verify yours is pure Sodium Thiosulfate. If your photographic fixer is spent (used) or a blend, visit this page. If your photographic fixer is ammonium thiosulfate, visit this page.
Always consult a manufacturer SDS. Read our Full Liability Disclaimer
Na₂S₂O₃•5H₂O
Hazard Level & Hazards:
When we create or update chemical disposal entries, our H-code hazard list is generous. We list hazards that may be just below regular cutoffs and would otherwise normally be unlisted. We do this to provide hobbyists a wider scope of safety risks that may still be relevant, despite normally being omitted. Our GHS icons, however, are congruent with typical legal labeling requirements of the respective chemical.
Low
- H000: Low hazard for normal industrial use
Disposal Method
Check local regulations. For small amounts, dilute with copious amounts of water and pour down the drain with the faucet running during and for 2 minutes afterwards. Avoid large quantities or undiluted product going down the drain, entering sewers or entering waterways.If mixed with other chemicals, contact your local household hazardous waste facility for information about local regulations. While biodegradable, high concentrations can affect aquatic life and water quality.
Neutralizing Agent
N/A. For solid spills: Wear PPE. Sweep or absorb the solid. For liquid spills: Wear PPE. Use an absorbent like kitty litter to absorb the liquid. Wipe with paper towels and dispose of all waste regularly but in a separate bag.
Storage Precautions
Keep in original container. If transfer is necessary, HDPE is ideal. Polypropylene is often used for short term. Keep container tightly closed. Dry. Do not store near acids.
Ideal Storage Container
HDPE. Review our chemical storage container guide for more information about container types and materials.
Other Names
hypo, sodium hyposulfite, anticlor, and thiosulfuric acid disodium salt.
Additional Tips
- Localized pH drops can ruin a batch of fixer before the gas even becomes a smell. When adding an acid (like a hardener) to a thiosulfate solution, never pour it in a stream. Use a high-shear mixing method. Stir the thiosulfate rapidly to create a vortex and drop the acid into the center. This prevents localized acidification, where the pH drops below 4.0 in one spot, causing a sulfur kick-out. Once sulfur precipitates (turns the water cloudy), it cannot be re-dissolved, and it can act as a sensitizer for respiratory irritation
- Sodium thiosulfate usually comes as the pentahydrate (Na₂S₂O₃•5H₂O). If a formula calls for sodium Thiosulfate without specifying, it’s usually the pentahydrate. If you substitute Anhydrous (water-free) powder, you must use roughly 64% of the weight specified for the pentahydrate.
- Sodium thiosulfate is like candy for certain sulfur-eating bacteria. If your clear solution develops a rotten egg smell or a cloudy biofilm over time, it has been colonized. Do not try to boil it clean. Heating a biologically contaminated thiosulfate solution can accelerate the release of H₂S and SO₂. Always stabilize long-term stock solutions with a tiny amount of Sodium Carbonate (0.2g/L) to keep the pH slightly alkaline and hostile to bacteria
- In a darkroom context, thiosulfate “fixes” silver by forming soluble complexes. There is a point of no return where the solution is so saturated with silver that it forms insoluble thiosulfates. These can’t be washed out of the paper/film and will eventually rot the media. Incomplete fixing leaves silver halides that can react with skin or surfaces. Use hypo check drops (mercuric iodide or potassium iodide) to test your solution. If it turns cloudy when the drop hits the water, the solution is exhausted and must be disposed of as hazardous heavy metal waste.
- Sodium thiosulfate pentahydrate is efflorescent. It loses its water molecules to the air in dry environments, turning it into a white crusty powder. The crust is more concentrated than the crystals. If you measure by volume (scoops) rather than mass (grams), you will accidentally create a stronger, more reactive solution than intended. Always store in an airtight container.
- Sodium thiosulfate is the antidote for chlorine bleach spills, but the order of operations matters. Never dump dry thiosulfate powder onto a liquid bleach spill. It can create a localized heat reaction. Dilute the thiosulfate in water first (10% solution), then slowly add it to the spill. This “wet-to-wet neutralization prevents the splashing and thermal spikes that cause skin burns or the release of gas
- Mixing anhydrous thiosulfate with water is exothermic (releases heat). Mixing pentahydrate is endothermic (the beaker will get ice-cold). Make sure you expect the correct temperature change. In my experience, the rapid change to cool can sometimes trick you into thinking it’s hot.
Incompatible Chemicals/Dangerous Combinations
- Strong Acids (e.g., Stop Bath, HCl) Toxic Gas – Reacts to release Sulfur Dioxide (SO₂), a colorless but choking gas. The solution will also “milk” or turn cloudy while solid sulfur precipitates out.
- Oxidizing Agents (e.g., Nitrates, Chlorates) Fire / Explosion – Can cause an exothermic (heat-releasing) reaction. If mixed as dry powders and heated, can become shock-sensitive and explode
- Sodium Hypochlorite (Bleach) (NaClO) Toxic Gas – Neutralizes the bleach but can generate Chlorine gas or Chloramine fumes if concentrations are high or if the solution becomes even slightly acidic
- Sodium Nitrite (NaNO₂) Explosion – If heated together to dryness, the mixture can undergo an explosion once the water of crystallization is driven off.
- Potassium Permanganate (KMnO₄) Instant Heat – Causes an exothermic reaction. The thiosulfate acts as a reducing agent that rapidly breaks down the permanganate, resulting in boiling or splashing
- Lead or Silver Salts Heavy Precipitation – Often used to fix silver, but mixing with high concentrations of metal salts creates dense, dark precipitates that can be difficult to clean and may be environmentally regulated