Overview
ISO 16265:2009 defines a laboratory procedure for determination of the methylene blue active substances (MBAS) index in water using continuous flow analysis (CFA). The standard targets concentrations in two working ranges (0.05–0.5 mg/L and 0.5–5.0 mg/L) and expresses results as equivalent sodium dodecyl sulfate (SDS) concentration. It is intended for diverse matrices including groundwater, drinking water, surface water, wastewater and leachates, and provides a method-defined estimate of anionic surfactant content.
Key topics and technical requirements
- Analytical principle: Automated mixing of sample with alkaline methylene blue to form ion-pairs, liquid–liquid extraction with trichloromethane, back-extraction/acidification and photometric detection at 650 nm ± 10 nm.
- CFA system components: low-pulse pumps, inert transport tubing, phase separator (gravimetric or membrane), flow-cell photometer, data acquisition and optional autosampler. Tubing in contact with trichloromethane must be solvent-resistant.
- Reagents and calibration: specified methylene blue reagents (alkaline and acidified), trichloromethane extraction solution, and SDS calibration standards. Multiple calibration points per decade are recommended; daily sensitivity checks (e.g., 0.25 mg/L or 2.5 mg/L depending on range) are described.
- Analytical ranges and flexibility: primary working ranges are 0.05–0.5 mg/L and 0.5–5.0 mg/L; the method can be adapted to other concentration decades on a case-by-case basis.
- Interferences and sample handling:
- Potential positive/negative biases from cationic ion-pairing compounds, high humic acid (>20 mg/L), non-MBAS surface-active chemicals (>50 mg/L), strong reducing agents (e.g., sulfite), and high inorganic anion loads.
- Filtration recommendations (avoid >100 µm particles), sample storage at 2–5 °C for up to 3 days, and optional preservation with formaldehyde (1% v/v of 40% solution) for short-term storage. Do not preserve with mineral acids.
- Safety and waste: trichloromethane and methanol wastes require proper disposal and normal laboratory safety practices.
Applications and users
ISO 16265:2009 is used by:
- Environmental and public health laboratories performing water quality monitoring.
- Wastewater treatment plants and industrial effluent testing for anionic surfactant control.
- Regulatory agencies and consultants assessing surfactant pollution and compliance.
- Manufacturers and service providers of automated continuous flow analysis (CFA) instrumentation.
The method is suited to high-throughput batch testing (CFA can process many samples per hour) and situations where a method-defined MBAS index is a practical estimator of anionic surfactant contamination.
Related standards
- ISO 3696 - Water for analytical laboratory use (grade 1)
- ISO 1042 - Volumetric flasks
- ISO 648 - Single-volume pipettes
Keywords: ISO 16265:2009, MBAS, methylene blue active substances, continuous flow analysis, CFA, anionic surfactants, water quality, sodium dodecyl sulfate, photometric detection.