3004
Anal. Chern. 1904, 56,3004-3005
recent report on a single-beamllock-insystem also shows these advantages (29). In comparison with the single-laser/collinear-beam configuration, the crossed-beam arrangement offers the following additional advantages (for short path length samples only): (1) Optical simplicity and reduced system costs. Because the pump radiation is spatially separated from the probe beam, a polarizer pair of high extinction ratios is not required. An inexpensive glass plate beam splitter suffices for the splitting of the laser beam. Also, the beam-recombining optic is not needed. (2) Improved detection limit (Table I). (3) Relatively insensitive to system vibrations. This is also due to the absence of the optical interference noise which is sensitive to mechanical vibrations. (4)Thermal lens background signals reduction. This arises from the fact that system background absorptions, except those from the interaction region, are not being probed in the crossed-beam geometry.
ACKNOWLEDGMENT We are grateful to Kenneth Street for technical assistance in constructing the flow cell and to Keith Jameson for reading this manuscript. LITERATURE CITED Gordon, J. P.; Leite, R. C. C.: Moore, R. S.; Porto, S. P. S.; Whinnery, J. R. J. Appl. Phys. 1985, 36, 3. Hu, C.; Whinnery. J. R. Appl. Opt. 1973, 72, 72. Long, M. E.; Swofford, R. L.; Aibrecht, A. C. Sclence 1978, 797, 183. Whinnery, J. R. Acc. Chem. Res. 1974, 7,225. Kliger. D. S. Acc. Chem. Res. 1980, 73, 129. Harris, J. M.; Dovichi, N. J. Anal. Chem. 1980, 52,695A. Dovichl, N. J.; Harris, J. M. Anal. Chem. 1979, 57,728. Dovichl, N. J.; Harris, J. M. Anal. Chem. 1980, 52,2338. Dovichi, N. J.; Harris, J. M. Anal. Chem. 1981, 53, 106. Carter, C. A.; Rady, J. M.; Harris, J. M. Appl. Spectrosc. 1982, 36, 309.
Carter, C. A.; Harris, J. M. Appl. Spectrosc. 1983, 37, 166. Carter, C. A.; Harris, J. M. Anal. Chem. 1983, 55, 1256. Carter, C. A.; Harris, J. M. Anal. Chem. 1984, 56. 922. Carter, C. A.; Harris, J. M. Appl. Opt. 1984, 2 3 , 476. Imasaka, T.; Miyaishi, K.; Ishibashi, N. Anal. Chim. Acta 1980, 7 75, 407. Miyaishi, K.; Imasaka, T.; Ishibashi, N. Anal. Chim. Acta 1981, 124, 381. Miyaishi, K.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1982, 54,2039. Mori. K.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1982, 54, 2034. Higashi, T.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1983, 55, 1907. Haushaiter, J. P.; Morris, M. D. Appl. Spectrosc. 1980, 3 4 , 445. Buffett, C. E.; Morris, M. D. Appl. Spectrosc. 1983. 37,455. Fujiwara, K.; Uchiki, H.; Shlmokoshi, F.; Tsunoda, K . 4 ; Fuwa, K.; Kobayashi, T. Appl. Spectrosc. 1982, 36, 157. IFuJiwara, K.; Lei, W.; Uchiki, H.; Shimokoshi, F.; Fuwa, K.; Kobayashi, T. Anal. Chem. 1982, 54,2026. IDovichi, N. J.; Harris, J. M. Anal. Chem. 1981, 53,689. Leach, R. A.; Harris, J. M. J. Chromatogr. 1981, 278, 15. Buffett, C. E.; Morris, M. D. Anal. Chem. 1982, 54, 1824. Sepaniak, M. J.; Vargo, J. D.; Kettier, C. N.; Maskarinec, M. P. Anal. Chem. 1984, 56, 7252. Buffett, C. E.; Morris, M. D. Anal. Chem. 1983, 55,376. Pang, T.-K. J.; Morris, M. D. Anal. Chem. 1984, 56, 1467. Yang, Y. Anal. Chem. 1984, 56,2336-2338. Betteridge, D. Anal. Chem. 1978. 50,832A. Stewart, K. K. Anal. Chem. 1983, 55,931A. Toiles, W. M.; Nibier, J. M.; McDonald, J. R.; Harvey, A. B. Appl. Spectrosc. 1977, 3 7 , 253. Levenson, M. J. Phys. Today 1977, 30 (5), 44. Harvey, A. B. Anal. Chem. 1978, 50,905A. Morris, M. D.; Wallan, D. J.; Ritz, G. P.; Haushaiter, J. P. Anal. Chem. 1978, 50, 1796. Owyoung, A. IEEE J. Quantum Electron. 1978. 74, 192. Woodruff, S. D.; Yeung, E. S. Anal. Chem. 1982, 54, 1174. Peiietier, M. J.; Thorsheim, H. R.; Harris, J. M. Anal. Chem. 1982, 54, 239. Jackson, W. 8.; Amer, N. M.; Boccara, A. C.; Fournier, D. Appl. Opt. 1981, 20, 1333. Chen, T. I.; Morris, M. D. Anal. Chem. 1984, 56,19. Leach, A. A.; Harris, J. M. Anal. Chem. 1984, 56,1481. ~
Received for review July 16, 1984. Accepted September 10, 1984. This research was supported by grants from 1983 Loyola University Summer Research Award, Research Stimulation Fund, and Small Research Grant.
Restoration of Unresponsive Fluoride Ion Selective Electrodes John W. Bixler* and Larry S. Solomon’ Department of Chemistry, State University College at Brockport, Brockport, New York 14420 The solid-state fluoride ion selective electrode was first described nearly 2 decades ago by Frant and Ross (1). This electrode has filled a genuine analytical need and, hence, ranks among the most popular potentiometric sensors. In commercially available models, the inner surface of the lanthanum fluoride sensing membrane is in wet contact with an internal reference, which is typically a miniature solid-state chloride ion selective electrode. This internal electrolytic contact is made with a solution containing both chloride and fluoride, often by means of a fibrous wick wetted with this solution. The most frequent cause of electrode failure is the loss of internal contact due to the evaporation or leakage of the internal solution. The inner surface of the transparent lanthanum fluoride crystal appears dry and opaque when this occurs. Response can be restored by penetrating the outer barrel and replenishing the solution (2))but it has been our experience that restoration by this method is only temporary. We have developed a restoration procedure based upon the creation of a solid-state internal Ag/AgF/LaF, interfacial structure which completely replaces the original internal reference system. This structure is illustrated in Figure 1. Present address: Department of Biochemistry, University of Iowa, Iowa City, IA 52242. 0003-2700/84/0358-3004$01.50/0
This type of contact has been described by Fjeldly and Nagy, who fused a AgF-LaF, admixture onto a LaF3 membrane at a high temperature, claiming that this fusion was necessary to obtain satisfactory mechanical adherence to the LaF3 crystal (3). They coated this junction with silverloaded paint or epoxy to complete the contact. Our procedure for creating a mechanically and electrically stable interface is much simpler than Fjeldly’s. By not removing the LaF, crystal from its original epoxy barrel, we take advantage of creating a mechanically strong epoxy-to-epoxy bond. EXPERIMENTAL SECTION Electrode Disassembly. The l/s in. thick outer barrel of an Orion 94-09A electrode was cut through just beneath the cable end cap with a fine-toothed hack saw. Care was taken not to cut into the barrel of the internal reference electrode, which was removed with the cable intact. (The internal electrode was prepared for use as a chloride sensor by polishing with an Orion 94-82-01 polishing strip and rinsing with distilled water.) A second cut was made through the outer barrel about 1 in. above the sensing membrane. Both parts of the outer barrel were rinsed with distilled water using a cotton-tipped applicator as a brush to dislodge all salts, A final rinsing was made with methanol and the barrel parts were allowed to air-dry. Silver Fluoride Preparation. A concentrated solution of silver fluoride in methanol was prepared on the same day it was 0 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
E
Figure 1. Structure of reconstructed fluoride electrode: (A) shielded
cable; (B) orlglnal outer electrode barrel; (C) sliver-loaded conductlng epoxy; (D) silver fluorlde layer; (E) lanthanum fluorlde senslng membrane.
to be used. Approximately 0.6 g of silver oxide (Fisher Purified) was combined with 0.2 g of 48% aqueow hydrofluoric acid (Baker analyzed) in a plastic centrifuge tube. After mixing and centrifugation, the supernatant solution was evaporated to dryness on a watch glass in a 110 "C oven. The residue on the watch glass was slurried with about 1mL of absolute methanol, the slurry was centrifuged, and the clear supernatant was drawn into a Pasteur pipet. Electrode Reassembly. Two or three small drops of methanolic silver fluoride were placed directly onto the inner surface of the lanthanum fluoride membrane. The methanol was removed by vacuum desiccation. The shorter piece of barrel containing the sensing membrane was packed with silver-loaded epoxy (Eccobond solder 57C, Emerson and Cuming, Inc., Canton, MA) by using a syringelike device constructed from a 13/,in. piece of 5-mm glass tubing and a no. 10 common nail with the tip removed. The syringe was loaded by pressing the open end into the conducting epoxy paste. It is important that the edge of the electrode barrel be kept free of conducting epoxy, since a direct electrical connection between the inside and outside walls of the electrode must be avoided. Therefore, the outside of the loaded syringe was wiped free of conducting epoxy by use of a tissue moistened with toluene. The epoxy was injected onto the inner membrane surface as soon as possible after the membrane was removed from the vacuum desiccator since the silver fluoride layer is extremely hygroscopic. This was done by placing the syringe inside the electrode barrel with the tip against the inner membrane surface and injecting conducting epoxy until the 1-in. barrel was nearly filled. Electrical contact was established by inserting the conductor from a coaxial cable salvaged from a broken glass pH electrode into the soft conducting epoxy. The cable shielding was left intact except a few millimeters were removed to prevent contact with the conducting epoxy surface. The longer piece of electrode barrel was slipped over the cable before inserting the conductor into the silver epoxy and was reattached to the shorter piece of barrel by cementing with transparent epoxy adhesive (Hardman, Inc., Belleville, NJ). The protruding cable was potted into the upper end of the barrel with epoxy putty to minimize mechanical strain on the electrical contact. Also, a flexible plastic sleeve was fitted around the upper end of the barrel to facilitate mounting the reconstructed electrode in the electrode holder of a Corning pH
3005
meter. The entire reconstruction procedure required about l/z h of labor but was generally spread over several days to allow for drying periods. Electrode Testing Procedure. Fifty milliliters of 0.05 M sodium perchlorate solution (reagent grade sodium perchlorate, deionized distilled water) were pipetted into a thermostated plastic cell. Measured portions of the standard sodium fluoride solution were added with a micrometer buret and the emf was measured after each addition using the fluoride electrode being tested, an Orion 90-02 double-junction reference electrode and a Corning Model 130 pH meter. The chloride electrodes recovered from the internal reference systems were tested by an analogous procedure using standard sodium chloride solution.
RESULTS AND DISCUSSION Electrodes restored by this procedure can be used as soon as the epoxy has hardened, but the apparent standard potential decreases several millivolts per day for a week or two, suggesting that interfacial changes occur as the conducting epoxy cures. Following this initial period, the apparent standard potential only changes a few millivolts in the ensuing 6 months. The restoration process does not alter the slope, stability, response range, or dynamic response properties of the electrode. It is our experience that slope, stability, and dynamic response seem to be markedly affected by the cleanliness of the sensing membrane but not by the nature of the internal reference system. In fact, the value of the apparent standard potential, which became 700 to 900 mV more positive after reconstruction, is the only parameter affected by this modification. The variation in the apparent standard potential among the restored electrodes undoubted reflects the fact that the conditions during the creation of the interface varied somewhat in each case. Three fluoride electrodes have been restored during the past 3 years using this procedure. All three restorations were successful on the first attempt and none of them has subsequently lost response. A significant fringe benefit of this procedure is the recovery of the internal chloride reference electrodes with their cables and pH meter jacks intact. Their response properties are those of silver sulfide-silver chloride based solid-state chloride ion selective electrodes. Registry No. AgF, 7775-41-9; Ag, 7440-22-4; fluoride, 16984-48-8; chloride, 16887-00-6. LITERATURE CITED (1) Frant, M. S.; Ross, J. W. Science 1966, 754, 3756. U.S. Patent 3431 182. (2) Stahr, H. M.; Ross, P. F.; Hyde, W. Mlcrochem. J . 1980, 25(2), 232. (3) Fleldly, T. A.; Nagy. K. J . f/ectrochem. SOC.1980, 127, 1299.
RECEIVED for review June 11,1984. Accepted July 16,1984. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. This work was presented in part at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, NJ, March-1984.
