MBAS assay

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. A surfactant detected by the color reaction is called a methylene blue active substance (MBAS).

After first acidifying a water sample (with boric acid, for example), one adds to it chloroform and a solution of methylene blue. Methylene blue is a cationic dye. The entire solution is then agitated to distribute these reagents throughout the water sample. The color of the water changes according to the amount of surfactants in the sample. The higher the concentration of surfactants, the deeper and richer the blue coloration.

MBAS assay is a standard technique for detecting anionic surfactants. These include carboxylates, phosphates, sulfates, and sulfonates. An MBAS assay alone does not, however, identify specific surfactants.

References

• Anionic Surfactant Detection
• ASTM D2330 - 02 Standard Test Method for Methylene Blue Active Substances
• Nollet, Leo M. L. Handbook of Water Analysis. 2nd ed. New York: Marcel Dekker, 2007.
• Regulation (EC) No 648/2004 of the European Parliament and of the Council of 31 March 2004 on detergents

See also

• Colorimetry
• Water chemistry analysis
• Water testing