Chemical formula: Na?S?O? (commonly found as the pentahydrate **Na?S?O?·5H?O**, commonly known as "hypo" or "sodium thiosulfate pentahydrate")  

1. Basic Properties

- Physical Properties:  

  - Colorless transparent crystals or white granules, easily soluble in water, slightly bitter taste.  

  - The pentahydrate (Na?S?O?·5H?O) is prone to efflorescence in dry air and loses water of crystallization above 33°C.  

- Chemical Properties:  

  - Reducibility: Can be oxidized by oxidizing agents (such as iodine, chlorine, H₂O₂), commonly used in titration analysis (e.g., iodometry).  

  - Complexation: Can form stable complexes with silver ions (Ag?), used in photographic fixing:  

  - Decomposition in acid: Decomposes under acidic conditions:  

 2. Main Applications

- Photography industry: Fixing agent (dissolves unexposed silver halides).  

- Medical field:  

  - Antidote for cyanide poisoning.  

  - Treatment of calcific urinary stones.  

- Chemical and environmental protection:  

  - Wastewater treatment (removal of residual chlorine, heavy metal ions).  

  - Complexing agent in the electroplating industry.  

- Laboratory:  

  - Standard reducing agent in iodometric titrations.  

  - Preparation of buffer solutions or indicators.  

3. Safety Precautions

- Toxicity: Low toxicity, but excessive intake may cause gastrointestinal discomfort.  

- Protective measures:  

  - Avoid dust inhalation; wear gloves and goggles during operation.  

  - Store in a cool, dry place to prevent efflorescence or deliquescence.  

- First aid treatment: Drink plenty of water after accidental ingestion and seek medical observation.