Solvent extraction-spectrophotometric determination of anionic surfactants in sea water

Abstract

A solvent extraction-spectrophotometric determination of anionic surfactants in sea water using ethyl violet is described. The anionic surfactant sodium dodecylsulphate (SDS) reacts with ethyl violet to form an ion associate which can be extracted into toluene. Some of the chloride also forms an ion associate with ethyl violet and is extracted simultaneously with SDS into toluene. As sea water contains large amounts of sodium chloride (about 0.5 M), the blank is very large. To eliminate the chloride interference, the toluene extract is washed with distilled water. The concentration of the surfactant can be determined by measuring the absorbance of the organic phase at 612 nm after washing. The calibration graph is linear over the range 0–9.0 × 10^–6M SDS (in toluene) and the apparent molar absorptivity is 9.6 × 10^–4 l mol^–1 cm^–1. As the method is free from interferences from major constituents of sea water, it can be applied to the determination of anionic surfactants in sea water at levels up to 0.15 p.p.m.