Some N-alkyl-naphthylazo pyridinium salts as new reagents for the spectrophotometric determination of anionic surfactants

Abstract

Eleven pyridinium azo dyes with straight-chain alkyl groups CnH2n+1−(n=6−18) and bromoalkyl groups BrCmH2m−(m=6−12) were synthesized with the intention of developing reagents for the determination of low levels of anionic surfactants in an aqueous medium. The effect of the alkyl chain length of these reagents on the reactivity with anionic surfactants such as sodium dodecylsulphate (SDS), sodium linear-dodecylbenzenesulphonate (DBS), sodium dodecylsulphonate (DS) and sodium laurate (SL) was studied. It was found that the alkyl chain length played an important role in the formation of ion associates and the composition of the ion associates formed. These reagents were classified into four groups with respect to the reactivity with anionic surfactants. The first group (n,m=6) reacts only with DBS. The second group (n,m=8) reacts with SDS, DBS and DS. The third group (n,m=10, 12) reacts with SDS, DBS and DS; however, the colour intensity of the DBS-ion associate was unstable. The fourth group (n=14, 16, 18) reacts with all anionic surfactants examined, and the composition of the ion associates with SDS and DS was 2:1 ([reagent]/[surfactant]) though that of the ion associates of the three reagent groups mentioned above was 1:1. The optimal conditions for the determination of anionic surfactants in river water with 1-octyl-4-(4-aminonaphthylazo)-pyridinium bromide was examined. The calibration graph was linear up to 3×10−6 mol/l, and the apparent molar absorptivity of the ion associate was 3.8×104 l mol−1 cm−1 (at 427 nm). The relative standard deviation for 2.4×10−6 mol/l SDS was 4.9%. Recoveries of 88–107% were found for 8.0×10−7 mol/l SDS in river water samples.