Optimization of the methylene blue assay for anionic surfactants added to estuarine and marine water

The existing rapid method for the determination of anionic surfactants in a queous solutions relies on the association of the cationic methylene blue d ye with the anionic surfactant to form an electroneutral complex that is su bsequently extracted into chloroform and determined spectrophotometrically. This methylene blue active substance (MBAS) assay suffers from salt interf erence in estuarine and marine waters due to the high concentration of chlo ride (Cl-) ions. The interference was shown to arise from the association o f Cl- ions with the cationic methylene blue dye, which, although weak, beco mes significant at the elevated concentrations of Cl- in marine water. Anal ysis of the kinetics of the formation and extraction into chloroform of com plexes of methylene blue with a typical anionic surfactant (sodium dodecyl sulfate, SDS) and with Cl- ions revealed that the efficacy of the extractio n step was critical in yielding reliable data. Based on these results, a mo dified, simple one-step MBAS method is described that overcomes interferenc e by salt.