ABSTRACT
Many anionic surfactants can react with a cationic dye such as methylene blue to form strong ion pairs that can be extracted by a suitable organic solvent and can be determined using colorimetric techniques. The anionic surfactants that respond to the methylene blue test are primarily the sulfonate and the sulfate ester type substances. On the other hand, soaps and the alkali salts of fatty acids used in certain detergents do not respond to the above test. This chapter presents the various anionic surfactants and their characteristic structural features. Such anionic surfactants that form ion pairs with methylene blue and that are extractable with chloroform are known as "methylene blue active substances" (MBAS). Other cationic dyes, such as crystal violet dye, may be used instead of methylene blue. The determination of anionic surfactants in aqueous samples involves the analytical steps described in the chapter.
