Overview
ISO 5813:1983 is an international standard developed by the International Organization for Standardization (ISO) that specifies a water quality testing method for the determination of dissolved oxygen using the iodometric (Winkler) method. This reference procedure is designed for all types of water with dissolved oxygen concentrations greater than 0.2 mg/L up to approximately double saturation (around 20 mg/L). The standard accounts for the presence of certain interfering substances to ensure accurate measurements.
Dissolved oxygen measurement is a critical parameter in assessing water quality, influencing aquatic life, water treatment processes, and environmental monitoring. The iodometric method under ISO 5813:1983 remains widely used due to its precision and reliability in various water types free from interfering compounds like oxidizable organic matter or sulphides.
Key Topics
- Winkler Iodometric Method: The procedure involves reacting dissolved oxygen with manganese hydroxide, followed by titration with sodium thiosulphate to quantify the iodine liberated, which is directly proportional to oxygen content.
- Scope of Application: Suitable for fresh, marine, or treated water with oxygen levels between 0.2 mg/L and 20 mg/L; best applied in waters free from oxidizable or reducing substances that could interfere with results.
- Interferences and Modifications:
- Recognizes interferences from organic material (e.g., tannins, humic acids) and sulphur compounds.
- Provides modified procedures where such interferences exist, including alternative electrochemical probe methods (ISO 5814).
- Procedures for handling oxidizing and reducing agents in samples to maintain accuracy.
- Sampling and Handling: Details on collecting water samples from various sources (surface, pipelines, varying depths) while avoiding contamination or oxygen changes.
- Reagents and Apparatus: Specifies analytical-grade reagents such as sulfuric acid, manganese sulfate, alkaline iodide-azide, sodium thiosulphate, and iodine solutions, along with specialized glass flasks used for titration.
- Calculation and Reporting: Formulas to derive dissolved oxygen concentration expressed in mg/L and guidelines to report results with relevant sample and environmental details.
- Special Cases: Describes how to deal with samples containing suspended matter capable of consuming iodine, using modified flocculation and separation techniques.
- Quality and Reproducibility: The standard ensures test reproducibility as demonstrated by low standard deviations in inter-laboratory tests.
Applications
- Water Treatment Facilities: Monitoring dissolved oxygen is essential for controlling biological processes in wastewater and drinking water treatment.
- Environmental Monitoring: Evaluating oxygen levels in lakes, rivers, and oceans to assess ecosystem health and detect pollution.
- Industrial Processes: Controlling water quality for cooling systems, boiler feedwater, and manufacturing processes requiring specific oxygen concentrations.
- Research Laboratories: Providing a standard method for scientific studies focusing on water chemistry and aquatic environments.
- Regulatory Compliance: Ensuring compliance with environmental regulations that specify limits on dissolved oxygen to protect aquatic life.
Related Standards
- ISO 5814: Water quality - Determination of dissolved oxygen - Electrochemical probe method
Provides an alternative method for oxygen determination that is more suitable where interferences affect the iodometric method.
- ISO 10523: Water quality - Determination of pH
Often used concurrently in water analysis to assess overall water chemistry impacting dissolved oxygen.
- ISO 5667 series: Water quality - Sampling
Critical for ensuring correct collection and handling of water samples prior to analysis following ISO 5813.
By adhering to ISO 5813:1983, organizations and laboratories can reliably determine dissolved oxygen levels in water, ensuring accurate water quality assessments essential for environmental protection, resource management, and industrial applications. This iodometric standard continues to be a cornerstone for water quality testing worldwide.