Sodium Thiosulfate (STS) in Raw Water Analysis: Role, Mechanism, and Importance in Accurate Testing

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Sodium Thiosulfate (STS) in Raw Water Analysis: Role, Mechanism, and Importance in Accurate Testing

Sodium Thiosulfate (Na₂S₂O₃) is an essential chemical reagent widely used in raw water analysis and microbiological testing. Its main function is to neutralize residual chlorine in water samples before microbial or chemical analysis. Chlorine is commonly used as a disinfectant in water treatment, but its presence can inhibit microbial growth, interfere with biochemical reactions, and lead to inaccurate analytical results. Hence, Sodium Thiosulfate plays a critical role in ensuring the accuracy and reliability of water testing.

What is Sodium Thiosulfate (Na₂S₂O₃)?

Sodium Thiosulfate is an inorganic salt composed of sodium (Na⁺) and thiosulfate (S₂O₃²⁻) ions. It is typically available as Sodium Thiosulfate Pentahydrate (Na₂S₂O₃·5H₂O), which appears as colorless crystalline solids, odorless, and highly soluble in water. It is known for its strong reducing properties and ability to react with oxidizing agents such as chlorine and iodine.

  • Chemical Formula: Na₂S₂O₃·5H₂O
  • Molecular Weight: 248.18 g/mol
  • Appearance: Colorless crystals
  • Solubility: Soluble in water
  • pH: Slightly alkaline in solution (7–9)

Role of Sodium Thiosulfate in Raw Water Analysis

During raw water sampling for microbial or chemical testing, chlorine present in treated water can destroy or inhibit the microorganisms intended for enumeration. To prevent this, Sodium Thiosulfate is added to neutralize the residual chlorine immediately after sampling. This ensures that microbial counts reflect the actual condition of water and not post-sampling chlorine activity.

1. Neutralization of Residual Chlorine

Chlorine acts as an oxidizing agent that kills microorganisms. Sodium Thiosulfate reacts with chlorine to form harmless chloride ions and sulfate compounds, effectively removing chlorine’s interference.

Na₂S₂O₃ + 4Cl₂ + 5H₂O → 2NaHSO₄ + 8HCl

This reaction ensures that chlorine is completely neutralized before microbiological analysis begins.

2. Preservation of Microbial Integrity

In microbiological water testing (e.g., Total Plate Count, MPN, or Membrane Filtration), the presence of chlorine can lead to false negative results. Sodium Thiosulfate prevents this by neutralizing chlorine at the time of sample collection, ensuring accurate microbial recovery.

3. Application in Chemical and Biochemical Analysis

In addition to microbial tests, Sodium Thiosulfate is used in titrimetric and redox reactions in analytical chemistry. It acts as a reducing agent and is widely applied in iodometric titrations and other quantitative analyses involving oxidizing agents.

Mechanism of Action

The reducing nature of Sodium Thiosulfate allows it to react with oxidizing compounds like chlorine, hypochlorous acid, or chloramines, converting them into non-reactive chloride ions. This process is rapid and stoichiometrically efficient, which makes STS a standard dechlorinating agent in analytical procedures.

Cl₂ + 2Na₂S₂O₃ → 2NaCl + Na₂S₄O₆

This mechanism ensures immediate chlorine removal from water samples before laboratory analysis begins.

Preparation of Sodium Thiosulfate Solution

For laboratory and field use, a 10% Sodium Thiosulfate solution is commonly prepared and added to sampling bottles in small quantities (e.g., 0.1 mL of 10% STS per 100 mL sample bottle) to neutralize up to 15 mg/L of residual chlorine.

10 g Na₂S₂O₃·5H₂O dissolved in 100 mL distilled water

Storage and Stability

Sodium Thiosulfate solutions are stable under normal laboratory conditions but should be stored in amber glass bottles to avoid decomposition by light. The solution must be freshly prepared or periodically verified for potency, as it may lose effectiveness over time.

Safety and Handling Precautions

  • Wear gloves and goggles while handling Sodium Thiosulfate solutions.
  • Store in cool, dark, and dry conditions.
  • Avoid contamination with strong acids or oxidizing agents.
  • Dispose of unused solutions as per environmental safety guidelines.

Advantages of Using Sodium Thiosulfate in Water Analysis

  • Ensures accuracy in microbial and chemical testing.
  • Prevents chlorine-induced microbial death post-sampling.
  • Simple, cost-effective, and rapid chlorine neutralization.
  • Compatible with various analytical and microbiological methods.
  • Recognized by international water testing standards (e.g., APHA, ISO, BIS).

Conclusion

Sodium Thiosulfate (Na₂S₂O₃) plays a crucial role in ensuring the reliability and precision of raw water analysis. By neutralizing residual chlorine, it maintains microbial viability and prevents analytical interference. Whether in pharmaceutical, industrial, or environmental laboratories, Sodium Thiosulfate remains an indispensable reagent for maintaining sample integrity and achieving accurate test results.

💬 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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