This part of ISO 7875 specifies a spectrometric method for the determination of anionic surfactants by measurement of the methylene blue index (MBAS) in aqueous media. The method is applicable to drinking water, surface water as well as waste water, for example for the determination of the primary degradation of surfactants under investigation in test systems containing natural or synthetic waste water. It applies for both laboratory scale and technical waste-water treatment plants. In the case of effluents originating from municipal waste-water treatment plants, the MBAS index comprises not only synthetic but also, to a considerable extent, natural anionic surface active substances This method is applicable to a range of concentrations from 0,1 mg/l to 5,0 mg/l and the limit of detection is about 0,05 mg/l for solutions of standard surfactants in distilled water. Under the experimental conditions, sulfonates and sulfates are the compounds chiefly measured, but some positive and negative interferences may occur (see clause 9).