Spectrophotometric determination of anionic surfactants in water after solvent extraction coupled with flow injection

Abstract

Anionic surfactants in water were determined by a spectrophotometric method involving flow injection coupled with solvent extraction. Eight cationic dyes were examined with several extraction solvents. Of the pairs of cationic dye and extraction solvent investigated, Methylene Blue and 1,2-dichlorobenzene were the most efficient. The ion associate formed between an anionic surfactant and Methylene Blue was extracted into the organic phase, the absorbance of which was measured at 658 nm. The carrier stream was distilled water and the reagent stream consisted of a cationic dye, sodium sulphate and acetate buffer (pH 5). A phase separator with a poly(tetrafluoroethylene) porous membrane (0.8 µm pore size) was used to separate the organic phase. The sampling rate was 20 samples per hour. The calibration graphs were linear up to 3 × 10–5M(8.7 mg l–1) or 7 × 10–5M(20.2 mg l–1) of anionic surfactant when the injection volumes were 300 and 100 µl, respectively. The relative standard deviation (n= 10) was 0.9% when 300 µl of 2.1 × 10–6M(610 µg l–1) sodium dodecylsulphate were injected. The detection limit corresponding to a signal to noise ratio of 3 was 1 × 10–8M(5 µg l–1) for the injection of 300 µl of sodium dodecylsulphate. The proposed method was used to determine anionic surfactants in river water.