Sodium Thiosulfate (STS): Properties, Preparation, and Applications in Pharmaceuticals and Water Treatment

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Sodium Thiosulfate (STS): Properties, Preparation, and Applications in Pharmaceuticals and Water Treatment

Sodium Thiosulfate (STS), chemically represented as Na₂S₂O₃, is an inorganic compound widely used as an antidote, reducing agent, analytical reagent, and dechlorinating agent. It plays a crucial role in various sectors including pharmaceuticals, photography, water treatment, and analytical chemistry. The most common form is Sodium Thiosulfate Pentahydrate (Na₂S₂O₃·5H₂O), a colorless crystalline solid highly soluble in water.

🔹 Chemical Identity and Structure

  • Chemical Name: Sodium Thiosulfate
  • Chemical Formula: Na₂S₂O₃
  • Molecular Weight: 158.11 g/mol (anhydrous), 248.18 g/mol (pentahydrate)
  • CAS Number: 7772-98-7
  • Appearance: Colorless or white crystalline powder
  • Odor: Odorless
  • Solubility: Soluble in water, insoluble in alcohol
  • pH (1% solution): 6.5 – 8.0

🔹 Preparation of Sodium Thiosulfate

Sodium Thiosulfate can be prepared by the reaction of sodium sulfite (Na₂SO₃) with sulfur at elevated temperature:

Na₂SO₃ + S → Na₂S₂O₃

Alternatively, it can also be synthesized by the oxidation of sodium polysulfide or by reacting sulfur dioxide with sodium sulfide solutions. The resulting compound is purified and crystallized as the pentahydrate form.

🔹 Chemical Properties of Sodium Thiosulfate

  • Decomposition: On heating above 100°C, it decomposes to sodium sulfate and sulfur dioxide.
  • Reducing Property: Acts as a mild reducing agent, capable of reducing halogens and other oxidizing agents.
  • Complex Formation: Forms complexes with silver, iodine, and other metals, which makes it useful in analytical chemistry and photography.

🔹 Reactions of Sodium Thiosulfate

1. Reaction with Iodine (I₂)

2 Na₂S₂O₃ + I₂ → Na₂S₄O₆ + 2 NaI

This reaction is the basis of iodometric titrations, used for determining oxidizing agents in analytical chemistry.

2. Reaction with Acids

Na₂S₂O₃ + 2 HCl → 2 NaCl + H₂O + SO₂ + S↓

This reaction produces sulfur dioxide (SO₂) gas and elemental sulfur. It demonstrates the instability of thiosulfate in acidic conditions.

3. Reaction with Silver Nitrate

2 Na₂S₂O₃ + AgNO₃ → Na₃[Ag(S₂O₃)₂] + NaNO₃

Used in photographic fixing processes to dissolve unreacted silver halides.

🔹 Applications of Sodium Thiosulfate

1. In the Pharmaceutical Industry

Sodium Thiosulfate is used as an antidote for cyanide poisoning. It converts toxic cyanide into less harmful thiocyanate, which can be safely excreted. It is also utilized in certain chemotherapy treatments to reduce side effects of cisplatin.

2. In Water Treatment

Sodium Thiosulfate acts as a dechlorinating agent, neutralizing residual chlorine or chloramine in water systems before it is released to the environment or used in sensitive processes like aquaculture and microbiology laboratories.

3. In Analytical Chemistry

Used as a titrant in iodometric titrations to quantify oxidizing substances like iodine, chlorine, and potassium dichromate. It is an essential reagent in chemical analysis and quality control labs.

4. In Photography

Historically, Sodium Thiosulfate was used as a fixer to remove unexposed silver halides from photographic films, ensuring a clear and stable image.

5. In Textile and Leather Industries

Employed in the removal of residual chlorine after bleaching processes, preventing damage and improving product texture.

🔹 Grades of Sodium Thiosulfate

  • Pharmaceutical Grade: High purity (>99.5%) for medical and laboratory use.
  • Industrial Grade: Used in water treatment, photography, and textile processing.
  • Analytical Reagent (AR) Grade: Suitable for analytical and titration purposes.

🔹 Safety and Handling

Sodium Thiosulfate is generally regarded as non-toxic and safe for laboratory use; however, certain precautions are recommended:

  • Storage: Store in a cool, dry, and well-ventilated area.
  • Protect from moisture: It may lose water of crystallization when exposed to air.
  • Personal Protective Equipment: Gloves, goggles, and lab coat recommended.
  • First Aid: In case of ingestion or eye contact, rinse with water and seek medical advice if necessary.

🔹 Analytical Quality Specifications

Test ParameterSpecification
AppearanceColorless crystalline powder
Assay (Na₂S₂O₃·5H₂O)≥ 99.0%
pH (5% Solution)6.5–8.0
Chloride≤ 0.01%
Sulfite≤ 0.01%
Heavy Metals (as Pb)≤ 5 ppm

🔹 Environmental Impact

Sodium Thiosulfate is environmentally safe when handled properly. It neutralizes chlorine and oxidizing agents, making it an eco-friendly choice for water treatment and wastewater management.

🔹 Conclusion

Sodium Thiosulfate (STS) is a versatile and essential chemical with diverse applications in pharmaceuticals, water treatment, analytical chemistry, and industry. Its reducing and dechlorinating properties make it invaluable for safe laboratory practices, industrial processing, and medical treatments. Proper handling and purity control ensure its optimal performance across all sectors.

💬 About the Author

Siva Sankar is a Pharmaceutical Microbiology Consultant and Auditor with extensive experience in sterility testing, validation, and GMP compliance. He provides consultancy, training, and documentation services for pharmaceutical microbiology and cleanroom practices.

📧 Contact: siva17092@gmail.com
Mobile: 09505626106

📱 Disclaimer: This article is for educational purposes and does not replace your laboratory’s SOPs or regulatory guidance. Always follow validated methods and manufacturer instructions.

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