Spectrophotometric determination of anionic surfactants in sea-water based on ion-pair extraction with bis[2-(5-trifluoromethyl-2-pyridylazo)-5-diethylaminophenolato]cobalt(III) as counter ion

Abstract

A simple and sensitive spectrophotometric method was developed for the extraction and spectrophotometric determination of low levels of anionic surfactants in sea-water. Anionic surfactants are extracted into toluene as an ion-pair with bis[2-(5-trifluoromethyl-2-pyridylazo)-5-diethylaminophenolato]cobalt(III){[Co(CF₃-PADAP)₂]⁺} and the absorbance of the organic phase is measured at 560nm. The cobalt complex cation is inert and stable over a wide pH range, and has a large molar absorption coefficient (ε= 1.06 × 10⁴ m² mol^–1 at 573 nm in water and ε= 5.83 × 10³ m² mol^–1 at 560 nm in toluene). The Beer–Lambert law was obeyed over the concentration range 0.01–0.4 mg dm^–3 of sodium dodecyl sulfate (SDS) and the detection limit was 0.005 mg dm^–3 on a 3σ_b basis. The relative standard deviations for artificial sea-water spiked SDS were about 2%. Electrolytes in sea-water do not interfere. The proposed method is simpler and more sensitive than the conventional Methylene Blue method even when the degradation products of anionic surfactants are present. The method was applied to the determination of anionic surfactants in sea-water samples without interference from matrix substances.