Fabrication of an Inexpensive lon-Selective Electrode A. Palanivel and P. Rlyazuddin The New College, Madras-BOO 014 India
An enormous amount of research is currently being conducted in the field of ion-selective electrodes (ISE's). This is evidenced by the publication of over 700 papers during 1978-80 and by the appearance of a new serial publication, ISE Reviews (I). A few ISE's made of graphite paste (2-4), porous graphite, and spectrally pure graphite rods coated with metal complexes (5-7) have been reported. The special-grade graphite rods are expensive and not readily available on the market. However, we found that a graphite rod extracted from a used dry cell is a completely acceptable substitute after it has been cleaned by abrasion followed by an overnight treatment with hydrochloric acid. preparation of Graphite (Ag,S-CUS) Electrode A 2.5-cm long graphite rod extracted from a D-type dry cell was inserted intoa tightly fitting polythene tuhe from which 2 cmof the rod was protruding. An Aralditeeseal wasapplied to make sure that no solution could seep into the tube. The rod was kept in a saturated solution of sodium sulfide for 1 hr. Then it was transferred ta a saturated solution of silver nitrate and left for another hour. The rod was removed and washed several times with twice-distilled water and ethanol until no Ag+ ions could be detected in the washings. Same steps were repeated using saturated copper sulfate solution instead of silver nitrate to precipitate copper sulfide on the same rod. The electrical contact'waseffe&id by putting a few grams of mercury into the polythene tube and connectingit to the pH meter with a thin copper wire enclosed in plastic sleeves (or coaxial cable from a broken glasselectrode). Potentials were measured against a saturated calomel electrode. A potassium nitrate (0.1 M )hridae - was used while workinc with silver solutions. Electrode Performance The graphite rod serves the dual roles of supporting, medium and conducting material. The electrode shows linear response in the range of 10-'-10-5 M toward silver ion (pH = 2-9) and copper ion (pH = %7) with slope of 57 and 23 mV per decade of concentration, respectively. The limit of detection is 10-"M for both ions. Response time is instantaneous for 10-1-10-3M solutions and increases to a maximum of 30 sec for 10-4-10-5M solutions. Graphite rods extracted from several brands of dry cells were tested. The difference was not more than f1-2 mV in the slope. The response time and concentration range were the same in all cases. No change in stability and reproducibility of potential was seen even after five months of continuous use.
920
Journal of Chemical Education
The electrode requires no maintenance other than cleaning with chloroform or dilute ammonia when it is subieded to complexometric and precipitation titrations. I t should be stored dry. No ISE responds exclusively to the ion that it is designed to measure, although it is often responsive to primary ions more than to others. The important interferences are mercuric, bismuth, and sulfide ions. Chloride, bromide, ferric, and cadmium interfere when they are present in large quantity. The graphite (AazS-CuS) electrode also Droves verv useful as an iidiiator electrode in the complexo~etrictitrition of ions such as calcium, zinc, nickel, and copper with EDTA and NTA, etc. and in the argentometric titration of halides. The cost and lifetime of an electrode are not related chemically, hut an electrode must last long enough to earn its keep. This is truly a zero-cost electrode since the graphite rod is extracted from a dead throw-away dry cell. Subjecting the electrode to various drastic and aggressive environments like high acidity, high alkalinity, and various nonaqueous media does not alter its performance. However, when it starts showing poor performance the sensor can he scraped off and a new coat can be easily applied again. Alternately, other types of sensors, such as CdSIAgzS, PbS/Ag& silver halide/Ag~S, AgSCNIAgzS, can be coated. Several inexpensive ISE's have already been reported in THIS JOURNAL (8-14). The most noteworthy feature of this electrode is its easv p re oar at ion. Students neither need anv assistance from thetkacber to fabricate it nor do they have go in search of chemicals not commonly available (9, 11, 13) nor do they have a pellet press (12,14). This electrode can he used with advantaw for teachine ISE methodolow "" in eraduate programs.
i m .c ~ . , a n d ~ ~ i ~AWL & ~chim. . . Arro.. 64. I i1971). (6) Lamm,C. G.,Hansen, E. H..and Ruzlcka, J., A m l . Chim. Acta., 59.403 (1973) (7) Haasan, S. S. M., and.Habib, M.M.,M i c m k m . J.,Z6,181(1981). (8) Lamb. R. E., Natuach, 0. F. S., O'Reilly. J. E., and Wstkina, N. J.. J. Czmw Eouc, (5)
50,432(1973).
(9) Grggs.A., Moody,G. J..and Thoman. J. D.R., J. C ~ M Eouc., . 51,54l(1974). (10) Wilmx, Jr., F., J. CHm. Eouc.,52,123 (1975). (111 Lloyd, B. W., O'Brien, F. L., m d Wilaon, W. D., J. C H m . Eouc.,53,328 (1976). (12) hieke, G. H..and Kuotz, M.J., J. CHEM.EDUC, 54,517 (1977). (13) Martin, C. R., and Freiacr. H., J. CHEM.EouC.,67.512 (1980). ( I 0 Pspastathopodos.D. S.,and Ksrayannis, M. I., J. CHBM.E D U C . , ~ (1980). ~,?~
