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Anal. Chem. 1 9 8 0 , 52, 1532-1534
ACKNOWLEDGMENT We acknowledge many helpful discussions with the late Robert W. Vaughan and dedicate this work to his memory. We are grateful to J. W. Rakshy, Jr., of Dow Chemical Company, Midland, Mich., for the sample of 0.095% cross-linked We thank J' D' Roberts for the to the 45'2 MHz Spectrometer. LITERATURE CITED ( 1 ) 8. W. Erickson and R. B. Merrifield in "The Proteins", 3rd ed., VoI. II., H. Neurath and R . L. Hill, Eds., Academic Press, New York, 1976, pp 257-527. (2) E. C. Blossey and D. C. Neckers, "SolicCPhase Synthesis", Halsted/Wiley, New York, 1975. (3) C. C. Leznoff,,,Acc. Chem. Res., 11, 327 (1978). (4) F. A. Bovey. High Resolution NMR of Macromolecules", Academic Press, New York, 1972, pp 118-129.
(5) . , K. Matsuzaki. T. Urvu. K. Osada. and T. Kawarmura. Macromolecules. 5, 816 (1972). (6) A. Allerhand and R. K. Hailstone, J . Chem. Phys., 56, 3718 (1972). (7) H. Sternlicht, G. L. Kenyon, E. L. Packer, and J. Sinclair, J. Am. Chem. SOC.,93, 199 (1971). (8) C. Birr, "Aspects of the Merrifield Peptide Synthesis", Springer, New York, 1978. (9) J. S. Waugh and L. M. Huber, J. Chem. Phys., 47, 1862 (1967). ( I O ) U. Haeberlen and J. S. Waugh, Phys. Rev., 175, 453 (1968). ( 1 1 ) J. Schaefer, R. A. McKay, and E. 0. Stejskal. J . Magn. Reson., 34, 443 (1979). (12) T. R. Steger, E. 0. Stejskal. R. A. McKay. 6. Stuits. and J. Schaefer, Tetrahedron Lett., 295 (1979). (13) J. Schaefer, E. 0. Stejskal and R. Buchdahl, Macromolecules, 10, 384 (1977). (14) M. Kainosho, Tetrahedron Lett., 4279 (1976). (15) M. Kainosho and H. Konishi, Tetrahedron Lett., 4757 (1976). (16) M. Kainosho and K. Ajisaka, Tetrahedron Lett., 1563 (1978). (17) J. D. Graham and J. S. Darby, J. Magn. Reson.. 2 3 , 369 (1976). (18) A. Pines and T. W. Shattuck, J . Chem. Phys., 61, 1255 (1974). (19) C. A. Fyfe, P. J. R. Lyerla, and C. S. Yannoni, J. Magn. Reson.. 31, 315 (1978). (20) J. Schaefer, Macromolecules, 4, 110 (1971). (21) P. Tekely and E. Turska, Macromol. Chem., 180, 211 (1979). (22) J. R. Lyerla and D. A. Torchia, Biochemistry, 14, 5175 (1975). (23) D. A. Torchia, M. A. Hasson, and V. C. Hascall, J. Biol. Chem., 252, 3617 (1977). (24) H. Slito, T. Ohki, and T. Sasaki, Biochemistry, 16, 908 (1977). (25) K. Yokota, A. Abe, S.Hosaka, I. Sakai, and H. Slito, Macromolecules, 11, 95 (1978). (26) H. Slito. E. Miyata, and T. Sasaki, Macromolecules, 11, 1244 (1978). (27) M. W. Duch and D. M. Grant, Macromolecules, 3, 165 (1970). (28) J. Schaefer, Macromolecules, 5, 427 (1972).
(29) J. Schaefer in "Topics in Carbon-13 NMR Spectroscopy", G. C. Levy, Ed., Wiley-Interscience. New York. 1974, pp 149-208. (30) R. A. Komoroski and L. Mandelkern, J. Polym. Sci., Polym. Lett. Ed., 14, 253 (1976). (31) R. A. Komoroski, J. Maxfield, F. Sakaguchi, and L. Mandelkern, Macromolecules, 10, 550 (1977), and ref. cited therein. (32) J. M. Stewart and J. D. Young, "Solid Phase Peptide Synthesis", W. H. Freeman and Company, San Francisco, Calif., 1969, p 27. (33) K. W. Pepper, H. M. Paisley, and M. A. Young, J. Chem. SOC.,4097 (19531. (34) G.A. Obh, D. A. Beai, and J. A. Oiah, J. Org. Chem., 41, 1627 (1976). (35) H. C. Brown and K. L. Nelson, J. Am. Chem. SOC.,75, 6292 (1953). (36) J. Schaefer and D. F. S. Natusch, Macromolecules, 5, 416 (1972). (37) D. Doddrell, V. Glushkco, and A. Allerhand, J. Chem. Phys., 56, 3683 (1972). (38) D. M. Grant and B. V. Cheney, J. Am. Chem. Soc., 89, 5315 (1967). (39) S. B. H. Kent, A. R. Mitchell, M. Engelhard, and R . 8.Merrifield, R o c . Natl. Acad. Sci. U.S.A.. 76, 2180 (1979).
S t a n l e y L. M a n a t t * David Horowitz Information Systems Research Section J e t Propulsion Laboratory California Institute of Technology Pasadena, California 91103 Robert Horowitz Fruit and Vegetable Chemistry Laboratory U.S. Department of Agriculture Science and Education Administration Pasadena, California 91106 Robert P. Pinnell* Joint Science Department Scripps, Pitzer, and Claremont Men's Colleges Claremont, California 91711 RECEIVED for review January 7, 1980. Accepted April 2, 1980. This work was supported in part by the Director's Discretionary Fund, the Caltech President's Fund, a Research Corporation Grant, Caltech NIH Biomedical Research Support Grant No. RR07003-14, and as one phase of research a t the Jet Propulsion Laboratory under Contract No. NAS7-100 sponsored by the National Aeronautics and Space Administration.
Polymer Membrane Electrode Based Potentiometric Ammonia Gas Sensor Sir: Commercially available potentiometric gas sensing electrodes based on internal pH probes (e.g., ammonia, carbon dioxide) are now commonly employed in a variety of analyses ( I ) . Response of such electrodes arises from diffusion of the analyte gas through a gas permeable membrane resulting in a p H change a t the surface of the internal glass membrane. Although widely used, these electrodes suffer inherent analytical limitations due to their technological design and the chemical nature of their response. Detection limits are predetermined by the respective equilibrium constants of the internal electrolytes ( 2 ) and, in certain instances, these detection limits are not suitable for a given analysis (i.e., blood ammonia at pH 7.4). In addition, the development of desirable miniature or microsize sensors has been limited by the preparative problems associated with these fragile glass systems. The purpose of this correspondence is to introduce a new type of potentiometric gas sensor which employs a polymer membrane internal sensing element and to demon0003-2700/80/0352-1532$01.OO/O
strate the practical advantages such an approach can offer. Specifically, we describe the development of a miniature ammonia gas sensor utilizing an internal polymer membrane ammonium probe. Previously, Montalvo ( 3 ) suggested the use of an ammonium cation glass electrode as an internal element for an ammonia sensor but no practical analytical results were obtained because of the unfavorable geometry of the glass membrane used. Others ( 4 ) have attempted to replace the inner pH glass membrane with silver/silver-suKde membranes, but with limited success. In this work, we have incorporated the antibiotic nonactin into an appropriate plasticizer-poly(viny1 chloride) (PVC) membrane electrode and then used this probe as the inner electrode in a gas sensing arrangement for dissolved ammonia. The preliminary data presented here will illustrate that the proposed electrode offers advantages of simplicity, reduced size, improved detection limits, and low cost over conventional glass membrane based ammonia gas sensors. 0 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
1533
L~ 2 -log
[cation] , M
Response and selectivity of nonactin-PVC membrane electrode in 0.01 M Tris-HCI, pH 7.5 Figure 1.
EXPERIMENTAL Apparatus. Potentiometric measurements were made using a Corning Model-12 pH meter in conjunction with a HeathSchlumberger Model SR-204 Recorder. Ammonium sensitive polymer membranes were preliminarily evaluated using an Orion Model 92-00 liquid membrane body and a saturated calomel reference electrode. All measurements were made at 25 "C in a thermostated cell. Reagents. Nonactin was obtained from Sigma Chemical Co. Poly(viny1 chloride), chromatographic grade, was a product of Polysciences, Inc. Plasticizers tested including dioctyl phthalate, dibutyl sebecate, dioctyl adipate, diethylhexyl phosphate, diethylhexyl sebecate, dipentyl phthalate, and diethyl phthalate were all commercial preparations from various suppliers. All aqueous solutions were made with deionized water. Preparation of Ammonium Sensitive Polymer Membrane. Simon (5)was the first to show that saturated levels of nonactin in certain organic liquid membranes can serve as selective ammonium sensors. Guilbault (6) developed a solid state ammonium electrode by incorporating extremely high levels of nonactin into silicone rubber membranes. Similarly, Morf and Simon ( 7 ) have recently reported the use of nonactin in PVC membranes. In preliminary studies for this work, at least 30 different PVC-nonactin membranes were cast and tested for ammonium response. The membranes differed in percent composition of nonactin (0.2-25%), plasticizer, and ratio of plasticizer to PVC. Optimal ammonium responsive membranes were prepared as follows: 65 mg of PVC, 1 mg of nonactin, 125 pL of dibutyl sebecate, and 1.5 mL of tetrahydrofuran (THF) were mixed thoroughly on a vortexer and cast into a 22-mm diameter glass ring placed on a microscope slide. The thickness of the final plastic membranes was 0.17 mm. Smaller diameter membranes were cut from this larger piece and evaluated for ammonium response using an Orion liquid electrode body. Figure 1 illustrates the response and selectivity of these PVC-nonactin membranes in 0.01 M Tris-HC1 (tris(hydroxymethy1)aminomethane-hydrogenchloride) buffer, pH 7.5 following preconditioning in 0.01 M NH,Cl for 12 h. The electrodes exhibited a nearly Nernstian response (58 mvldecade) toward ammonium over a wide concentration range. Selectivity over other cations was similar to that reported for the earlier liquid membrane systems using nonactin ( 5 ) . A miniature ammonium probe was prepared by cutting a 2-mm diameter membrane and attaching it to the end of a disposable plastic pipet tip having a small piece of Tygon tubing (2-mm diameter) at the end. The membrane was sealed to the Tygon tubing with a paste made of THF, dibutyl sebecate, nonactin, and PVC (that used to cast the membrane). The inside of the plastic tube was filled with 0.01 M NH4C1and fitted with a Ag/AgCl reference wire. Preparation of Ammonia Gas Electrode. The final ammonia gas electrode was constructed as shown in Figure 2a. The internal ammonium sensor was placed into a slightly larger plastic pipet tip which had been filled with 0.01 M Tris-HC1 buffer, pH 7.5. The end of this pipet was covered by a gas permeable membrane.
.. "3
Figure 2. (a) Schematic diagram of miniature ammonia sensor: (A) Ag/AgCI electrodes, (B) internal buffer, (C) 0.01 M NH,CI, (D) Tygon tubing, (E) PVC membrane, (F) O-ring, (G) gas permeable membrane. (b) Expanded view of processes taking place at sensing tip: (H)
polymer-nonactin membrane, (I) buffer film, (J) gas permeable membrane
This gas membrane was held in place by a plastic O-ring. Gas membranes used were either Celgard 2500 microporous polypropylene (Celanese Corp.) or polyktrafluoroethylene (W. L. Gore, Inc.). When the internal ammonium probe was pressed into the outer pipet, a thin film of internal buffer formed between the gas membrane and the polymer membrane. Final electrical connections were made by placing a second Ag/AgCl electrode in the bulk portion of the buffer solution. Calibration curves for ammonia were made by making standard additions of NH,C1 to 25 mL of 0.005 M NaOH or Tris-HC1 buffer, pH 8.0.
RESULTS A N D DISCUSSION This new ammonia gas sensing electrode is based on the measurement of ammonium ions formed inside the film of buffer solution sandwiched between the polymer ammonium sensitive membrane and the gas permeable membrane. An expanded view of the sensing tip and the reactions taking place are shown in Figure 2b. Ammonia in the sample diffuses across the gas permeable membrane and is converted t o corresponding ammonium ions by the controlled pH condition of the buffer. Diffusion continues until the partial pressure of ammonia gas is equal on both sides of the membrane. A buffer trap effect (3) essentially concentrates the amount of ammonium formed in the thin film according to the Henderson-Hasselbalch equation: T o achieve the desired production of ammonium, the internal buffer should have a p H favorable toward the formation of ammonium ions (i.e., pH C9.3). In addition, the buffer chosen should not interfere significantly with the response of the ammonium polymer electrode. Consequently, buffer systems containing sodium or potassium must be avoided. Final electrodes were, therefore, constructed using 0.01 M Tris-HC1 or 0.1 M Tris-HC1, pH 7.5, since the nonactin membranes were found to have excellent selectivity over protonated Tris. Figure 3 shows a typical comparison of low level response curves obtained for this new type of ammonia sensor vs. a commercial Orion Model 95-10 ammonia probe. It can be seen that the polymer membrane based ammonia sensor has en-
Anal. Chem. 1980, 52, 1534-1535
1534
1270
-=-=T--\
90%
-~
,
6 -iog
5 [ N H ~ M~
4
90
Figure 3. Comparison of low level ammonia response curves obtained for miniature polymer membrane based gas electrode and commercial Orion (95-10) probe, in 0.005 M NaOH
hanced detection limits as a result of the buffer trap effect. Curvature a t higher ammonia concentrations indicates a change of pH in the buffer film due to the poor buffer capacity of the 0.01 M Tris-HC1 buffer. Increasing the ionic strength of the internal solution to 0.1 M effectively extends the linear response range up to M NH,. Typical slopes for the electrodes prepared thus far have been in the 40-50 mV/decade range. This sub-Nernstian response appears due to the lack of ideal geometry at the sensing tip (Le., NH3 gas diffusion into the bulk buffer solution) and on changes in the nonactin electrode response when in contact with the hydrophobic gas membrane. The latter was demonstrated by an experiment in which (NH4)2S04 standards prepared in Tris-HC1 buffer were placed in the external electrode body and the internal electrode was then reinserted. The slopes obtained for the range of 10-5-10-2 M ammonium were sub-Nernstian (50 mV/decade). This suggests that the microenvironment present a t the sensing tip alters the response characteristics of the polymer membrane. Response times of the electrodes to reach steady-state potentials were a function of the pressure exerted on the gas membrane by the internal plastic probe and on the concentration of ammonia being detected. Increasing the pressure of the internal electrode on the gas membrane made the buffer film thinner, resulting in faster electrode response. Typical response times observed ranged from 10 min a t ammonia levels lo4 M. This suggests that the preconcentration process is quite rapid and response times are dependent on the rate of diffusion of ammonia through the gas permeable membrane. In addition, reversal of this process and complete reestablishment of the starting electrode potential was accomplished in 1G15 min by placing the electrode in a well-stirred ammonia-free 0.005 M NaOH solution. Steady state potentials observed at given ammonia levels remain constant for approximately 10 min before the loss of ammonia from the sample causes a negative drift in the measured potential.
The electrode was also checked for ammonia response using an external sample pH of 8.0. As expected the improved detection limits over the commercial sensor were similar to that found in 0.005 M NaOH. This suggests that this new ammonia probe may provide a simple and inexpensive means of preparing bioselective sensors using ammonia-producing biocatalysts (Le., enzyme and bacterial electrodes), where operation at close to physiological pH conditions is required. A surprising aspect of this work was the low levels of nonactin necessary to prepare the ammonium polymer membrane electrode. Earlier work with nonactin (5,6) suggested that a very high concentration of nonactin was necessary to make functioning electrodes. In this work, increasing nonactin levels up to 25% by weight in the PVC membrane did not alter the response or resistance levels (2 x lo8 a)of the electrode. At concentrations above 3 % , the membranes cast were no longer clear, indicating that the nonactin has only minimal solubility in the plasticizer-PVC matrix. Consequently, increasing nonactin levels does not improve ion transfer onto the polymer membrane. This fact allows one to prepare the internal ammonium probe using only a few pennies worth of the costly antibiotic, and, therefore, the resulting gas sensors are virtually disposable units. The ammonia sensors prepared thus far are operational for at least 1 week without any significant change in their potentiometric response characteristics. After this time period, evaporation of the internal buffer solution tends to change the absolute potentials observed and fresh buffer is required to regain the original potential values. More careful design of the electrode assembly should eliminate this problem and make these sensors useful for much longer time periods. This report has demonstrated the concept that polymer membranes can be used as internal elements in the preparation of simplified gas electrodes. Studies are currently under way to fully examine and further optimize the response characteristics of this sensor. In view of the significant advantages offered, this new ammonia sensor should provide an attractive alternative to current ammonia detection systems.
LITERATURE CITED (1) "Analytical Methods Guide"; Orion Reseach Inc., Cambridge, Mass., 1978. (2) Hansen, E. H.;Larsen. N. R . Anal. Chim. Acta 1975, 78, 459-462. (3) Montalvo, J. G. Anal. Chim. Acta 1973, 65, 189-197. (4) Anfatt, T.; Graneli, A.; Jagner, D. Anal. Chim. Acta 1975, 76, 253-259. (5) Simon, W. Pure Appl. Chem. 1971, 25, 811-823. ( 6 ) Guilbault. G. G.; Nagy, Geza Anal. Chem. 1973, 4 5 , 417-419. (7) Morf. W. E.; Simon, W. I n "Ion-Selective Electrodes in Analytical Chemistry", Volume I, Freiser. H., Ed.; Plenum Press: New York, 1978; p 211.
Mark E. Meyerhoff Department of Chemistry The University of Michigan Ann Arbor, Michigan 48109
RECEIVED for review March 27,1980. Accepted May 19,1980. This paper was presented a t the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 11, 1980, Atlantic City, N.J.
Hydrogen Sulfide Removal with Boric Acid Sir: In the determination of thiosulfate- and tetrathionate-sulfur by cyanolysis, hydrogen sulfide can interfer following its oxidation to the elemental state (Equations 1 and 2). Elemental sulfur reacts with cyanide (Equation 3). In the
determination of thiosulfate and tetrathionate by cyanolysis (Equations 4 and 5), ferric iron is added to form a ferric thiocyanate complex (Equation 6) which is measured spectrophotometrically.
This article not subject to U.S. Copyright. Published 1980 by the American Chemical Society
