A methylene blue active substances assay, or MBAS assay, is a colorimetric analysis test method that uses methylene blue to detect the presence of anionic surfactants (such as a detergent or foaming agent) in a sample of water. An anionic surfactant detected by the color reaction is called a methylene blue active substance (MBAS).[1]
After first acidifying a water sample (with boric acid, for example), one adds to it chloroform and a solution of methylene blue et al.[2] Methylene blue is a cationic dye. The entire solution is then agitated to distribute these reagents throughout the water sample.[2] The color of the water changes according to the amount of anionic surfactants in the sample. The higher the concentration of surfactants, the deeper and richer the blue coloration.
MBAS assay is an ASTM International standard technique for detecting anionic surfactants.[3] These include carboxylates, phosphates, sulfates, and sulfonates. An MBAS assay alone does not, however, identify specific surfactants. ASTM withdrew the standard (ASTM D2330-02) in 2011 pending a review and update of the method, which was last approved in 2003.[3].