Flow injection determination of anionic surfactants with cationic dyes in water bodies of central India

Abstract

A new, simple and specific flow injection analysis (FIA) procedure for the determination of anionic surfactants, viz., sodium lauryl sulfate (SLS), sodium dodecyl sulfonate, sodium hexadecyl sulfonate and sodium dodecyl benzenesulfonate, with cationic dyes, viz., Brilliant Green (BG), Malachite Green, Methylene Blue, Ethyl Violet and Crystal Violet, in water bodies, viz., ponds, tube wells, rivers and municipal wastes, of central India (east Madhya Pradesh) is described. It is based on the precipitation of the cationic dyes with the anionic surfactant due to formation of an ion-associate species within the pH range 5.5–8.0. The apparent molar absorptivity of the ion-associate species formed with various anionic surfactants and cationic dyes is in the range (0.60–1.50) × 10⁴ l mol^–1 cm^–1 at λ_max 590–665 nm. Among them, the pair BG⁺–LS^– was selected for detailed investigation. The detection limit (amount causing absorbance >3s) of the method with BG is 100 ppb SLS and the sample throughput is 50 h^–1. Optimization of FIA and the analytical variables in the precipitation and determination of SLS with BG is described. The method is free from interferences from almost all ions which are commonly present with the surfactant. The proposed method was applied to the mapping of SLS pollution levels in the various water bodies. All surface waters and municipal waste waters and some ground waters lying near the sources were found to be contaminated with SLS beyond permissible limits.