An Application of Nonfunctional Glass Electrodes Tessa Brandt, Denise Barnes, Brad Messer, Kurt Johnson, and Stephen Lawrence Rockhurst College, 1100 Rockhurst Road, Kansas City, MO 64110 Chloride Ion Seiective E l e c t r o d e
In our department we have divided the glass electrodes into three major categories that include: those that work, sometimes work, and those electrodes that do not work. Within time, most of the electrodes fell into the latter two categories. Rather than dispose of them, students in the Instrumental Analysis course decided to prepare a chloride ion selective electrode (ISE) employing the inner element of a AgIAgCl reference electrode removed from nonfunctional or broken electrodes from several manufacturers. The procedure for making the external assembly has been previously described' and was followed with regard to the physical dimensions of the glass tubing used. However, the chloride ion membrane was prepared in a different manner that involved the dissolution of 1.0 g of Fisher Reagent Grade poly(methy1 methacrylate) in akinimum amount of Fisher ACS Certified tetrahvdrofuran ('I'HF). Inco this solution, the students added dropwise approximately 0.4 g of chloride ion exchanger (Fisher Scientific, 13-641-903). This mixture was poured into a polypropylene cap (Nalgene, i d . ~~
~
~
Figure 2. Typical calibration curve for a series of KC1 standard solutions using the student-built chloride-lon-selective electrode shown in Figure 1. The imernai filling solution was 0.1 M KCi. Comparable values were obtained win? ~tandardsolution~ of NaCI.
PLUG CONNECTOR
INNER ELEMENT
ELECIRODE EJBQNT
EXTERNAL GLASS BODY SEVER POWDER S u m CHLORalE C r n O N RFIAWER N E F N U f U L N G SOLU7ION
Figure 1. Diagramof an assembledchloride-ion-selective eleclrde. Reprinted in pert with permission from Leeds and Nathrup, a unit of General Signal.
50 mm) that was subseauentlv covered with fdter paper and allowed to evaporate i n the hood, thus forming &eiatively uniform membrane. Disks were cut from themembrane with a No. 11 cork borer, and, in some cases involving irregular shapes the membrane was gently stretched over the end of the 8 mm4.d. glass hody. I t was discovered that membranes formed in aglass evaporating dish were brittle and frequentIv cracked when attemotine to remove or cut them to the appropriate size. since these membrane diameters were much lareer than the hodv of the electrode.. thev . were secured using a piece ofsulfur-free Latex tubing (Fisher Scientific. 14-178E3aooroximatelv 1 cm in lenzth withan internal diameter slight& smaller than that of the electrode body diameter. Final assembly involved insertion of the AgIAgC1 internal element from nonfunctional Leeds and Northruo (Model numbers 117203 and 1199-69) electrodes into the 10-cmlength memhrane-covered glass tube that was previously filled witha0.1 M potassium chloridenolution. Parafilm was initially used tosecure theinternal electrodeassembly to the externd glass hody. The complete electrode assembly is shown in Figure 1. Prior to use, the asseml~ledelectrode was immersed overnieht in a 0.1 M KC1 solution. As displayed i n Figure 2, a linear response using the relative millivolt scale was measured with a Pope pH1pIon meter (Model 1502) for 0.1-1.0 X 10-4 M standard KC1 and NaCl solutions. Referencing of these potentials were to a double junction electrode (Fisher, 13-620-47). Concentrations outside this range revealed deviations from linearity. Steady state potential readings o f f 3 mV were obtained within 3045 s and continued for over 75 min. Electrodes of this type
' Lloyd. B. W.: O'Brlen. F. L.: Wllson, W. W. J. Chem. Educ. 1976,
53.328-330.
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were also used in the titration of silver and chloride ions but with variable success. Althoueh this t w e of membrane has been described for a coated wGe n i t r a i k l ~ ~we z , were unable to locate references in the literature that involved construction of a chloride ISE using nonfunctional AgIAgC1 electrodes and chloride imnreenated methyl methacrylate membranes. tensions ofthe above procedure involve student analysis of unknown solutions, changing the concentration and components of the internal filling solution, subntituting either a saturated calomel electrode' or a student-constructed internal electrode prepared by electrolyzing AgCl on silver wire. These variations have been performed, and comparable results to those reported have been obtained. Different types of membranes were tried but with limited success. These include agar impregnated with varying amounts of chloride ion exchanger and membrane tubing (Spectrapor, Spectrum Medical Industries, Inc.). The former gave erratic voltage readings, while the response of the
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Journal of Chemical Education
latter changed as a function of time. These increasesldecreases in the voltage were not apvarent until the electrode was immersed in the solution to be measured for 15-20 min. I t may be assumed that the agar inhibits charge migration, while the pore size of the membrane allows diffusion across the barrier and in time significant mixing of the solutions . occurs. Due to the success achieved with the above electrode membrane, the students will attempt to prepare nitrate2, calcium3.4, fluoride, and mixed ion electrodes using derivations of the procedures described above. Electrode response will be measured using a modified version of an inexpensive student-built pH1pIon meter5. Kneebone, B.: Frelser, H. Anal. Chem. 1973,45,449-452. Ansaldl, A,: Epstein, S. I. Anal. Chem. 1973, 45, 595-596. Mocdy, G. J.; Oke, R. B.: Thomas, J. D. R. Analyst 1970,95,910912.
Slevers, D. J. Chem. Educ. 1982,58281.
