Sodium Thiosulfate (Na₂S₂O₃)
Versatile inorganic salt used in photography, water treatment, medical treatments and industrial applications
Overview
Sodium Thiosulfate (Na₂S₂O₃, CAS No: 7772-98-7) is an inorganic compound in colorless to white crystalline form. It is used as a fixer in the photography industry, chlorine remover in water treatment, antidote in cyanide poisoning in medicine and in gold mining.
Sodium thiosulfate has high water solubility (70.1 g/100 mL at 20°C) and the pentahydrate form (Na₂S₂O₃·5H₂O) is commonly used. This sulfur-containing compound exhibits strong reducing properties and reacts with iodine to form sodium tetrathionate.
Applications
Sodium Thiosulfate has a wide range of industrial, medical and commercial applications:
Photography
Known as "hypo" in traditional photography and used as a fixer. Dissolves silver halides to fix films and papers.
Medical Applications
Used as an antidote in cyanide poisoning. Also used in calcium channel blocker poisoning and treatment of some skin diseases.
Water Treatment
Used as a chlorine remover in drinking water and swimming pools. Neutralizes excess chlorine to make water safe for consumption after disinfection.
Mining
Used as an alternative to cyanide in gold and silver leaching processes. Offers an environmentally friendly alternative and safer operation.
Analytical Chemistry
Used as a standard reagent in iodometric titrations. Reacts with iodine to give accurate results.
Leather and Textile
Used in leather processing and decolorizing dyes in the textile industry. Neutralizes the effect of chlorinated bleaches.
Technical Specifications
| Property | Value | Unit | Test Method |
|---|---|---|---|
| Chemical Name | Sodium Thiosulfate | - | - |
| CAS Number | 7772-98-7 (anhydrous), 10102-17-7 (pentahydrate) | - | - |
| EC No | 231-867-5 | - | - |
| Formula | Na₂S₂O₃ (anhydrous), Na₂S₂O₃·5H₂O (pentahydrate) | - | - |
| Molecular Weight | 158.11 (anhydrous), 248.18 (pentahydrate) | g/mol | - |
| Appearance | White crystalline powder or colorless crystals | - | Visual |
| Odor | Odorless | - | - |
| Purity | ≥99.0 (anhydrous), ≥98.0 (pentahydrate) | % | ISO 424 |
| Na₂S₂O₃ Content | ≥99.0 (anhydrous), ≥98.0 (pentahydrate) | % | ISO 424 |
| Sulfate (SO₄) Content | ≤0.1 | % | ISO 3423 |
| Water Content | ≤0.2 (anhydrous), 36.0-38.0 (pentahydrate) | % | Karl Fischer |
| pH Value (5% sol.) | 6.0-9.0 | - | ISO 1842 |
| Solubility (Water, 20°C) | 70.1 (anhydrous), 210 (pentahydrate) | g/100 mL | ISO 918 |
| Density | 1.67 (anhydrous), 1.69 (pentahydrate) | g/cm³ | ISO 787 |
| Melting Point | 48.3 (pentahydrate, dehydrates), anhydrous form at 100°C | °C | ISO 6321 |
| Heavy Metals (as Pb) | ≤10 | mg/kg | ISO 172 |
Safety Information
Hazard Symbols and Statements
Irritant
Personal Protective Equipment (PPE)
- Dust mask (in high dust concentrations)
- Chemical-resistant gloves
- Goggles (against dust or solution splash)
- Lab coat or protective clothing
Storage Conditions
- Store in a cool, dry and well-ventilated place
- Keep in original packaging, tightly closed
- Pentahydrate form should be stored below 50°C
- Keep away from acids and strong oxidants
- Protect from moisture, prevent water contact
Fire Fighting
- Non-combustible substance
- Decomposes at high temperatures releasing toxic gases (sulfur oxides)
- Suitable methods: Water spray, dry chemical powder, CO₂
- Firefighting personnel must use full respiratory protection
Handling and Storage
Transportation
- Ensure containers are tightly closed
- Prevent moisture contact
- Transport separately from acids and oxidants
- Have absorbent material available for spills
- Provide ventilation during transport
Storage
- Store in dry, cool and well-ventilated areas
- Recommended storage below 50°C for pentahydrate form
- Display appropriate signage in storage area
- Protect from direct sunlight
- Store on elevated shelves
Spill Response Measures
- Evacuate exposed personnel from area
- Use appropriate personal protective equipment
- Collect spill by dry method (sweeping)
- Avoid contact with acids
- Wash spilled area with plenty of water
Regulatory Information
GHS Classification
- Eye Irritation Category 2
- Skin Irritation Category 2
Important Regulations
- REACH: Registered
- EPA: Listed in TSCA inventory
- FDA: Approved in pharmaceutical grade USP/BP/EP standards
- European Pharmacopoeia: Suitable for medical use
- WHO Essential Medicines List: Listed as cyanide antidote
Compliance Certificates
- ISO 9001:2015 Quality Management System
- ISO 13485:2016 Medical Device Quality Management System
- USP/BP/EP Pharmacopoeia Standards (for medical grade)
- Food grade certificate (for specific batches)
Environmental Information
Environmental Impact
Sodium thiosulfate can cause oxygen depletion in aquatic environments at high concentrations. However, it is naturally degradable and non-persistent in the environment. Direct mixing into water sources should be prevented. It decomposes into sulfate and sodium ions in soil.
Biodegradation
- Decomposed by microbial activity in soil and water
- Participates in natural sulfur cycle
- High biological oxygen demand (BOD)
Waste Management
- Classified as non-hazardous waste
- Neutralization recommended for large amounts of waste
- Do not discharge to sewers or surface waters
- Dispose in accordance with local regulations
- Consider recycling or reuse
Frequently Asked Questions
Why is sodium thiosulfate used in photography?
Sodium thiosulfate, known as "hypo" in photography, is used as a fixer. It dissolves light-sensitive silver halide crystals (AgBr, AgCl), removing them from film or paper. This process makes the image permanent and unaffected by further light exposure.
How does sodium thiosulfate work in cyanide poisoning?
Sodium thiosulfate converts cyanide ions (CN⁻) to less toxic thiocyanate (SCN⁻) in cyanide poisoning. This reaction is catalyzed by the rhodanese enzyme. It is usually used together with sodium nitrite to increase effectiveness.
What is the difference between anhydrous and pentahydrate forms of sodium thiosulfate?
Anhydrous form (Na₂S₂O₃) contains 100% active substance, while pentahydrate form (Na₂S₂O₃·5H₂O) contains approximately 36% water. Pentahydrate form is more commonly used and stable at room temperature. Anhydrous form is hygroscopic and tends to absorb moisture.
How to prepare and store sodium thiosulfate solutions?
Sodium thiosulfate solutions should be prepared using distilled or deionized water. Solutions should be stored in airtight containers in a cool and dark place. To prevent microbial growth in long-term storage, 0.1% sodium benzoate can be added to solutions.
What is the shelf life of sodium thiosulfate?
When stored under proper conditions (dry, cool and sealed packaging), sodium thiosulfate has a shelf life of 3-5 years. Pentahydrate form may dehydrate over time or sulfur formation may be observed on the surface. Solutions can be stored for weeks to months depending on stability.