1394 (10) (11) (12) (13)
(14) (15) (16)
(17) (18) (19) (20) (21) (22) (23) (24) (25) (26) ~I
(27) (28)
Anal. Chem. 1982, 54, 1394-1399 Smyth, M. R.; Osteryoung, Janet Anal. Chem. 1977, 4 9 , 2310-2314. Florence, T. M. J. Electroanal. Chem. 1979, 9 7 , 219-236. Vaneesorn, Y.; Smyth, W. F. Anal. Chlm. Acta 1980, 117, 183-191. Osteryoung, Janet; Whittaker, J. W.; Smyth, M. R. "proceedings of the Conference on Electroanalysis in Hygiene, Environmental, Clinical, and Pharmaceutical Chemistry"; Smyth, W. F., Ed.; Elsevier: London, 1980: no 413-422. . __ S&th:W. F.; Davison, I . E. "Proceedings of the Conference on Eiectroanaivsis in Hvaiene. Environmental. Clinical. and Pharmaceutical Chemihy"; S m $ t , W. F., Ed.; Eisevier: London, 1980; pp 271-286. Paiecek, E. Anal. Left. 1980, 13, 331-343. Brainina, Kh. 2 . "Striming . . - Voitammetrv In Chemical Analysis": Wiiev: New York, 1974. Vydra, F.; Stutk, K.; JuiBkovL, E. "Electrochemical Stripping Analysis"; Wiley: New York, 1976. Shimizu, K.; Osteryoung, R. A. Anal. Chem. 1981, 53, 584-588. Paiecek, E.; Jeien, F.; Manousek, 0. Collect. Czech. Chem. Commun. 1980, 45, 3460-3471. Paiecek, E.; Jeien, F. Collect. Czech. Chem. Commun. 1980, 45, 3472-348 1. Paiecek, E. Anal. Biochem. 1980, 108, 189. Paiecek, E.; Jelen, F.: Hung, MacAnh; Lasovskk J. Bioelectrochem. Bloenerg., in press. Webb, J. W.; Janik, 6.; Eiving, P. J. J. A m . Chem. SOC. 1973, 9 5 , 8495-8505 . .. ..... Vetteri, V. Collect. Czech. Chem. Commun. 1977, 31, 2105-2126. Vetteri, V. Abh. Dtsch. Akad. Wlss. Berlin, K1. Med. 1966, 493. Eivina. P. J. I n "Tonics in Bioeiectrochemistrv and Bioeneraetics": . Miiazzc G., Ed.; Wiiey: London, 1976; Vol. l;pp 179-286. Dryhurst, G. "Electrochemistry of Biological Molecules"; Academic Press: New York, 1977; ChaDters 3-5, DD 71-319. Paiecek, E. I n "Proceedings of the Conference on Electroanalysis in
(29) (30) (31) (32) (33) (34) (35) (36)
(37) (38) (39) (40) (41) (42)
Hygiene, Environmental, Clinical, and Pharmaceutical Chemistry"; Smyth, W. F., Ed.; Eisevier: London, 1980; pp 79-98. OSteryOUng, Janet; Kirowa-Eisner, E. Anal. Chem., 1980, 52, 62-66. Flanagan, J. 8.; Takahashl, K.; Anson, F. C. J. Electroanal. Chem. 1977,-81, 261-273. Vetteri, V. J. Electroanal. Chem. 1968, 19, 169-173. Retter, U.; Vetterl, V.; Jehring, H. J. Electroanal. Chem. 1973, 5 7 , 391-397. Vetterii V. Bloelectrochem. Bloenerg 1978, 3 , 338-345. Kinoshita, H.; Christian, S. D.; Dryhurst, G. J. Electroanal. Chem. 1977, 83, 151-166. Retter, U. J. Electroanal. Chem. 1978, 8 7 , 181-188. Kinoshita, H.; Christian, S. D.; Kim, M. H.; Barker, J. G.; Dryhurst, G. I n "Electrochemical Studies of Biological Systems"; Sawyer, D. T., Ed.; American Chemical Society: Washington, DC, 1977; ACS Symp. Ser. 38, pp 113-142. Brabec, V.; Christian, S. D.; Dryhurst, G. J. Electrochem. SOC.1978, 125,1236-1244. Fianagan, J. 6.; Takahashi, K.; Anson, F. C. J. Nectroanal. Chem. 1977, 85,257-266. Schleich, T.; Biackburn, 8. J.; Lapper, R. D.; Smith, I.C. P. Blochemistry 1872, 11, 137. Paiecek, E.; Frary, 8. D. Arch. Blochem. Siophys. 1966, 115, 431-436. Jacobsen, E.; Lindseth, H. Anal. Chlm. Acta 1976, 86, 123-127. de Vrles, W. T.; Van Daien, E. J. Nectroanal. Chem. 1967, 14, 315.
.
RECEIVED for review November 24, 1981. Accepted April 1, 1982. This work was supported by the Office of Naval Research.
Galactose Oxidase Enzyme Electrode with Internal Solution Potential Control Jay M. Johnson,"
H. Brian Halsall, and William R. Heineman
Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 4552 1
The analytical appllcations of a novel enzyme electrode based on galactose oxidase which incorporates solution potential control In the enzyme thin layer are described. Under diffusion llmltlng condltlons the relative sensitivlty to certain substrates can depend qulte dlfferently upon solution potential. This allows the measurement of certain pairs of substrates in the same solution by maklng measurements at two preselected control potentlals. Two-substrate measurements on galactose-glycerln, galactose-lactose, and galactose-stachyose are described as well as the dependence of measurement errors on various parameters. The lncorporatlon of solution potenilal control also Improves the sensltlvity and the dynamlc range of the galactose oxidase electrode. The lower detection llmlts for galactose and glycerin are 0.02 and 0.04 mM, respectlvely. The upper limlis of the llnear range are about 70 and 400 mM, respectively.
Immobilized enzyme electrodes, both potentiometric and amperometric, are becoming increasingly prevalent as analytical tools. A number of reviews describing the properties of these devices are available (1-4). Coupling the selectivity of enzyme reactions through an electroactive product or reactant to an electrochemical device has the advantages of speed, sensitivity, and economy. Clark and Lyons are usually credited with describing the first enzyme electrode (5),and Present address: Yellow Springs I n s t r u m e n t Springs, OH 45387.
Co., Inc., Yellow
since then, electrodes for the measurement of amino acids (6), ethanol (7), glucose (8),cholesterol (9),phosphate (IO),uric acid (11),galactose (12),xanthine (13),and many others have been reported. In the case of amperometric enzyme electrodes, an oxidase is often involved since either O2consumption or H 2 0 2production can be easily measured electrochemically. However, the measurement of H202production allows a more favorable signal-to-noise ratio than the measurement of O2 consumption. Many oxidases including xanthine oxidase (14), alcohol oxidase ( 1 5 ) , ascorbate oxidase (16), and galactose oxidase (17) are not all specific and will accept a variety of substrates, and of course in the past an enzyme electrode could be no more specific than the enzyme used in its construction. Galactose oxidase (E.C. 1.1.3.9) is extremely interesting because many of its substrates are of analytical import. These include galactose, lactose, glycerin, dihydroxyacetone, and glyceraldehyde (17, 18). We have previously demonstrated how galactose oxidase can be reversibly turned on and off by oxidizing and reducing it, respectively, using a thin-layer electrochemical cell and a mediator titrant (19). Also, data were presented which indicate that the solution potential dependence of activity (half-wave potential) for galactose oxidase is relatively independent of the substrate with which the determination is made. However, at very high enzyme concentrations the apparent half-wave potentials for the various substrates can be distorted from the true value by an extent dependent upon the degree to which the diffusion of substrate into the enzyme thin-layer is rate limiting. The degree of distortion from the true value is primarily dependent upon how rapidly the substrate is turned over by the enzyme and in the case of galactose oxidase there is such diversity in
0 1982 American Chemical Society 0003-2700/62/0354-1394$01.25/0
ANALYTICAL CHEMISTRY, VOL. 54, NO. 8, JULY 1982
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the rates of turnover for the various substrates that the apparent half-wave potentials can be extremely different for the various substrates (19). The purpose of this report is to demonstrate how solution potential control in conljunction with this phenomenon can be used to improve the relative specificity of an amperometric galactose oxidase electrode. The enzyme electrode described is very similar to one reported earlier except that the present device is much more useful due to the ability to control relative specificity and also sensitivity (12). Solution potential control is accomplished through the use of a gold membrane electrode which is located in the enzyme thin layer of the membrane covering the H202sensitive amperometric electrode. EXPERIMENTAL SECTION The enzyme electrode used in this study was exactly the same as described previously (configuration11)and was also constructed in exactly the same manner (19). Analytical Procedure. One-Substrate Measurements. Measurements performed on aqueous solutions containing various concentrations of a single substrate were made in the following manner: (A) the appropri,atepotential was set on the gold control electrode, (B) 25 pL of the substrate solution to be measured was injected with a YSI Syringepet into the sample chamber, (C) after a steady-state H202current was achieved (0.5-1.0min depending on the substrate), the H202current was recorded, and (D) finally, the sample chamber was cleared and the background current allowed to reach n constaint level before this procedure was repeated on the next solution. In all cases, a solution which was near the middle of the range in concentration was used as a standard and the other solutions were treated as “unknowns”. The solution used as LL standxd was injected before and after the solutions treated as “unknowns”. The currents obtained for the standard before and after injection of a given “unknownn were averaged, and this average value was used to calculate the concentration of the particular substrate in the “unknown”. Finally, the values calculated for a given series of ”unknowns”were then plotted against the concentration to which tlhe “unknowns”were made up, thereby obtaining a slope and an intercept (y = concentration to which the solution was made up; x = concentration measured). All measurements were made at 25 “C. Two-Substrate Measurements. Measurements performed on aqueous solutions containing various known concentrationsof two different substrates were made essentially as described above at each of two different appropriate control potentials. The major difference was the necessity of using two standards for each measurement at each pobential, each containing a different one of the two substrates being measured. Two equations in two unknowns were then used to solve via Cramers rule for the concentrations of each of the substrates in the “unknown”. These equations were of the general form ax(a:l + aYb)= R,, b,O:) + b,(y) = Rbl where a, and ay are the sensitivities to substrates x and y, respectively, obtained at potential a and b, and by are sensitivities to substrates x and y obt,ained at control potential b. x and y are the “unknown” concentrations of substrates x and y, and R , and Rb are the responses obtained for unknown 1 at potentials a and b, respectively. All measurements were made at 25 “C. Apparatus. The apparatus used in this study was exactly the same as that described previously (19). Reagents. All reagents were analytical grade unless specified otherwise. Lactose monohydrate was obtained from J. T. Baker Chemical Co. Potassium EDTA was obtained from Eastman. Glyceraldehyde was purchased from Aldrich Chemical Co, Raffinose pentahydrate W i l l purchased from Pfanstiehl Laboratories. Other reagents used were exactly the same as previously described (19). Standards. All standards were prepared in deionized water containing 2 X loA3M K2 EDTA. Standards made in this way were stable for several months stored at room temperature. Mediator Titrant. In all cases potassium ferricyanide (E”’ r +0.215 V VS. SCE) was used as a mediator titrant when solution
1395
C-GALACTOSE
VOLTS
VS
SCE
Potentlostatic activity voltammograms for glycerin and galactose determined on a typical 570 IU/kL galactose oxidase membrane. The net steady state (S.S.) H,Op current was determined by using Injections of either 10 mM glycerin or 3 mM galactose. The mediator titrant was 1 mM ferricyanide plus 1 mM cobalt terpyridine. Figure 1.
potential was controlled at or above +0.090 V vs. SCE. At solution potentials below +0.090 V vs. SCE, bis(terpyridine)cobalt(lI) chloride ( E O ’ = -0.010 V vs. SCE) was used. Bulk Solution. The bulk solution in which measurements were done and in which membranes were stored between uses was a pH 7.3,0.07 M phosphate buffer, which was 2 mM in mediator titrant, 0.05 M in sodium chloride, 3 mM in sodium bromate, 6 mM in sodium benzoate, and 0.01 mM in cupric chloride dihyrate (12). RESULTS AND DISCUSSION Figure 1 illustrates how the solution potential dependence of galactose oxidase activity is apparently different for different substrates at an enzyme concentration of about 570 IU/pL in the membrane. The points above +0.1 V were determined by a potentiostatic technique as described previously (19). The points below +0.10 V were corrected for changes in background response by using discrete determinations of net HzOzcurrent a t each applied potential (one injection of substrate per determination). The least-squares fit of these data to the Nernst equation for glycerin is Eapp = +0.174 +0.02466 In ([O]/[R]); and for galactose, Ea,,, = +0.070 + 0.02905 In ([O]/[R]). The correlation coefficients were 0.994 and 0.992 for galactose and glycerin, respectively. Glycerin is such a poor substrate that the response of the H20z electrodes to glycerin is not diffusion limited even at this relatively high enzyme concentration and thus the apparent half-wave potential for glycerin is not significantly different from the true half-wave potential measured at much lower enzyme concentrations (19). Galactose, on the other hand, is a much better substrate and therefore the electrode response to galadose is diffusion limited at high control potentials. This effect distorts the activity dependence upon solution potential and thus the apparent half-wave potential (+0.070 V vs. SCE) for galactose is much lower than the true half-wave potential measured a t low enzyme concentrations (19). Response Characteristicsof the Enzyme Electrode. As can be seen in Table I, the use of internal solution potential control via the control electrode allows the sensitivity to any given substrate to be controlled and maintained reversibly over as much as a factor of 10 depending upon how close the set potential on the control electrode is to the apparent half-wave potential for that substrate. Also, as would be expected, in general the errors (accuracy and precision) and the detection limits increase in inverse proportion to the sensitivity. Thus, using this technique the working range of the enzyme electrode can be rapidly increased by as much as a factor of 10 and
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possibly further simply by changing the control electrode potential. This effectively results in a 10-fold increase in the dynamic range of the enzyme electrode. In this system, the potential on the gold control electrode must be such that the system is sufficiently sensitive toward whichever substrate is introduced. Also, the linearity of the response to any given substrate is dependent on several factors, the most important of which are the Michaelis-Menten constant for the particular enzyme substrate reaction, the degree to which peroxide production by the enzyme is limited by diffusion of substrate through the outside membrane to the enzyme layer (12),and the sensitivity of the electrode to the substrate. The latter parameter is important since H 2 0 2 measurement is done by using two electrodes and this can result in polarization a t approximately 150-200 nA. In most cases this apparently limited the linear range for the measurement of any given substrate. Furthermore, the enzyme kinetics and the relative importance of diffusion and the rate of the enzyme reaction are similarly important in determining the sensitivity and the speed to steady state of the system for any given substrate (20). A few trends which illustrate these points are apparent in the data of Table I. For example, stachyose, raffinose, and galactose are all known to be very good substrates (17) and this is reflected in the similar and high average relative sensitivities shown in Table I. Also, the approach to steady state for these substrates is quite rapid. Lactose, on the other hand, is a very poor substrate and this results in a lower relative sensitivity and a slower approach to steady state (21). The same general trends can be observed in the data taken on the three-carbon substrates. Thus the better the substrate the more important is diffusion in determining the response characteristics to any given substrate assuming a constant enzyme concentration. Two-Substrate Measurements. Table I1 shows the results obtained when various pairs of substrates were determined by making measurments a t each of two solution potentials. In each case the first set potential at which data were taken was +0.290 V vs. SCE. The second set potential at which data were taken is given in Table I11 for any given pair. Two general trends in these data can be observed. First, the relative error increases with increasing concentrations of the second substrate listed in the column headed "in presence of". This is true for both accuracy and precision. Second, the relative errors in the determination of a pair of substrates by this technique tend to increase as the separation in the apparent half-wave potentials of the members of the pair decreases. In the case of the determination of galactose in the presence of stachyose (or vice versa) the separation of the apparent half-wave potentials is only 0.020 V and the relative errors are approaching five to eight times the errors involved in the determination of galactose or stachyose alone (Table I). Furthermore, i t is also generally true that the member of any given pair of substrates which has the higher apparent half-wave potential has larger relative errors associated with its determination than the substrate with the lower apparent half-wave potentid. For example, the data show that galactose can be recovered more accurately and precisely in the presence of glycerin than glycerin can be recovered in the presence of galactose. There are two reasons for this. First, the absolute sensitivity to galactose is greater a t both potentials where measurements are made and, secondly, the relative decrease in sensitivity when going from the higher potential to the lower potential is much greater for glycerin than galactose. Table I11 illustrates the sensitivities and predicted errors involved in the two substrate measurements of Table 11. It is apparent from these data that the overall relative error in t h e determination of a pair of substrates increases as the
separation in half-wave potential decreases. Of course, the way in which the errors are proportioned between two members of a pair is dependent only upon the respective relative sensitivities at each of the two set potentials used and these become more similar the closer the apparent half-wave potentials of the two substrates are. For example, both the actual errors and the predicted errors in the determination of galactose in the presence of glycerin are very small compared to the actual and predicted errors in the determination of glycerin in the presence of galactose. The errors in the determination of galactose in the presence of lactose, however, are greater (relative to the error in the determination of lactose in the presence of galactose) than in the case of galactose in the presence of glycerin. This trend is even more apparent in the determination of galactose in the presence of stachyose. In general, then, the predicted relative errors agree a t least qualitatively with the actual errors observed. As mentioned above there should be some dependence of the errors involved in these two-substrate measurements on the second set potential. This dependence, however, will depend upon the specific substrates involved and how close their apparent half-wave potentials are to each other. In the case of glycerin and galactose Table IV illustrates this dependence. The predicted relative error for the glycerin determination appears to approach a minimum at about +0.140 V vs. SCE. The predicted relative error for galactose, on the other hand, decreases continuously from a second set potential of +0.200 V to +0.090 V vs. SCE. The trend in the precision calculated for these actual data as a function of the second set potential agrees closely with the trend in predicted error. However, the average recovery of glycerin decreases as the second set potential is lowered while the average recovery of galactose improves. A second set potential of +0.140 V vs. SCE appears to be a good compromise between these various effects. The optimization of this technique for the measurement of real samples would to some degree depend upon the specific pair of substrates of interest and their expected concentration ranges. For example, if the expected concentration ranges were such that the measurement would be limited by sensitivity, then the concentration of enzyme in the enzyme layer could be increased to increase sensitivity. Of course, there is a limit to the enzyme concentration attainable in the enzyme layer and, furthermore, the increase, if any, in sensitivity observed for any given substrate for a particular increase in enzyme will depend upon to what extent the substrate is diffusion limited. The question of interferences involved in the measurement of real samples with this electrode, without internal solution potential control, has been addressed to some degree since the accurate measurement of galactose in plasma and whole blood has been described (12). Also, the interference of selected reducing agents was determined to be minimal in that study. It is apparent that the enzyme electrode described here is even more immune to interference caused by some reducing substances since the control electrode can oxidize these directly or indirectly through the mediator titrant before they can reduce the enzyme and thereby change sensitivity. However, the possible interference of some component of a real sample in the maintenance of solution potential control as described here will need to be addressed when measurements on real samples are attempted. This will be the subject of a future communication. The basic design of the system described excluding the solution potential control ability is essentially the same that incorporated in the Yellow Springs Instrument Co. Models 23A and 27 analyzers. These instruments are commercially available for the measurement of glucose, sucrose, lactose, galactose, and ethanol. An interesting observation with regard
Anal. Chem. 1982, 5 4 , 1399-1402
to the the use of these enzyme electrodes is that even though there are more sensitive, precise, and accurate methods available for the measurement of these compounds, these devices provide a very rapid and simple measurement procedure involving minimal sample preparation without greatly compromising the quaility of the results (22). In conclusion we have described a technique which can be used to improve both the sensitivity and specificity of an enzyme electrode. We have demonstrated how various pairs of substrates can be determined by using this technique and that relative errors increase as the separation in apparent half-wave potentials decreases until at a1 separation of about +0.020 V the errors approach five to eight times what they are for the determination of a single substrate. Furthermore, the response of this electrode is so rapid that correlations can be made concerning the kinetics of the enzyme reaction and the response characteristics of the electrode for any given substrate. Finally, we sugpect that other enzymes may behave similarly and that solution potential control may ultimately be used to control the specificity of enzyme electrodes based on these enzymes under the appropriate rate limited conditions.
Guilbault, G. G.; Lubrano Anal. Chim. Acta 1974, 6 9 , 183. Clark, L. C., Jr. I n "Biotechnology and Bioengineering Symposium Series No. 3, Enzyme Engineering"; Wingard, L. B., Jr., Ed.; Wiiey: New York, 1972; pp 377-394. "Instructlon Manual, YSI Model 23A Glucose Analyzer"; Yellow Springs Instrument Co., Inc.: Yellow Springs, OH, 1976. Clark, L. C., Jr.; Emory, C. R. I n "Ion and Enzyme Electrodes in Bioiogy and Medicine"; Kessier, M., Clark, L. C., Jr., Lubbers, D. W., Silver, I . A., Simon, W., Eds.; University Park Press: Baltimore, MD, 1976; pp 161-172. Guilbault, G. G.; Nagy, G. Anal. Chim. Acta 1975, 78, 69-80. Guilbault, G. G.; Nanjo, M. Anal. Chem. 1974, 4 6 , 1769. Taylor, P. J.; Kmetec, E.; Johnson, J. M. Anal. Chem. 1977, 49, 789-794. Sittampalam, G. Masters Dissertation, Bowling Green State University, Bowling Green, OH, 1977. Bray, R. C. I n "The Enzymes"; Boyer, P. D., Ed.; Academic Press: New York, 1952; Vol. XII, p 344. Van Dijken, J. P. Ph. D. Dlssertation, University of Groningen, Groningen, The Netherlands, 1976. Malmstrom, B. G.; AndrQasson, L. E.; Reinhammer, B. I n "The Enzymes"; Boyer, P. D., Ed.; Academic Press: New York, 1952; Vol. X I I , p 574. Kosman, D. J.; Ettinger, M. J. I n "Metal Ions in Biology"; Spiro, T., Ed.; Wiley: New York, in press. Hamilton, G. A.; Adoif, P. K.; DeJersey, J.; duBois, G. C.; Dyrkacz, G. R.; Libby, R. D. J . Am. Ch8m. SOC. 1978, 100, 1699-1912. Johnson, J. M.; Halsall, H. B.; Heineman, W. R. Anal. Ch8m. 1982, 5 4 , 1377-1383. Mell, L. D.; Maloy, J. T. Anal. Ch8m. 1975, 4 7 , 299-307. Avlgad, G.; Amaral, D.; Asenslo, C.; Horecker, B. L. J . Biol. Chem. 1962, 237, 2736-2743. Huntington, J. L. Food Product Development 1978, 12, 78-79. (22) Cochran, W. G.; Snedecor, G. W. "Statistical Methods"; Iowa State (23) University Press: Ames, IA, 1980; p 91.
LITIERATURE CITED (1) Bowers, L. D.; Carr, F'. W. Anal. Ch8m. 1978, 48, 544-558. (2) Weetal, H. H.; Hersh, L. S. Anal. Chem. 19174, 46, 602-615. (3) Bowers, L. D.; Carr, IP. W. "Immobilized Enzymes in Analytical and Clinical Chemistrv": Elvina. P. J., Winefordner, J. D.. Eds.: Wlley: New York, 1980. (4) Rechnltz, G. A. Ch8m. Eng. News. 1975, Jan 27, 29-35. (5) Clark, L. C., Jr.; Lyons, C. Ann. N . Y . Acad. Sci. 1982, 102, 29-45.
1399
RECEIVED for review September 24, 1981. Resubmitted and accepted March 25, 1982.
Potentiometric Estimation of Molybdate Ion with a Solid Membrane Electrode W. U. Mallk,' S. K. Zirivastava," and Amla Bansal Department of Chemistry, University of Roorkee, Roorkee -247672,
A new solid membrane! electrode has been reported for the measurement of molybdate ions In the concentration range to 5 X M. The electrode works at pH 6 or higher. The response time Is less than a minute and it remains stable for more than 15 min. Except for some monovalent ions, the electrode shows selectvlty for molybdate ions over other anIons. Efforts have also been made to utilize It as an Indicator electrode in titrations iniwolving molybdate ions (Moot-) with titanium( I V ) chlorlde and cerlum( I V ) ammonium nltrate.
The development of a fluoride solid-state lanthanum fluoride electrode stimulated a widespread interest in the preparation of better and better electrochemical sensors. Apart from investigations reported on the electrodes for cation activity measurements, halide (1-3), sulfide ( 4 , 5 ) ,phosphate (6, 7), and other anion (8-10) selective electrodes have also been a subject of interest in recent years. In continuation to our work on some cation selective solid membrane electrodes (11-13), a zirconium molybdate membrane electrode showing selectivity to rnolybdake ions has been prepared and its Present address: ViceXhancellor, Kashmir University, Srinagar
(J & K).
India
electrochemical performance is described in this paper. EXPERIMENTAL SECTION All reagents used were of A.R. grade. Zirconium molybdate gel was prepared by slowly adding zirconium oxychloride (0.2 M) to sodium molybdate (0.2 M) in equal amounts. The compound so obtained was left to age for about 20 h, filtered, and washed thoroughly to remove adsorbed ions. It was then dried at 40 "C in an air oven. Preparation of Membranes. Homogeneous membranes of zirconium molybdate could not be prepared. Heterogeneous membranes obtained by embedding polystyrene in zirconium molybdate gel were found to be quite satisfactory. The following method was employed to prepare the membrane. Polystyrene granules were heated in a glass tube in sulfuric acid bath at 200 "C. The molten mass was allowed to cool down to room temperature and the polystyrene rod, so obtained, was ground to fine particles first and then in a mortar and pestle to give a 50 mesh sieve product. Membranes were obtained by mixing polystyrene and inorganic gel in the ratio 15:85, by weight, and heating the homogeneous mass in a die kept in a metallurgical specimen mount press, at 110 "C under a pressure of 6500-7000 psi. The optimum quantity of polystyrene added for preparing membrane was determined by trial and error. Membranes prepared in this way were quite stable and did not show any dispersion in salt solutions. Membranes were converted to the desired ionic form by immersing in a solution of electrolyte (0.01 M) for 2-3 days, the
0003-2700/82/0354-1399$01.25/00 1982 American Chemical Society
