Feature Article Preconcentration and Electroanalysis of Copper(II) in Ammoniacal Medium on Nontronite/Cellulose Acetate Modified Electrodes Jyh-Myng Zen,* Hsu-Fang Wang, Annamalai Senthil Kumar, Hsueh-Hui Yang, and Venkataraman Dharuman Department of Chemistry, National Chung-Hsing University, Taichung 402, Taiwan. e-mail: jmzen@dragon.nchu.edu.tw Received: February 26, 2001 Final version: May 16, 2001 Abstract Nontronite/cellulose acetate-coated glassy carbon electrodes (NCA/GCEs) were evaluated for the preconcentration and electroanalysis of Cu 2 in ammoniacal medium by square-wave voltammetry. The NCA/GCEs exhibit good accumulation ability for the [Cu(NH 3 ) 4 ] 2 complex ion via the ion-exchange ability of nontronite. A permselective membrane,celluloseacetate,wasappliedtostrengthenthemechanicalstabilityofthenontronitecoatingonGCEand to prevent the interference from surface-active compounds. The experimental parameters were studied in detail. The NCA/GCEsshowgoodreproducibilitywitharelativestandarddeviationof2.83%for15successivedeterminationof 20ppbCu 2 . Under optimized conditions, a linear calibration plot was obtained up to 4 ppm Cu 2 with a detection limit(S/N  3)of1.73ppbafterapreconcentrationtimeof2minutesinpH10ammoniasolution.Thepracticalutility of the NCA/GCEs was illustrated by the measurement of Cu 2 in tap water, groundwater, and pond water. Keywords: Copper, Nontronite, Clay-modified electrode, Cellulose acetate 1. Introduction Stripping voltammetric determinations of trace Cu 2 through the complexation with ligands such as pyrogallol red, alizarin red S, catechol, 8-hydroxyquinoline, thiourea, CSN  ,4-[(4-diethylamino-2-hydroxyphenyl)azo]-5-hydroxy- naphthalene-2,7-disulfonic acid, salicylaldehyde thiosemi- carbazone, salicylideneamino-2-thiophenol, and ammonia havebeenreported[1±13].Inconsiderationofavailability, cost, and complex-stability constant, copper-ammonia sys- tem seems to be a relatively good choice [14]. Walcarius et al. used silica modified carbon paste electrodes to determine Cu 2 in ammoniacal medium [12]. Their results demonstrated that silica particles displayed an attractive abilitytoefficientlypreconcentratetraceCu 2 fromammo- nia solution according to a combined acid-base and com- plexation reaction: 2(SiOH)  Cu(NH 3 ) 4 2 >(SiO) 2 Cu(NH 3 ) 2  2NH 4  . In addition, a chemically modified carbon paste electrode incorporated with zeolite was successfullyusedtodetermineCu 2 inammoniacalmedium [11].PreconcentrationofCu 2 canbeachievedrapidlyusing zeolite-modified electrodes and then be observed voltam- metrically as a redox characteristic of Cu(NH 3 ) 4 2 . Never- theless, these electrodes are questionable in consideration with repeatability, operational and long-term stability, and solutionmatrixeffect. Clay-modified electrodes have attracted considerable attention for their applications in electroanalytical chem- istrysimilartozeoliteduetoappreciablecationicexchange properties [15±40]. Various clay materials, such as mont- morillonite, kaolinite, vermiculite, and silica, have been used as a matrix for Cu 2 detection due to their specific adsorption effect [12, 25±27]. The sensitivity, calibration range,andmechanismofdetectionaredifferentdepending on the nature of clay used. On the other hand, nontronite (SWa-1, [(Si 7.25 Al 0.75 )(Fe 3 2.75 Al 0.85 Mg 0.33 Ti 0.05 )O 20 (OH) 4 X  0.98 ]) was proved to possess excellent electroanalytical and catalytic properties over other clays like kaolinite, illite, bentonite, vermiculite, and montmorillonite in our recent studies [28±31]. X-ray diffraction patterns confirmed that the d 001 interplanar distance (i.e. the basal spacing) of the coatedclayis12.9äwhichiscomparabletothe10äzeolite [32]. Surprisingly, electroanalytical studies for heavy metal assays using nontronite-clay modified electrodes are very limited.Wereporthereanewnontronite/celluloseacetate- coated glassy carbon electrode (NCA/GCE) for the deter- mination of Cu 2 in ammoniacal medium by square-wave voltammetry (SWV). Note that, in the present work, ammonia solution was used both for complexing agent as well as for supporting electrolyte. Similar to silica and zeolite modified electrodes, clay also was considered to be suffering from operational stability, regeneration of the electrodesurface,andmatrixeffects.Tosolvethisproblem, wecombinedtheclayelectrodewithpermselectivecellulose acetate (CA) membrane as an outer-layer coating to stabilize and improve the performance of the nontronite- coated GCEs (SWa-1/GCEs) [29, 30]. The rigid structure and porous nature of the CA membrane possesses a hydrophilic layer and is expected to improve the stability andtoreducethemembraneresponsetowardlargehydro- phobicanionsandinturntheinterferenceeffectoforganic moeities. Necessary solution phase and instrumental parameters were systematically optimized to improve the detection of analyticalsignals.Majorinterferenceeffectwascheckedby testing with various organic compounds (e.g., methyl 99 Electroanalysis 2002,14,No.2 ¹WILEY-VCH Verlag GmbH, 69469 Weinheim, Germany, 2002 1040-0397/02/0201-0099 $ 17.50+.50/0