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 aqueous solutions relies on the association of the cationic methylene blue dye with the anionic surfactant to form an electroneutral complex that is subsequently extracted into chloroform and determined spectrophotometrically. This methylene blue active substance (MBAS) assay suffers from salt interference in estuarine and marine waters due to the high concentration of chloride (Cl-) ions. The interference was shown to arise from the association of Cl- ions with the cationic methylene blue dye, which, although weak, becomes significant at the elevated concentrations of Cl- in marine water. Analysis of the kinetics of the formation and extraction into chloroform of complexes of methylene blue with a typical anionic surfactant (sodium dodecyl sulfate, SDS) and with Cl- ions revealed that the efficacy of the extraction step was critical in yielding reliable data. Based on these results, a modified, simple one-step MBAS method is described that overcomes interference by salt.