Two-step solvent extraction flow injection system for the determination of anionic surfactants by spectrophotometry

An automated how injection procedure for the determination of anionic surfactants with two-step extraction was developed, based on the official methylene blue method. A washing step was used for on-line treatment of the extracted products with methylene blue to eliminate interferences. An improved gravitational phase separator was used to overcome the limitations of the membrane separator, and an effective separation of the two phases was performed by gravity separation in a conical cavity constructed of PTFE and stainless steel. Use of PTFE pump tubing allowed reliable and reproducible withdrawal and propulsion of organic extractant at low how-rates. The calibration graphs were linear up to 6.0 mg/l of sodium dodecyl sulfonate in aqueous solution when 100 mu l of sample was injected, a relative standard deviation of 4.6% (n = 7) was achieved. The recoveries of the procedure were in the range of 89-107%. The results obtained for anionic surfactants in waste water samples agreed well with those obtained by the standard methylene blue batch method.