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).
The biphasic solution is then agitated to distribute these reagents throughout the aqueous and organic phases.
The color saturation of the chloroform increases with the concentration of anionic surfactants.
MBAS assay is an ASTM International standard technique for detecting anionic surfactants.