Overview
EN 25813:1992, titled "Water quality - Determination of dissolved oxygen - Iodometric method (ISO 5813:1983)," is a European Standard developed by CEN. It specifies a modified Winkler titration procedure for the accurate determination of dissolved oxygen in water. This iodometric method is the reference standard for quantifying dissolved oxygen concentrations ranging from 0.2 mg/l up to double saturation, approximately 20 mg/l, in waters free of interfering substances.
The standard is applicable to a wide range of water types, including surface waters, distribution pipelines, and varying depth samples. EN 25813:1992 is harmonized with ISO 5813:1983, ensuring global consistency in water quality analysis.
Key Topics
- Iodometric Winkler Method: The procedure involves the reaction of dissolved oxygen with manganese(II) hydroxide, forming an oxidized compound that liberates iodine. This iodine is then titrated with sodium thiosulphate to determine oxygen concentration.
- Modified Procedure for Interferences: Adjustments are made in the standard to correct for oxidizing or reducing substances, nitrites, and suspended matter that could interfere with the measurement. For complex samples, an annex provides a method involving flocculation with aluminum hydroxide.
- Reagents and Apparatus: The standard details preparation of analytical-grade reagents, such as manganese sulfate, alkaline iodide-azide solution, potassium iodate, and sodium thiosulphate. Specific glass flasks calibrated for volume and airtight stoppers are recommended for sample collection.
- Sampling Techniques: Sampling protocols ensure minimal oxygen loss or air bubble inclusion, with specialized methods for surface water, deep water, or pipeline samples.
- Quality Control: Daily standardization of titration solutions is required. The method’s reproducibility has been validated across multiple laboratories, with standard deviations within 0.03 to 0.05 mg/l for air-saturated water.
- Handling Special Cases: Procedures for samples containing oxidizing or reducing agents include parallel sampling and correction titrations. If interfering substances are present, an electrochemical probe method (ISO 5814) is recommended.
Applications
This standard is essential for laboratories and environmental agencies conducting water quality monitoring, including:
- Environmental Monitoring: Assessing dissolved oxygen in rivers, lakes, reservoirs, and coastal waters to evaluate aquatic ecosystem health.
- Water Treatment Plants: Ensuring oxygen levels meet regulatory standards during treatment and distribution processes.
- Industrial Effluent Testing: Monitoring discharged waters to prevent oxygen depletion in receiving bodies.
- Research and Development: Studying oxygen solubility and bioavailability in freshwater and marine systems.
- Quality Assurance: Serving as a reference method for calibrating electrochemical oxygen sensors and probes.
Overall, EN 25813:1992 supports compliance with environmental legislation and promotes sustainable water resource management by providing a reliable and internationally recognized procedure for dissolved oxygen determination.
Related Standards
- ISO 5813:1983 – The international counterpart specifying the iodometric Winkler method for dissolved oxygen determination.
- ISO 5814 – "Water quality - Determination of dissolved oxygen - Electrochemical probe method," recommended for waters with interfering substances.
- EN 25812 – Guidance on sampling methods for water quality analysis.
- ISO 3696 – Specifications for water used in laboratory analysis, ensuring reagent purity in testing.
- ISO 5667 series – Standards covering water sampling techniques appropriate for various water bodies and conditions.
Summary
EN 25813:1992 defines a rigorous, scientifically validated iodometric titration method for measuring dissolved oxygen in water samples. Through detailed procedures to mitigate common interferences and robust sampling instructions, this standard provides accurate and reproducible results critical for water quality assessment worldwide. Its alignment with ISO standards ensures compatibility and reliability in global water quality monitoring efforts.
