Differential Pulse Voltammetry and Additive Differential Pulse Voltammetry with Solvent Polymeric Membrane Ion Sensors J. A. Ortun ˜ o,* ,† C. Serna, ‡ A. Molina, ‡ and A. Gil † Department of Analytical Chemistry and Department of Physical Chemistry, Faculty of Sciences, University of Murcia, 30071-Murcia, Spain The ion transfer across the water-solvent polymeric membrane interface is investigated by using a new device based on a modification of a commercial ion-selective electrode body that permits the accommodation of a platinum counter electrode inside the inner filling solution compartment and, therefore, use of a four-electrode potentiostat with ohmic drop compensation. This device is used here to apply two different double potential pulse techniquessdifferential pulse voltammetry and additive differential pulse voltammetryswhich are more advanta- geous than other voltammetric techniques, such as nor- mal pulse voltammetry or cyclic voltammetry, for the determination of the characteristic electrochemical par- ameters of the system. This is due to the concurrence of two factors in these double potential pulse techniques, the peak-shaped response together with a considerable reduction of undesirable current contributions. Electrochemistry at the interface between two immiscible electrolyte solutions (ITIES), also named liquid-liquid electro- chemistry, is an emerging field of research, which serves as a simple model of biomembranes and as a basis of innovative analytical methods and industrial applications. 1,2 The application of electroanalytical techniques to ITIES allows the determining of Gibbs energies of transfer of charged compounds from an aqueous to an organic phase and partition coefficients of ionizable drugs. These parameters allow the quantifying of the lipophilicity, which is a molecular property that expresses the relative affinity of a species in organic and aqueous phases. This is the most used property in the design of drugs through the establishment of quantitative structure-activity relationships. 2,3 Most drugs un- dergo passive diffusion through biological membranes, and it has been traditionally accepted that only neutral compounds are able to cross biological membranes. However, later research has shown that transmembrane diffusion of ions does ocurr. 4 Most research done on ion transfer at ITIES has focused on the ion transfer at one single interface for which a reference and a counter electrode were immersed in the organic phase as well. For supported, gelified, and polymeric membranes, it is easier to immerse these electrodes (or alternatively one single electrode acting as both) in an aqueous solution in contact with the inner side of the membrane. Two interfaces are involved in this approach, which is used by most authors. 5-12 Our interest focuses on ion transfer at the interface between aqueous samples and plasticized poly(vinyl chloride) (PVC) mem- branes similar to those used in ion-selective electrodes. Armstrong and Marcus 5 and Horva ´th and Horvai 13 carried out pioneering voltammetric studies of ion transfer at these membranes using cyclic voltammetry (CV). Jadhav et al. 8,14 proposed the voltam- metric and amperometric transduction for solvent (or plasticized) polymeric membrane ion sensors. They used CV and normal pulse voltammetry (NPV) and a three-electrode potentiostat. In this paper, a modification of a commercial ion-selective electrode body is proposed that permits the accommodation of a platinum counter electrode inside the inner filling solution compartment, while maintaining the advantages of the original body such as the facility to replace the membrane and the water tightness of the sensor. The modified sensor can be coupled to a four-electrode potentiostat with ohmic drop compensation. This device is used here to apply two different double potential pulse techniques, differential pulse voltammetry (DPV) and additive differential pulse voltammetry (ADPV), as advantageous alterna- * To whom correspondence should be addressed. E-mail:jortuno@um.es. † Department of Analytical Chemistry. ‡ Department of Physical Chemistry. 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