Cetyldimethylethylammonium-Selective PVC Electrode Based on its Ion-Associate with Phosphotungstic Acid Sayed S. Badawy  , Yousry M. Issa, Waheed F. El-Hawary, and Manal S. Ashour Department of Chemistry, Faculty of Science, Cairo University, Giza, Egypt Abstract. A PVC membrane electrode selective for cetyldimethylethylammonium (CDEA) ion was pre- pared. The active element is a plasticized poly vinyl chloride membrane containing a dissolved ion associate complex of CDEA with phosphotungstic acid (PTA). The electrode showed a near-Nernstian response within the CDEA  concentration range 2:34  10 6 to 1:96  10 4 M at 25  1  C, good selectivity, and is usable within the pH range 3.5±9.0. The isothermal temperature coef®cient of the electrode amounted to 0.00012 V/  C. The electrode has been used for the direct determination of cetyldimethylethylammonium bromide either by the standard addition method or by potentiometric titration against PTA. Key words: Cetyldimethylethylammonium bromide; phospho- tungstic acid; ion-selective electrode; PVC membrane electrode. Cetyldimethylethylammonium bromide (CDEA) is a very important cationic surfactant having many analytical and technological applications. It was used for the determination of metal ions by spectrophoto- metry [1] and ¯ow injection analysis [2]. It has also several environmental [3], biochemical and biological [4], medical and clinical [5], and industrial applications [6]. In spite of the wide analytical and technological applications shown by CDEA, only four analytical methods have been reported for its determination. It has been determined by gas chromatography-mass spectro- metry (GC-MS) [7], and by electron ionization (EI) and mass analyzed ion kinetic energy (MIKE) spectro- metric [8] techniques. Such techniques are slow because of the necessary extensive sample pretreatment and the several manipulation steps before the ®nal result of the analysis is obtained. Only one potentio- metric method was proposed for the determination of CDEA by titration with Orange IV [9]. The indicator electrode was a C-electrode coated with PVC mem- brane containing triheptyldodecylammonium iodide and the reference electrode was a S.C.E. In the present work a PVC-plasticized membrane electrode, selective for the cetyldimethylethylammon- ium cation was constructed and its performance characteristics were studied. The electrode is based on the incorporation of the cetyldimethylethylammon- ium-phosphotungstate (CDEA) 3 PT ion associate in a poly vinyl chloride (PVC) membrane plasticized with dioctylphthalate (DOP). The electrode is used as a sensor for the determination of CDEA  in pure solutions. The phosphotungstate anion, used as the counter ion, is one of the most important anions used for the construction of many membrane ion-selective electrodes [10, 11]. Experimental Materials and Reagents All materials used were chemically pure including cetyldimethyl- ethylammonium bromide, CDEA (Sigma), phosphotungstic acid, PTA (Fluka), dioctylphthalate plasticizer, DOP (Fluka), tetrahydro- furan, THF (Aldrich or Merck), and polyvinyl chloride, PVC, of relatively high molecular weight (Aldrich). The solutions were prepared using bidistilled water. Stock concentrated solutions (10 1 or 10 2 M) of CDEA were prepared by accurate weighing and dissolving in bidistilled water and more diluted solutions (10 3 to 10 5 M) were prepared by appropriate dilution. The electrode body was usually ®lled with an internal solution containing 10 1 M NaCl and 10 3 M CDEA. The selectivity of the electrode was tested with Mikrochim. Acta 136, 1±7 (2001)  To whom correspondence should be addressed