A new ion-selective electrode for anionic surfactants

The new ligand 7-methyl-7,13-di-octyl-1,4,10-trioxa-13-aza-7-azonia-cyclopentadecane (L-1) has been designed, synthesised and used as ionophore in the development ion-selective electrodes for anionic surfactants. Different PVC-membrane anionic-surfactants-selective electrodes were prepared by using L-1 as ionophore and bis(2-ethylhexyl)sebacate (BEHS), dibutyl phthalate (DBP) and nitrophenyl octyl ether (NPOE) as plasticizers. The PVC-membrane electrode containing L-1 and NPOE (electrode E1) showed a Nernstian response to lauryl sulfate with a slope of -59.5 mV per decade in a range of concentrations from 1.3 x 10(-6) to 6.8 x 10(-3) M and a detection limit of 6.0 x 10(-7) M. The electrode E1 also showed a reasonable response to other alkyl sulfates and alkylbenzene sulfonates, whereas it does not respond to carboxylates and to cationic and nonionic surfactants. A similar electrode to El but additionally containing the cationic additive n-octylammonium bromide was also prepared (electrode E2) and compared with the response of El. Selectivity coefficients for different anions with respect to lauryl sulfate were determined by means of the fixed interference method considering lauryl sulfate as the principal anion and using a concentration of 1.0 x 10(-2) mol dm(-3) for the correspond- interfering anion. The selectivity sequence found for the electrode E1 was: LS- > SCN- > ClO4- > CH3COO- > I- > HCO3- > Br- > NO3- > NO2- > Cl- > IO3- > phosphate > SO32- > C2O42- > SO42-. Electrode E1 showed remarkably better selectivity coefficients than electrode E2. (c) 2006 Elsevier B.V. All rights reserved.