Feature Article Potentiometric Determination of Non-Ionic Surfactants by Liquid Membrane Electrodes Marco Giannetto, Claudia Minari, Giovanni Mori* Dipartimento di Chimica Generale ed Inorganica, Chimica Analitica, Chimica Fisica, Universita ¡ di Parma, Parco Area delle Scienze 17/A, 43100 Parma *e-mail: mori@unipr.it Received: July 2, 2002 Final version: November 1, 2002 Abstract It is well known that non-ionic surfactants (NIS) influence remarkably the potentiometric measurements with liquid membrane ion selective electrodes (ISEs), interfering particularly on performance of ISEs for earth-alkali metals, for which the loss of selectivity with regard to alkali metals has been documented. These studies indicate that such interferences are due to the extraction of surfactants within the membrane, where a competition takes place between the originally present ionophore and the surfactant which also acts as a ligand for alkali metals. The interpretation of such phenomena enabled one to exploit this interference for analytical purposes by membrane/solution extraction experiment monitored by UV measurements and by impedance FRA analysis on coated wire electrodes. Using Ca/ Mg ISEs based on the neutral ionophore ETH 4030, it has been established that the logarithm of the Ca/Mg over Na potentiometric selectivity constant is linearly correlated with the concentration of NIS like Tegopren 5863 and Triton X-100. The proposed method has been applied for the development of a new potentiometric analytical procedure for the determination of Tegopren 5863 in synthetic seawater (SSW), ranging from 0.25 to 5 ppm. Our procedure consists in the exposure of the electrode to stirred SSW containing the surfactant; the progressive extraction of Tegopren 5863 causes a growth in electrode×s sensitivity to Na  and K  , losing selectivity for Ca 2 and Mg 2 . In turn this induces an increase of EMF, as all these ions are present in the studied matrix. The potential drift was monitored for 15 hours, showing that the process reaches thermodynamic equilibrium after about 12 hours of exposure. This method presents a value of 210 ppb of Tegopren 5863 as detection limit. Keywords: Non-ionic surfactants, Liquid membrane ion selective electrodes, Ca/Mg ISEs, Tegopren 5863, Seawater 1. Introduction Surfactants and their biodegraded metabolites play a dangerous role in the environment [1]; NIS are widely employed as additives for detergents in industrial and domestic fields, so that their determination has been the object of many articles with several analytical approaches, including chromatographic [2], spectrophotometric [3] and potentiometric [4] methods. Well-known polyethoxylate surfactants, a subclass of NIS, when present in aqueous samples strongly affect the selectivity, sensitivity and detection limits of liquid mem- brane electrodes towards cationic species [5]. Experimen- tally, in presence of interfering alkali cations, remarkable increases of the EMF values of earth-alkali ion-selective electrodes(ISEs) are observed upon the addition of surfac- tant to the test solution leading to dramatic changes in performance of the devices. Although several articles [6 ± 8] report about the study and interpretation of such phenom- ena, the exploitation of the interference for analytical purposes has not yet been proposed; the effects of interfer- ence, in fact, can be suitable for the potentiometric determination of NIS, if a reliable relationship can be stated between the response of polymer membrane ISEs and the concentration of surfactant in the sample solution. The aim of this article is to set up an analytical method for quantitative determination of surfactants based on such a phenomenon. It has been shown how the surfactants inducing changes in performance of the ISEs contain both polyethoxylic chains and lipophilic aromatic/aliphatic moieties [5]. The lipophilic chains of surfactants allow their extraction within the non-polar solvent of the membrane, while the ethoxyl units show binding properties towards s-group metals. Lipophilicity and complexing capacity of surfactants were then considered in order to characterize their inter- ference with potentiometric behavior of ISEs. Potentiometry, as is well known, is a very suitable technique for environmental analysis, as it is cheap, easily automatizable and remotizable. In fact much work has been done for the potentiometric determination [9] of NIS, but most articles deal with determinations in pure or tap water. The here-developed method is suitable for seawater. The study was focused on Triton X-100, and Tegopren 5863 (Chart 1). 1598 Electroanalysis 2003, 15, No. 20 ¹ 2003 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim DOI: 10.1002/elan.200302727