Abstract Bis(diarylphosphine oxide) naphthalene com- pounds are used as novel ionophores in plasticized poly(vinyl chloride) matrix membrane sensors for barium ions. The most favorable sensor was 1,2-bis(diethyl- phenylphosphine oxide)naphthalene containing potassium tetrakis(4-chlorophenyl)borate as lipophilic salt and o-ni- trophenyloctyl ether as plasticizer for ion-selective elec- trode membrane construction. The electrode showed excel- lent properties. It gave a linear response with a Nernstian slope of 30 mV per decade within the concentration range 10 –1 –10 –5 mol L –1 BaCl 2 . The electrode exhibits a high se- lectivity towards Ba 2+ with respect to Li + , Na + , K + , Rb + , Cs + , NH 4 + , Ag + , Mg 2+ , Ca 2+ , Sr 2+ , Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Cd 2+ , Pb 2+ , Al 3+ , La 3+ , and Ce 3+ ions. The elec- trode response was stable over a wide pH range (3–11). The lifetime of the electrode was about 2 months. It was successfully applied to the determination of Ba 2+ contents in some rocks. Introduction Electrically neutral, lipophilic ion-complexing agents of low relative mass are known to behave as ionophores or ion carriers [1]. These neutral carriers have the capability of selectively extracting ions from aqueous solutions into a hydrophobic membrane phase and transporting these ions across barriers by carrier translocation. The neutral carriers, when incorporated into a water-immiscible mem- brane, an appropriate organic solvent or an inert matrix, function as cation-selective electrodes and often exhibit a nearly-Nernstian response to the primary ions. The poten- tiometric selectivity of these sensors among different ions is largely dictated by the complexation specificity of the carrier molecules involved, but it may also be influenced by the membrane solvent and by other parameters [2]. Se- lectivities are also influenced by the plasticizer employed and the relative concentrations of ions to which the sensor is exposed. Barium ion-selective electrodes (ISE s ) find little appli- cation in terms of actual barium determination [3–10], but they can be used for the indirect titration of sulfate[11–14] and particularly for the analysis of nonionic surfactants [15–17], including the measurements of their critical mi- celle concentrations [15, 16] and the number of oxyethyl groups [18]. They can also be used for the determination of lithium [19]. Electrically charged monodentate organophosphorus compounds (OPC s ) are suitable as sensors in K + -selective electrodes [20, 21] and Ca 2+ -selective electrodes for clini- cal applications [22]. Neutral bidentate OPC s have suc- cessfully been used as sensors in ion-selective electrodes of high stability and selectivity [23–26]. These carrier lig- ands induce useful selectivities for Ca 2+ , Mg 2+ , Ba 2+ and UO 2 2+ . In the present work attempts are made to correlate the response of the membrane with the structural charac- teristics of its chemically active constituents. In order to improve both the membrane response toward barium ions and the mechanical strength, a number of novel neutral bidentate OPC s have been tested as components in Ba 2+ - sensitive membrane sensors. Experimental Apparatus. All potentiometric measurements were made at 25 ± 1 °C with a Fisher Scientific computer aided pH/millivoltmeter (Model 450). A PVC membrane electrode based on OPC s in conjunction with a double junction Ag/AgCl/KCl (satd.) reference electrode (Cole-Parmer Co. 7425 N) was employed. A Fisher Accumet Model 825 MP pH-meter (accuracy ± 0.001 pH) with a glass pH electrode (Fisher electrode No. 13–639–90) was used for all pH measurements. The zeta potential measurements were made using a Rank Brothers Microelectrophoresis Mark II employing a rec- tangular glass cell. Reagents and materials. Analytical reagent grade chemicals and doubly distilled water were used for preparing all aqueous solu- tions. High molecular weight poly(vinyl chloride) (PVC) and Mohamed Besis Saleh Neutral bidentate organophosphorus compounds as novel ionophores for potentiometric membrane sensors for barium(II) Fresenius J Anal Chem (2000) 367 : 530–534 © Springer-Verlag 2000 Received: 20 August 1999 / Revised: 8 February 2000 / Accepted: 9 February 2000 ORIGINAL PAPER M. B Saleh Chemistry Department, Faculty of Science, Minia University, Minia, Egypt e-mail: rumenia@enstinet.eg.net