Charles R. Martin a n d Henry Freiser University of Arizona Tucson. A.Z 85721
Coated-Wire Ion Selective Electrodes and Their Application to the Teaching Laboratory
Although a number of contemporary undergraduate ana-
lnid chemistw texts describe ion selective electrodes (ISEs) i d their significant role as electrochemical probes, laboratoj exercises with these useful devices are not as generally encountered in the curricula as their importance and interest deserves, because of the relatively high cost of commercial ISEs. Use of the so-called coated-wire ISE which is not only inexpensive but also very easy to fabricate should do much to overcome this situation. In the course of a study of a series of liquid membrane ISEs (1.2). . . we became intrieued bv the solution develoned bv two Japanese scientists for the failure of Cu2S-plated copper to act as a sulfide-resnonsive electrode. Hirata and Date (3) rlweloped a umell-behat'ed electrode hy embeddmg a ('u rond u c u ~ in r a disc made bv incor~orat~ne finelvdivided C u 8 in a polymeric matrix. hipired b y thisrour research team (4) attempted to prepare electrodes by dipping the tip of a platinum wire in a solution of poly(vinylchloride) (PVC) in cyclohexanone or tetrahydrofuran (THF) containing also a suitable soluble electroactive material and allowing the resulting thin film to air dry overnight. Electrodes prepared in this manner, which we dubhed coated-wire ion selective electrodes (CWISEs), were found to give Nernstian potential resnonses to chanees in activities of the desienated ions. CWISES responsive to Ca2+ (5), various anioLs including amino acids (6),anionic surfactants (7),phenobarbital (81, K+ (9), and high molecular weight quaternary ammonium ions (10) have since been nrenared and evaluated. In all cases. the CWISEs have givei responses as good as or better than analogous conventional ISEs. The unique feature of the CWISE is that it does not require the internal reference solution and electrode present in conventional ISEs of the liquid or polymer membrane type. While from a theoretical viewpoint this presents some problems in interpreting the mechanism of operation of the CWISE, from a practical point of view this lack of internal reference components gives the CWISE some interesting advantages over conventional type ISEs. First, CWISEs are easier to construct and reauire much less construction time than conventional polymer membrane ISEs. This ease of construction makes the ('\VISE ~articularlvattractive for use in undereraduate or graduate~lahorato~courses in instrumental anal& Second, CWISEs are far less expensive than conventional ISEs. This difference is particulariy true when the cost of the CWISE is compared to the cost of commerciallv available ISEs. In addition, however, the CWISE is even iess expensive than the polymer membrane ISE recommended for instructional purposes hy Craggs, Moody and Thomas (11). Third, CWISEs may be easily miniaturized, making possible intercellular or other in uiuo electrochemical measurements. This paper describes the procedure for construction of a nitrate CWISE and sueeests exneriments for evaluation of electrode response and&stration of typical analytical applications of ISEs. In addition, modification of the procedure so that CWISEs responsive to other anions can be prepared are described. The electrode response is evaluated by generating a calibration curve and determining selectivity coefficients. Analytical applications described include determination of NO, in ambient air using the nitrate CWISE and determination of the solubilitv constant of KC10d.usine . product . a perchlorate CWISE.
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512 I Journal of ChernicalEducation
Preparation of Coating Solutlon The CWISEs described are based on the liquid ion exchange material, Aliquat 336s (tricaprylylmethylammonium chloride), which may be obtained from Henkel Corporation, Kankakee, Illinois. The first step in preparing a nitrate CWISE is to convert the Aliauat-C1- salt to the Aliauat-NO? salt. If CWISEs responsive 'to anions other than ~ 0are~ desired. a 1M solution of the desired anion should be substituted f i r NaN03 in the procedure below. The procedure is as follows: 1) Twelve ml of Aliquat-C1- and 8 ml of decanol are placed into a 40 ml extraction vial. The vial is fitted with a polyethylene liner and then shaken (a mechanical shaker is most convenient) for approximately 10 min to effect mixture of the two solutions. 2) The Aliquat-decanol solution is then shaken with successive 10 ml aliquots of 1M NaN03 to effectexchange of C1- for NOS-. Each 10 ml aliquot is shaken with the Aliquat-decanolsolution for about 10min. The phases are then allowed to separate and the lower aqueous phase removed with a micropipet. The progress of the exchange reaction is checked by adding several drops of 0.1 M AgNO3 to the aqueous phase. The appearance of an AgCl precipitate indicates that the exchange reaction is incomplete. Generally, 5-10 aliquots of the 1 M NaNO? solution are required to effect complete exchange. 3) After exchange, the Aliquat-decanolphase is shaken with three 10ml Aliquats of deionized water to remove superfluous NaNOs salution. As hefore, each aliquot of aqueous phase is shaken with the Aliquat-decanol solution for about 10 min, the phases allowed to separate, and the aqueous phase removed with a micropipet. 4) After washing, the Aliquat-decanol solution is centrifuged at 1500 rpm for 1hr and the upper phase separated and stored in a clean extraction vial. 5) Two ml of the Aliquat-Nos--deeanol solution is'thoraughly mixed with 32 mlof a 5 (WN)%solution of PVC inTHF. Thoroueh mixing may he effected conveniently by shaking for approximateiy 10 min. This solution is the coating solution. ~~
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The inner copper conductor of an RG 5 8 N coaxial cable, which has been fitted with an Amphenol (Danbury, CT) 74868 UG-89AN BNC connector makes a convenient substrate for the CWISE. The PVC-based coating solution is applied to this substrate. The preparation of the coaxial cable andthe coating procedure is described below. 11 Thecoaxial cable ir prepared by euttinganapprurimately 2.ft section and attaching the HNC connector to oneend. RNC ronnrrrors are used because they are the least expensive shielded connector available. Since the pH/mV meter used to measure the potential will not, however, accept a BNC jack, a patch cord having an Amphenol 36775 connector on one end and a US. Standard ISE plug on the other is needed. The ISE plugs may he obtained from Orion Research, Inc. (Cambridge,MA). The center conductor is soldered to the tip of the plug; the outer braid is soldered to the case. 2) Approximately 2 in. of the outer insulation at the other end of the cable is then removed with a razor blade. Care must be exercised so as to avoid cutting the inner insulation.The exposed braided outer copper conductor is then cut away with a wire cutter. 3) Approximately 1in. of the inner insulation of the coaxialcable is then carefully removed. The tip of thecopper wire thusexposedis sanded flat with a fine erade emerv cloth. 4, TheexporedCu ;ire is then iashed with laboratory detergent and water, thoroughly rinsed xith deionized water, nnd drwd wtth acrt
