Ion-selective electrodes for anionic surfactants using a new aza-oxa-cycloalkane as active ionophore Original Research Article

The new cyclic aza-oxa-cycloalkane 7,13-bis(n-octyl)-1,4,10-trioxa-7,13 diazacyclopentadecane (L1) has been synthesised, characterized and its interaction with anionic surfactants studied. Protonation constants of L1 and stability constants of L1 with lauryl sulfate, sulfate and phosphate were determined by potentiometric titration in 1,4-dioxane:water (70:30, v/v, 0.1 mol dm−3 KNO3, 25 °C). L1 forms stable complexes with lauryl sulfate in a wide pH range. Different PVC-membrane anionic surfactant-selective electrodes were prepared using L1 as ionophore and o-nitrophenyl octyl ether (NPOE), bis(2-ethylhexyl) sebacate (BEHS) or dibutyl phthalate (DBP) as plasticizers. The PVC-based membrane electrode containing L1 as carrier and NPOE as plasticizer (electrode E1) showed a Nernstian response with a slope of 57.7 ± 0.2 mV per decade over a range of lauryl sulfate concentration from 3.3 × 10−6 to 6.7 × 10−3 M with a detection limit of 2.2 × 10−6 M. For the sake of comparison, a PVC-based membrane electrode containing L1, NPOE and tetra-n-octylammonium bromide as cationic additive (electrode E2) was also prepared and studied. Selectivity coefficients for different anions with respect to lauryl sulfate were calculated. Electrode E1 showed remarkably better selectivity coefficients than electrode E2, probably due to favourable interaction of lauryl sulfate with L1. Electrode E1 was used for the determination of anionic surfactants in several mixtures, and the results obtained were compared to those found using a commercially available electrode.