Overview
ISO 7875-1:1996 defines a spectrometric method for the determination of anionic surfactants in water by measuring the methylene blue index (MBAS). The procedure is applicable to drinking water, surface water and wastewater, and can be used both at laboratory scale and in technical wastewater‑treatment plant investigations. The method reports MBAS as an indicator (summary parameter) of natural and synthetic anionic surface‑active substances.
Key topics and technical requirements
- Method principle: formation of ion pairs between methylene blue and anionic surfactants in alkaline medium, extraction into an organic phase and spectrophotometric measurement at 650 nm.
- Analytical range & sensitivity: applicable from 0.1 mg/L to 5.0 mg/L MBAS; limit of detection ≈ 0.05 mg/L for standard surfactant solutions.
- Primary analytes: sulfonates and sulfates are chiefly measured; positive and negative interferences can occur and are addressed in the standard.
- Reagents & standards: methylene blue (neutral and acidic), chloroform (toxic - use with caution), ethyl acetate, buffer (pH 10), ethanol/methanol and a dodecylbenzene sulfonic acid methyl ester stock standard (saponified to the sodium salt) for calibration.
- Apparatus: spectrometer (650 nm; 10 mm and 50 mm cells), gas‑stripping (solvent sublation) apparatus for separation/concentration, standard laboratory glassware and pH meter.
- Sample handling: guidance on sampling references ISO 5667 series; preservation options include cooling to 4 °C, addition of 1% (v/v) of 40% formaldehyde (up to 4 days) or saturation with chloroform (up to 8 days). Note: suspended solids may adsorb surfactants and affect recoveries.
- Calibration & quality control: use of working standards and blank tests; calibration typically prepared from the approved stock standard and periodic re‑calibration recommended.
Applications and users
ISO 7875-1:1996 is used where monitoring of anionic surfactants is required:
- Water quality laboratories performing routine monitoring of drinking water, surface water and effluents.
- Wastewater treatment operators evaluating primary degradation or surfactant loads in treatment systems.
- Environmental consultants and regulators assessing compliance and pollution sources.
- Researchers studying surfactant fate, biodegradation or method development.
Practical benefits include a standardized, reproducible MBAS measurement procedure and compatibility with large and small scale sampling and treatment studies.
Related standards
- ISO 7875-2 (Determination of non‑ionic surfactants)
- ISO 5667-2 and ISO 5667-3 (water sampling and sample preservation)
- Prepared by ISO/TC 147 (Water quality) - Subcommittee SC 2
Keywords: ISO 7875-1:1996, MBAS, methylene blue index, anionic surfactants, water quality, spectrometric method, wastewater, drinking water, surfactant determination.