Anal. Chern. 1982, 5 4 , 27 R-44 R (53D) Farnia, G.; Roffia, S. ,J. Electroanal. Chem. 1981, 122, 347-352. (54D) Moorhead, E. D.; Stephens, M. M.; Bhat, G. A. Anal. Left. 1980, 13, 167-179. (55D) Jaeger, C. D.; Bard, A. J. J. Am. Chem. SOC. 1980, 102, 5435-5442. (56D) Malpas, R. E.; Itaya, K ; Bard, A. J. J. Am. Chem. SOC. 1981, 103, 1622-1 627. (57D) Nlgrey, P. J.; MacInnes, D., Jr.; Nairns, D. P.; MacDlarmld, A. G.; Heeger, A. J. J. Nectrodwm. SOC.1981, 128, 1651-1654. AMPEROMETRIC DETECTION OF ENZYMATIC REACTIONS
(IE) Ianlello, R. M.; Yacynyi:h, A. M. Anal. Chem. 1981, 53, 2090-2095. (2E) Kamln, R. A.; Wilson, a. S. Anal. Chem. 1980, 5 2 , 1198-1205, (3E) Bourdlllon. C.; Bourgeoi!r, J. P.; Thomas, D. J. Am. Chem. SOC.1980, 102, 4231-4235. (4E) Coulet, P. R.; Sternberg, I?.; ThBvenot, D. Biochlm. Blophys . Acta 1980, 612, 317-327. (5E) Pfeiffer, D.; Scheller, F ; Janchen, M.; Berterman, K.; Weise, H. Anal. Lett. 1980, 13, 1179-1200. (6E) Bertrand, C.; Coulet, P. R.; Gautheron, D. C. Anal. Chim. Acta 1981, 126, 23-34. (7E) Kulys, J. J. Anal. Left. 1981, 14, 377-397. (8E) Blaedel, W. J.; Wang, J. Anal. Chem. 1980, 5 2 , 1426-1429. (9E) Lobel, E.; Rishpon, J. Anal. Chem. 1981, 5 3 , 51-53. (10E) Blaedel, W. J.; Engstrom, R. C. Anal. Chem. 1980, 52, 1691-1697. (11E) Durliat, H.; Comtat, M. Anal. Chem. 1980, 5 2 , 2109-2112. (12E) Kulys, J. J.; Svlrmlclk,as, G. J. S . Anal. Chim. Acta 1980, 117, 115-1 20. (13E) Kulys, J. J.; Samallus, A. S.;Svirrnickas, G. J. S. F€BS Lett. 1980, 114, 7-10. (14E) Schubert, F.; Kirsten, 12.; Scheller, F.; Mohr, P. Anal. Lett. 1980, 13, 1167-1 178. SPECTROELECTROCHEMISTRY
(IF) Hubbard, A. T. Acc. Chem. Res. 1980, 13, 177-184. (2F) Mlller, C. W.; Karwelk, D. H.; Kuwana, T. Anal. Chem. 1981, 5 3 , 23 19-2323. (3F) Fujlshima. A,; Masuda, H.; Honda, K.; Bard, A. J. Anal. Chem. 1980, 52. 882-885. --- - - (4F) Davidson, T.; Pons, 6.S.;Bewick, A.; Schmidt, P. P. J. flecfroanal. Chem. 1981, 125, 237-241. (5F) Beden, 6.; Lamy, C.; Beiwick, A,; Kunlrnatsu, K. J. Nectroanal. Chem. 1981, 121 I 343-347. (6F) Pemberton, J. E.; Buck:, R. P. J . Phys. Chem. 1981, 8 5 . 246-262. (7F) Malpas, R. E.; Bard, A. J. Anal. Chem. 1980, 52, 109-1 12. (8F) Fuiishlma, A., Maeda, Y.: Honda, K.; Brilmyer, G. H.; Bard, A. J. J. Nectrochem. SOC. 1980, 127, 840-846.
--.
(9F) Baumgartner, C. E.; Marks, 0. T.; Aikens, D. A,; Richtol, H. H. Anal. Chem. 1980, 5 2 , 267-270. (IOF) Prulksma, R.; McCreery. R. L. Anal. Chem. 1981, 53, 202-206. (11F) Skully, J. P.; McCreery, R. L. Anal. Chem. 1980, 5 2 , 1885-1889. (12F) Pruiksrna, R. J. Nectroanal. Chem. 1980, 114, 147-152. (13F) Robinson, R. S.;McCreery, R. L. Anal. Chem. 1981, 5 3 , 997-1001. (14F) Stankovich, M. Anal. Biochem. 1980, 109, 295-308. (15F) Bralter-Toth, A.; Dryhurst, 0. J. Nectroanal. Chem. 1981, 722, 205-2 13. (]6F) Bancroft, E. E.; Bllount, H. N.; Hawkrldge, F. M. Biochem. Biophys. Res. Commun. 1981, 101, 1331. (17F) Crawley, C. D.; Hawkrldge, F. M. Biochem. Biophys. Res. Commun. 1981, 99,516-522. (18F) Cotton, T. M.; Schultz, S. G.; van Duyne, R. P. J. Am. Chem. SOC. 1980, 702, 7960-7962. (19F) Yates, D. A.; Szentlrmay, R.; Kuwana, T. Anal. Biochem. 1980, 270-280. (20F) Denis, M.; Neau, E.; Blein, J. P. Bioelectrochem. Bioenerg. 1980, 7 , 757-773. (21F) Powell, L. A.; Wightman, R. M. J. Electroanal. Chem. 1980, 106, 377-390. (22F) Kadish, K. M.; Shlue, L. R.; Rhodes, R. K.; Bottomley, L. A. Inorg. Chem. 1981, 2 0 , 1274-1277. (23F) Bewlck, A.; Mellor, J. M.; Pons, 8. S . Nectrochlm. Acta 1980, 2 5 , 931-941. (24F) Sorlle, M.; Smith, 0. P.; Norvell, V. E.; Mamantov, G.; Klatt, L. N. J. Electrochem. SOC. 1981, 128, 333-338. MISCELLANY (10) McIntire, G.L.;Blount, H. N.; Stronks, H. J.; Shetty, R V.; Janzen, E. G. J . Phys. Chem. 1980, 8 4 , 916-921. (20) Martigny, P.; Mabon. G.; Simonet, J.; Mousset. G. J Nectroanal. Chem. 1981, 121, 349-354. (3G) Gagne, R. R.; Koval, C. A.; Llsensky, G. C. Inorg. Chem. 1980, 19, 2654-2855. (4G) Aoki, K.; Osteryoung, J. J . Electroanal. Chern. 1981, 125, 315-320. (5G) Kakihana, M.; Ikeuchl, H.; Sato, G. P.; Tokuda, K. J. Electroanal. Chem. 1980, 108, 381-383. (6G) Rubinson, K. A. Anal. Chem. 1981, 53, 932-934. (7G) Bellamy, A. J. Anal. Chem. 1980, 5 2 , 607-608. (8G) Ikeuchl, H.; Shiwa, Y.; Tsujimoto, H.; Kakihana, M.; Takekawa, S.; Sato, 0. P. J. Electroanal. Chem. 1980, 1 1 1 , 287-294. (9G) Kisele, H. Anal. Chem. 1981, 5 3 , 1952-1954. (1OG) Britz. D. Anal. Chem. 1980, 5 2 , 1166-1167. (110) Petrii, 0. A.; Khomchenko, I. G. J. Electroanal. Chem. 1980, 106, 277-286. (12G) Homolka, D., et ai Anal. Chem. 1980, 5 2 , 1606-1610. (13G) Cheng, H. Y.; Whlte. W.; Adams. R. N. Anal Chem. 1980, 5 2 , 2445-2448.
Ion-Selective Electrodes Mark E. Meyerhoff" and Yvonne M. Fratlcelll Department of Chemistry, The University of Michigan, Ann Arbor, Michigan 48109
The field of ion-selectiveelectrodes (ISEs) continues to grow at an ever increasing rate. This review covers ISE literature published between the f d of 1979 and the fall of 1981. Some overlap may exist with the 1980 biennial ISE review for articles which appeared late in the fall of 1979. Our survey is by no means an all-inclusive coverage of the literature, and ISE specialists are urged noit to take this review as the final word on what has or has not, been done over the past 2 years. Indeed, our intent here is to present the more significant developments in the field encompassing new electrode systems, techni ues, theory, and novel applications. We have primarily limitea our coverage to journals written in English although in some cases foreign journals are cited when such articles appear to offer significantly new information. A similar treatment is given to patents. The format used here is, for the most part, a continuation of that used in previous ISE biennial reviews. Highlights in the field are emphasized in the text portion while less exciting yet still interesting developments, etc. are incorporated into appropriate tables. References utilized here were obtained from four main sources: a manual search of major research journals, a manual search 0003-2700/82/0354-27R$06.00/0
of Analytical Abstracts, a computer search of Chemical Ab-
stracts, and reprints and/or publication lists provided by ISE researchers throughout the world.
BOOKS, SYMPOSIA, JOURNALS, AND REVIEWS Several comprehensive ISE monographs have been published over the past 2 years. Volume I of "Ion-Selective Electrode Methodologies", edited by Covington (I&), offers information on ISE types (A. K. Covington), instrumentation (P. R. Burton), measurement techniques (R. J. Simpson), standards for ISE work (A. K. Covington), glass membrane electrodes (A. K. Covington), liquid ion-exchange electrodes (A. K. Covington and P. Davison), polymer membrane electrodes (G. J. Moody and J. D. R. Thomas), coated wire electrodes (R. W. Cattrall), and crystalline solid-state electrodes (R. P. Buck). Volume I1 of the same title (19a) includes chapters on gas sensors (M. Riley), enzyme electrodes (P. Vadgama), medical research involving ISEs (D. M. Band and T. Treasure), and a summary of analytical ISE methods (E. 0 1982 American
Chemical
Society
27 R
ION-SELECTIVE ELECTRODES
Pungor, G. Nagy, and K. Toth). The concise in each chapter of this set of books serves to review fundamental concepts and state-of-the-art techniques known prior to 1979. The second volume of Freiser’s “Ion-Selective Electrodes in Analytical Chemistry” has also appeared ( 2 3 ~ ) .This volume complements Volume I by offering excellent chapters on potentiometric enzyme techniques (R. K. Kobos), coated Freiser), chemically sensitive field effect tranwire ISEs (H. sisters (J. Janata and R. J. Hiller) and an exhaustive compilation of ISE literature prior to 1978 (R. P. Buck, J. C. Thompson, and 0. R. Melroy). In addition, Bailey’s book (4a), first published in 1976, has been revised into a second edition. This concise monograph is highly recommended for those just getting started in the field. Similarly, Morf has authored a brand new book (48a) which emphasizes the fundamental principles of ISEs as well as basic membrane transport in general. Ion-Selective Electrode Reviews (59a, 62a),a journal started in 1979, continues to offer timely summaries of contemporary ISE research topics. Volumes I and I1 of the journal are now available in book form for home or office collections (58u,614. “Medical and Biological Applications of Electrochemical Devices”, edited by J. Koryta (35a),has also appeared. While not limited solely to classical potentiometric ISE types of devices, the book does contain several well-written summary chapters on relevant topics. These include discussions on biomedical applications of liquid membrane electrodes (P, C. Meier, D. Ammann, W. E. Morf, and W. Simon), clinical applications of solid-state electrodes (C. Fuchs), microelectrodes for single cell measurements (J.L. Walker), monitorin activity changes in excitable tissues by ISEs (P. Hanik, Sykova, N. Kriz, and F. Vyskoal) and a review of enzyme electrodes (G. G. Guilbault). Aside from individual books, a concise chapter describing the theory of membranes in general and their incorporation into cation and anion responsive ISEs has been authored by Lakshminarayanaiah
If
(36~).
Several conference proceedings concerning ISE research were also published. Most notable among these were the Proceedings of a Meeting on the Theory and Applications on Ion-Selective Electrodes in Physiology and Medicine ( 3 8 ~ ) (Dortmund, July 28-30, 1980) and the Proceedings of a Conference Organized by the Electroanalytical Group of the Chemical Society, London (36a) (London, April 17-20,1979). In the latter, papers on the clinical applications of new liquid-PVC membrane electrodes (D. Ammann, H. B. Jenny, P. C. Meier, and W. Simon) and the design of calcium selective electrodes (G. J. Moody and J. D. R. Thomas) were the highlights. At the Dortmund Conference, numerous specialized research papers were given and these will be cited individually where appropriate later in this review. Finally, Sensors and Actuators, a new journal, has come upon the scene (42u,43a). This journal is dedicated to providing original research articles concerning the development of solid-state transducers, of which certain ISEs are among (e.g., solid-state membrane electrodes, ISFETs, coated wire electrodes, etc.). Additional references to general as well as specialized ISE reviews, predominantly published in journals or within books not specifically about ISEs, are presented in Table I.
GENERAL DISCUSSIONS CONCERNING TECHNIQUES, APPARATUS, AND THEORY We consider here work relating to the general application of ISEs for analytical measurements both from a practical as well as a fundamental standpoint. Consequently, references relating to general measurement techniques (including titrations), general selectivity considerations, detection limits, novel apparatus (including reference electrodes), etc. will be covered without regard for the specific type of ISE used. Two papers openly addressed some of the problems associated with the use of ISEs, particularly with regard to selectivity, temperature, and response time limitations (22b, 42b). This is indeed refreshing and it emphasizes an important point, namely, that one must carefully examine various factors before successfully applying ISE technology to a given analytical situation. The notion that an ISE can be placed into any sample for direct measurement purposes is erroneous and it often results in disenchantment on the part of first time ISE users. More emphasis should be given to alerting no28R
ANALYTICAL CHEMISTRY, VOL. 54, NO. 5, APRIL 1982
Table I. Additional ISE Reviews (General and Specialized) no.
subject
ref
1 general ISEs
2 3 4 5 6 7 8
9 10 11
12 13 14 15 16 17 18
19
8a, 9a, 16a, 22a, 24a, 31a, 57a, 67a bibliographies loa, l l a , 12a, 13a, 45a, 47a ISEs in flowing systems 52a, 53a, 64a, 65a micro-ISEs and applications l a , 2a, 3a, l a , 20a, 37a, 54a, 63a, 66a, 70a enzyme, bacterial, and 14a, 25a, 26a, 27a, tissue bioelectrodes 32a, 39a, 40a, 41a ISEs in environmental, 21a, 29a, 30a, 55a, industrial, and water 60a analyses neutral carrier and inorganic 15a, 34a ion-exchanger based ISEs surface effects on solid28a state ISEs surfactant analysis 5a drug analysis 17a boron measurements in 6 9a reactor coolants ISEs in cystic fibrosis 4 6a studies ISEs in clinical chemistry 6a glass electrodes 50a ISEs in tetraphenylborate 68a titrations ISEs in nonaqueous 51a solvents nonflowing automation 4 9a of ISEs ISEs in fermentation 33a processes application of ISEs in 44a dental and mineralized tissue programs
nexperienced ISE users to potential pitfalls. In connection with measurement techniques, Selig continues to explore the feasibility of using commercial ISEs to potentiometrically sense alkylammonium ions and the subsequent use of these electrodes in tetraphenylborate titrations of the organic cations (41b) as well as the analysis of surfactants and soaps (40b). This work is based on the fact that several commercial liquid membrane electrode systems (e.g., fluoroborate, perchlorate, calcium, etc.) actually have surprisingly large responses to many alkylammonium species. Potentiometric titration analyses based on equilibrium reactions have also been applied by using various ISEs (6b) for the determination of weak acids and bases as well as for other ligands with electron donating ability (e.g., amino acids, polypeptides, etc.). A variety of ISEs were used in a novel two indicator electrode arrangement for the differential titration of halide mixtures (20b). Dual ISE arrangements in semiautomated gradient titration systems have also been reported for chloride determinations (4b). The advantageous use of ISEs for binding association measurements was exemplified by the development of a simple educational experiment involving a sodium electrode (9b). With regard to more general measurement techniques, Mascini (24b)has reviewed the use of addition, Gran’s plots, titration, etc., methods for potentiometric determinations, while Horvai and Pungor (13b, 14b, 15b) have provided an elaborate three part series of articles dealing with the comparative precisions of these various techniques. Johansson and Gran (19b) further described a simple method for calculating the equivalence volume from a titration method based on an extension of the Gran 1method. Similarly, a general mathematical method for simultaneously determining the equivalence volume and slope of an ISE used in a titration has been reported (18b). Perhaps the most important innovation with regard to ISE apparatus within the past 2 years has been the incorporation of redox reference electrode systems within ISE configurations. This was pioneered by Ross at Orion Research (33b) and led
ION-SELECTIVE ELECTRODES
Ymh E. Il.r.mOn. AsWard RotsMor of chemclby at me Unkasny ol Mkh@agan. re. celvsd hls B A Dsgw h chrmisby horn Lehman Collegs (CUNY Syaem) in 1974 f m at me State Unlvasny and his ph D W of New Yovk at Bunab In 1979 where he dld his doctoral research wtm ProleG A Rechnltl He was a b a posldo*o~aI felbw in Professor Rechnltl s research awo at me universw Of Galaware belor0
b b d e W i issays lncludin~enzyme l a M mmpetiiive binding techniques. He is B member Of UW ACS. AAAS. AACC. and the Alpha Chi Slgma c h e w ice1 fraternity. V r o m YW* F r M . gnduata shldsnt at
me Unhwsny 01 Mkhgan. rewmc ha B.S.
Wee hom me univmrsny of PUMo Rico.
6-
R b F i e h s Campus, In 1972. I n met Sam0 year. she stalled wovkhxl tu me h b a l Analytical Laboratq of me Aglculhnai Exwlment Station as B Boll and plant tissu~ chemist. In 1974 she was given a lsave of absence to complete her M.S. Dsgree in Phpkal Chemlsby at !ha Unlvedty of P u r to Rice where she dld research wlth Or. Oerald Stevenson. she mmpleted her M.S. Degree h 1975 and retuned to the Cenbal Anahmeal Laboratmy to assume its SUWNC sbn I n 1977 rht Pined Uw ranks of Eli Uty Induobies. Inc., as a SsnloT Chemist lor Tednlcal Swvkx h BlahemC CBI Prodwlbns Gapnment. I n 1978 she entered gaduate chemlsby Fagam at the University of Michgan where she is c m n w wovklng on her R.D. Degw in Analytical chemlsby & supvision of ROIMark E. Meyemon. she ts B member ot ACS. college 01 chemists 01 PWO R~CO. phi Lambda Vpsibn. HOnaary Chsmleal Society. and Alph CM Sbma chem iCal haternity (Vice President. Alpha Bsta Chapter. 1980).
-7
- 7
r-7
\
me
me
to the development of the so-called "Ross pH electrode". While simple glass pH electrodes are not within the scope of this review, the potential application of this concept into other ISEs could add performance benefits similar to those found with the new pH electrodes (e.g., im roved stability under normal and abnormal operating con&ions). Passive membrane dosimeters, which r uire no individual calibration, have also been introduced hye8rion to be used with ISEs (26). The ease of preparing homemade ISEs has once again been demonstrated in an educational exercise (386) while the current state-of-the-art with regard to ISE equipment was discussed in an article by Crow (86). A new ISE body for pressed pellet type solid-state electrodes was introduced (226) with its main advantage being that membranes with different compaitions (and therefore selectivities) can be intercban ed rapidly. Designs for using copper, cadmium, and fluoride I& as stable reference electrodes were examined (396) and the construction of simple polymer membrane electrodes for simultaneous calcium and potassium determinations was described (16). Work on improved reference electrodes for ISE measurements has also been reported. A new short path liquid junction incorporating a porous ceramic layer has been developed (236) while a plate shaped Ag/AgCl reference electrode has been designed and used for the determination of fluoride in microliter volumes (76). A unique nonclogging water permeable membrane-based reference electrode has also been patented (346). The need for improved reference systems was documented by Illingworth (276)when he ascrihed common errors in pH measurements to liquid junction potential variations in modem combination pH electrodes. Additionally, microcapillaiy reference electrodes used in conjunction with an inverted fluoride electrode have been applied for measurements in microliter as well as nanoliter volumes ( a b ,456). The advantageous use of microprocessor-controlled and other automated ISE instrumentation has grown rapidly in recent years as a result of the lower cost and increased availability of such systems. Moody and Thomas have reviewed the applications of microprocessor-based pH meters
(29b)while others have used similar on-line instrumentation for titration equilibrium studies (IOb), automated soil pH measurements (Zlb), construction of pH stat/titrator systems (476, 2-56), the automated measurement of residual chloride in water (326) and in conjunction with enzyme electrode de. terminations via the use of first and second derivative techniques (436). Theoretical and mechanistic studies regarding speeifc ISE types will be presented in subsequent sections of this review under the class of ISE examined. However, some basic theoretical work concerning practical applications of ISEs needs to be mentioned here. For example, equations describing the theoretical and experimental detection limits of ISEs as well as factors influencing their non-Nemstian behavior have been systematically examined by Midgley in several articles (266, 276, 286). Methods of extending the working range of ISEs via the use of improved sensors and better calibration techniques were outlined by Thomas (44b). The concept of detection limits and the expressions used for its calculation was further addressed by Midgley (256). Several studies concerning the response times and selectivity characteristics of various classes of silversilver halide electrodes were reported. The types of electrodes investigated included electrodes of the second kind (typical Ag/AgCl reference electrodes) as well as classical pressed pellet type AgzS based membranes and anodized Ag/AgCl electrodes based on pressed silver pellets. In an attempt to correlate response times to the thickness of AgCl coverage, impedance characterizations of the latter type were performed by Rhodes and Buck (306). These workers (316)also examined bromide interferences on such an electrode which turns out to be a result of ion-exchange kineticz at the electrode surface. Buck (56) also describes the math and theory involved in general impedance measurements for pH and Ag Ag hallde electrodes in a separate article. The question oifbromide .' effects on Ag/AgCI electrodes of the second kind were further examined by Sandifer (366) who subsequently found that a cellulose acetate coverage of the electrode surface minimizes bromide as well as uric acid interferences (376). A model for the treatment of selectivity coefficients for classical solid-state ISEs which takes into account bulk species concentrations, time, temperature, and stirring rates has been offered (166). The theoretical attention given to the various Ag/Ag halide electrode systems is clearly a result of their increased application for physiological measurements. Consequently, a recent patent (216) concerned itself with the elimination of bromide interferences in the measurement of chloride in fluid samples by ISEs via the use of selective complexing agents. Finally, an elaborate discussion on the calculation of monoand bivalent ion selectivities by the Nikol'skii equation and the Nernst-Planck electrodiffusion concept was presented (36). Good correlation between both methods for several IS& were obtained only when using the fixed interference experimental technique.
GLASS AND SOLID-STATE ELECTRODES In this section we review work done with glass membrane electrodes, conventional solid-state ISEs (including the LaF-fluoride electrode and Ag*/AgX or AgzS YS pressed pellet types) and what we term 'nonconventiond" solid-state IS&. This latter group will include heterogeneous membranes involving crystalline materiala suspended in polymer matrices (so-called Pungor type electrodes) as well as solid-state conductors (e& glass, carbon, silver, etc.) which have been chemically modified with crystalline-based membranes or coatings (these fall into the broad category of chemically sensitive semiconductor devices (CSSDs)). In addition, several papers relating to the development of solid-state pH sensitive electrodes are also included. While in the strictest sense, such pH electrodes are not ISEs, their incorporation in this review seems warranted. Glass Electrodes. Without a doubt, the pH responsive g k membrane electmde continues to be the most widely used ISE. As mentioned earlier, it is not within the scope of this review to cover the vast number of routine and nonroutine applications for which the pH glass electrode has been used. However, several informative articles relevant to new designs or arrangements of pH glass membranes have appeared and should be mentioned. For example, an inexpensive pH microcell glass membrane assembly has been developed which ANALYTICAL CHEMISTRY, VOL. 54, NO. 5. APRIL 1982
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ION-SELECTIVE ELECTRODES
utilizes lot-type glass membranes to reduce alkaline errors for micro-pH measurements (81c). Tests describing how to assess pH glass electrodes' performances and electrical resistances have been published (23c) in connection with British standards for such devices. Questions concerning storage, aging, and regeneration of pH lass electrodes have been addressed in a German paper (95 as has the question of whether to use a gelled-solid or liquid internal electrolyte in pH electrodes (31c). It is suggested that the liquid internal electrolyte systems are more suitable for precise measurements. A good historical review of the development of pH glass electrodes has also appeared (25c). With regard to other glass electrodes, a Japanese group has investigated the specific response characteristics of several glass membranes (46c) and reported that the sodium glass electrode displays instantaneous transient responses to K*, Rb', Cs', and NH4+as well as to some multivalent ions. These transient responses were explained on the basis of diffusion potentials in the hydrated layer of the glass. Electrodes prepared with different glass compositions did not exhibit transient behavior and this was presumed to be due to a thicker hydrated layer in those membranes. This transient response question was also examined by others ( 1 2 2 ~in) relation to potential potassium errors in the clinical analysis of sodium. It was found that transient response to K+ could be eliminated by exposing the hydrated layer of the glass to dilute NH4HF2 (dissolves hydrated layer). This result seems to conflict with the theory of the Japanese workers. Whatever the cause of this transient response, the effect can be useful analytically as demonstrated by Vesely (121c) who determined sulfate by differential titration with barium and a sodium glass ISE. The use of new glass membrane materials, chemically treated or not, continues to be an active area of research. Chalcogenide glass materials composed of arsenic/selenide or selenide/telluride were used to prepare copper(I1) and lead(I1) selective sensors (83c). Although they are Nernstian in response over a wide concentration range, these electrodes appear to function via a redox potential mechanism rather than a classical glass ion-exchange process. A lead responsive glass electrode with limited selectivity was developed by using a glass material composed of AgAsS2-PbIz (7c), A hydrogen peroxide sensitive electrode was prepared by coating an Fe or Ti doped glass membrane with powdered graphite (9Oc). In view of this electrode's dependence on 02,it can be assumed that this glass electrode also functions via a redox mechanism. Some selected a plications of glass membrane ISEs are summarized in Tafle 11. Canventional Solid-state ISEs. The europium doped LaF3 crystal-based fluoride selective electrode still enjoys wide use due to its excellent selectivity over other common monovalent and divalent anions. Attempts to improve electrode response by doping the crystal with calcium rather than europium did not offer any advantages with the exception of . sintered CaF2 rare earth fluoride lower cost ( 1 2 3 ~ )However, mixtures proved useful in preparation of a calcium ISE ( 6 8 ~ ) . The sluggish response times of aged commercial fluoride electrodes were attributed to the loss of internal electrolyte contact with the internal portion of the LaF, crystal (111~). This can be readily overcome by cutting open the top of the electrode body and adding fresh internal electrolyte. Another approach used to circumvent this problem involved the use of a solid-state internal contact to the LaF3 crystal (29c). Others have also reported their attempts at making an all solid-state fluoride electrode ( 1 2 8 ~ ) .Improvements in the detection limits for fluoride measurements were noted by Coetzee and Martin (22c) when they investigated the response of the fluoride electrode in nonaqueous solvents. Their data suggest an indirect relationship between solubility of the L a 3 membrane in various solvents and the resulting detection limit,s (e.g., the more insoluble, the better). Improvements of up to 2 to 3 decades in detection limits can be realized when using potentiometric titration methods in organic solvents. Several investigators have tried to determine the optimum choice of buffers to use with fluoride electrodes. Monofluorophosphate interferences can be minimized by using acetate buffers (114c) while the best results for fluoride determinations in water were accomplished with a Tris-ammonium citrate buffer (77c). Trojanowicz ( 1 1 8 ~suggesb ) that a thorium-buffered standard can yield Nernstian response 30R
ANALYTICAL CHEMISTRY, VOL. 54, NO. 5, APRIL 1982
down to nmol/L levels, although he claims that the practical detection limit is still in the bmol L range as predetermined by contamination. Some selecte recent applications of the fluoride electrode appear in Table 11. Numerous investigations involving solid-state ISEs based on pressed-pellet precipitates have been undertaken in the past 2 years. The simplest of this ISE type is the Ag2S membrane electrode used to detect silver and sulfide ions. Rhodes and Buck (96c) continued their impedance studies of such an electrode in an attempt to determine the factors influencing the response times. They concluded that the dissolution of membrane salts is the rate limiting step in the establishment of steady-state potentials. Radic and Milisic (93c) investigated the use of an Ag2S electrode to determine lead under solution conditions not condusive for the use of a Ag,S PbS-based lead electrode. By utilization of the initial rate o potential change upon addition of lead(II), the rate of PbS formation could be monitored and related to the lead(I1) concentration in solution. The Ag,S/PbS-based lead selective electrode has also been the focus of several fundamental studies. The effect of organic solvents was studied in several papers (13c, 14c, 1OOc) and, as expected, increasing the dielectric constant of the sample solution causes greater electrode potentials and an increased exchange of lead onto the electrode surface. The latter was demonstrated by radiotracer studies with 210Pb. Ion exchange at the lead electrode surface was supported by ESCA studies which clearly showed hydration at the outermost layers of the membrane (12c, 130c). The cleansing action of EDTA and HCIOl was also confirmed by this ESCA study. Interestingly, it was also proposed that ESCA measurements ultimately could be used to determine selectivity data for such electrodes by determining equilibrium exchange constants for various cations directly at the electrode surface. Finally, the effect of membrane thickness and surface ratios on the lead electrode's response and potentials was studied (16c). The A &3/CuS-based copper ISE has also been the subject of consiierable attention. Coetzee and co-workers (21c) demonstrated, by ESGA measurements, that upon exposure to copper(I1) the copper in a membrane of composition Ag1,~6Cu0.46S was really in the form of copper(1). Moreover, the failure of such a copper ISE in acetonitrile solutions is due to an exchange of copper(I1) for copper(1) at the membrane surface as well as a leaching of silver from the electrode. The electrode did, however, function well in other organic solvents. Coetzee and Istone also used the same copper(I1) responsive membrane electrode to determine free energies associated with copper(I1) transfer from water to organic solvents (20c). Chloride interference with the copper ISE has been theoretically and experimentally studied (44c, 45c). Westall et al., (125c) maintain that in high chloride concentrations (Le., seawater) the cupric selective electrode actually becomes responsive to cuprous ions with an associated slope of 59 mV decade. This is apparently due to the reduction of bulk so ution: copper(I1)to copper(1) at the electrode surface caused by the stability of the chloride complexes. In another study, the copper(I1) electrode was shown to be responsive to a number of copper inorganic cation complexes induced . it was found that the by pH changes ( 1 2 4 ~ ) Consequently, copper ISE also responds to CuOH+, C U ~ ( O H ~and ) ~ +CuH, C03+,and therefore the electrode lacks specificity at pH X3.30. A copper ISE prepared with a Cu4,80SC U ~ , membrane ~~S composition wai3 found to have reasonab e selectivity toward copper(I1) (64c) but it could also be used to determine V03-. With regard to other conventional solid-state ISEs, the effect of iron(II1) on cadmium ISEs (Ag2S/CdS) was investigated ( 1 0 1 ~and ) it was determined that iron(II1) is a serious interference probably due to its oxidation of membrane sulfide ions. Cyanide was determined in the presence of mercaptans with a AgS/AgCN-based electrode by prior oxidation of the thio compounds (5c). An interesting patent has appeared describing the advantageous use of an Ag/AgI electrode as a working electrode in a polarographic arrangement for gas determinations (88c). The construction of a solid-state iodide electrode for teaching purposes was reported (86c) as well as the use of an iodide selective membrane (Ag2S/AgI) for mercury(I1) measurements (752). In addition, Selig ( 1 0 5 ~ ) reported that many of the conventional solid-state precipitate-based ISEs respond to organic anions and cations and, once again, as in the earlier case of liquid membrane elec-
d
d
i
i
ION-SELECTIVE ELECTRODES
Table 11. Applications ;,f Glass and Solid-state ISEs electrode used no. 1
sodium-glass
2 3 4 5
silver-glass fluoride (LaF, cryst)
6 7 8
9 10 11
12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51
copper (A.g2S/CuS)
iodide (Ag2S/AgI)
chloride (Ag,S/AgCl)
lead (Ag28/PbS) cadmium (Ag,S/CdS) sulfide or silver (Ag,S)
bromide (Ag,S/AgBr) cyanide (A.g,S/AgCn) several of the above
determ of sodium in human breast milk in aluminous materials as ref electrode for determ thermodyn. pK values of bases determ of Hg by titration with dithiooxamide determ of fluoride in rocks in concsulfuric acid in solns of amines in zinc electrolytes in coffee in biolotical & environ. samples in fertilizers & wastewater at subnanogram levels in blood determ of fluoracil in blood determ of monofluoroacetate in biological tissues determ in aluminum determ of barium in glass materials in titrations of selenium in fluoride uptake studies by dental tissues determ of copper in archaeological & corrosion samples in fresh waters in concentrated electrolytes in presence of Co and Ni determ of Cu(I1) & Zn(I1) by titration with EGTA in titrations of fulvic acid for binding constant measurements for pK, measurements of amines titration of palladium(I1) kinetics of periodate-iodide rxn. for sulfide determ in wastewater titration of ascorbic acid iodide and iodophor deterrn in milk determ of mercury in presence of iron(II1) microdeterm of arsenic by titration with Ce(1V) catalytic determ of Mo(V1) and V(I1) microdeterm of chloride determ of chloride in cheese in beer in rocks determ of sulfur in aliphatic compounds selenium in organics by titration determ of nitrates by reduction with cadmium metal determ of sulfate in fertilizers determ of sulfide in environmental samples trace analysis of sulfide by titration with Pb(NO,), applications in clinical chemistry determ of Ag and Cu by titration with rubeanic acid determ of bromide in rocks determ of cyanide in natural waters determ of pharmaceuticals containing bound halogens determ of inorganic and organic thiophosphates
trodes, he demonstrated ,their application in tetraphenylborate titrations. Several new membrane formulations for solid-state ISEs were also reported. Unfortunately, little in the way of practical analytical devices was obtained from these studies, primarily because of the poor selectivity characteristics of the resulting electrodes. For example, silver citrate and lead oxalate were used, along with AgzS, to prepare citrate and oxalate responsive membranes that possessed only marginal selectivities (25c, 27c). Silver iodomercurate(I1) (Agz(Hg14))was used to construct a pressed pelbet silver membrane electrode which had no clear-cut advantages over the conventional models (104~)while others reported membrane compositions responsive to phosphate (49c, 50c) and sulfate (74c) without successfully eliminating common anion interferences. Of course, the development, of truly selective phosphate and sulfate membrane electrodes would be enthusiastically welcomed in many disciplines and additional research aimed at developing such devices needs to be undertaken. Some se-
ref
application
36c 84c 97c, 117c 88c 85c, 115c 8c 108c 48c 71c 89c 17c 24c 18c 15c 28c 61c, 119c 91c 4c ll0c 120c 112c, 113c 47c 41c 103c 6c 76c, 82c, 99c 43c 32c 61c 80c 73c 126c 109c 19c 57c 92c 35c 87c I C
38c 1oc 37c llc 127c 30c 12c 40c 2c 95c, 129c 94c 51c
lected applications of the pressed-pellet type ISEs are given in Table 11. Nonconventional Solid-state ISEs. A wide variety of nonconventional solid-state ISEs have been prepared and studied over the past 2 years. Metal wires, carbon rods, metal disk electrodes, etc. have all been utilized as internal conductive substrates onto which inorganic precipitates or ionexchange resins have been coated (so-called “CSSDs”). Alternatively, the precipitates or solid exchangers have been incorporated into various polymer type matrices (e.g., epoxy, silicone rubber, poly(viny1 chloride), etc.) and cast into traditional membrane form. For example, an ammonium sensing electrode was prepared (63c) by impregnating the salt, (NH4)3(PMo12040), into a methacrylate powdered-based membrane, while a sulfate sensitive electrode was made by suspending BaS04 and AgzS in a PVC matrix (70c). Dowex 50-W-X-4 ion-exchange resin in silicone rubber was used in cobalt(I1) (58c) and copper(I1) (59c) responsive electrodes while a mixture of HgS/CuS in a silicone grease membrane ANALYTICAL CHEMISTRY, VOL. 54, NO. 5, APRIL 1982
31 R
ION-SELECTIVE ELECTRODES
(107c) also showed response to copper(I1). A novel nitrate electrode was developed by using precipitated nitron-nitrate in an Araldite support (epoxy adhesive) (60c) and metal ferrocyanide membranes were used as chloride responsive electrodes ( 6 6 ~ ) .Araldite supported silver-stearate membranes served as silver selective electrodes without mercury interferences (unlike the case with the conventional silver electrode) (52c). A fairly selective cesium electrode was developed (55c, 56c) by using pressed disks of zeolite ion exchangers (of the mordenite type) in an epoxy-based support. The novel channel structure of the zeolite material makes this membrane electrode concept one of the more intriguing new approaches in the area of solid-state membranes. Takasaka and Suzuki (116c) reported a rare earth sensitive electrode (responds to cerium(II1)) using cerium(1V) oxide suspended in an Araldite membrane coated onto a copper disk electrode. In view of the 59 mV/decade slope, it is apparent that this electrode functions via a redox type mechanism. Thallium molybdoarsenate in Araldite membranes served as a very stable thallium(1) selective electrode (53c, 54c). Hassan and Habib (39c, 42c) precipitated copper diethyldithiocarbamate (DDC) and AgzS within a graphite rod to obtain a sensitive DDC electrode. This electrode was elegantly used in the potentiometric titration of heavy metal mixtures with DDC. Because of the significant differences between the stability constants for the DDC-metal complexes, selective titration breakpoints were observed for each metal. In other work, a sulfite responsive electrode was prepared by coating a platinum wire with AszS3 (106~);a (cobalt ethylenediamine)3+responsive electrode based on an epoxy containing chromium ferrocyanide (102c) was studied; a chloride selective electrode was constructed by using a AgCl coating of a silver-silver telluride wire (69c); and a phosphate sensor based on a parchment supported cobalt(I1) phosphate was reported (3c). In these latter examples, selectivity of the resulting electrodes limit their potential applications. Finally, a few words about nonglass solid-state pH sensors. The fragile and costly nature of pH glass has prompted intense research into the development of alternative pH sensitive electrode materials, particularly for applications in severe environmenb and for preparation of practical microelectrodes useful in biological studies. Consequently, a miniature palladium-palladium oxide pH electrode has been introduced (62c) and suggested for use in biological samples. Grubb and King (34c) have prepared such electrodes which were, unfortunately, inactivated by strong reducing agents such as ascorbic acid. The antimony pH sensor was studied in the presence of some complexing ligands (33c) and it was found that many buffers are not suitable for calibration purposes because of their complexation properties. Measurements of pH at elevated temperatures have been accomplished with palladium hydride electrodes (65c) as well as yttria-stabilized zirconia membranes (78c, 79c). The latter type of electrode is apparently not sensitive to the redox environment of the sample thus making it the most attractive, for practical purposes, of all the metal type pH electrodes. Perhaps an even more attractive alternative is the polymer type pH electrode developed by Simon which will be covered in the next section.
LIQUID AND POLYMER MEMBRANE ISE’S In this section we review literature relating to the development, study, and application of liquid and polymer membrane ISEs (neutral carrier and ion-exchanger based). As evidenced by the rather larger number of references cited, research activities involving this ISE class are at an all time high. This is, in part, due to the unique interdisciplinary nature of ion transport phenomena in liquid membranes. Often membrane-related studies in other disciplines (e.g., biophysics, physiology, biochemistry, engineering, etc.) can lead to advances in the design of liquid membrane ISEs suitable for analytical use. In our coverage, we separate neutral carrier based ISEs from ion-exchanger types but do not differentiate between strictly liquid membranes and polymer supported liquid membranes. However, it should be noted that response properties and selectivitiescan change somewhat when incorporating a particular ion transport molecule into a different membrane environment (e.g., from pure liquid membrane ISE to polymer membrane ISE). In addition, specific clinical applications and instrumentation involving this class of ISEs (mainly sodium, potassium and calcium 32R
ANALYTICAL CHEMISTRY, VOL. 54, NO. 5, APRIL 1982
electrodes) will be covered separately in a subsequent section of this review as will noninternal solution type liquid membrane electrodes (Le., coated wire, ISFETs, and other CSSDs). Neutral Carrier Based ISEs. Simon and co-workers in Zurich continue to lead the way in the development and study of neutral carrier based ISEs. Over the recent years, this group has published several important papers. Perhaps their most exciting development has been the preparation of two different hydrogen ion sensitive neutral carrier membrane electrodes. Both 3-hydroxy-N-dodecylpicolinamideand tri-n-dodecylamine have been employed as carriers in polymer membrane type pH electrodes (334 99d). In the case of the picolinamide derivative, addition of tetrakis(pchloropheny1)borate to the organic membrane enabled pH measurements to be made in highly acidic solutions (pH lo0 and the resulting lithium electrode could prove quite valuable in monitoring lithium drug treatments for manic depression. Similarly, the new calcium electrode based on the neutral carrier (R,R)N,N’-bis((ll-ethoxycarbonyl)undecyl)-~,~’-4,5-tetramethyl-3,6-dioxaoctanediamide(ligand ETH 1001) allows for calcium measurements to be made in highly acidic samples (not possible with conventional alkyl phosphate based ionexchanger calcium electrodes) thus enabling the direct determination of total calcium in blood by prior dissociation of bound calcium under acidic conditions. Both of these new electrodes were made in polymer membrane form using poly(viny1 chloride) (PVC) and appropriate plasticizers. In view of the growing use of such neutral carrier electrodes, Oesch and Simon (934 reported on the expected lifetime of such electrodes in general. Theoretical models and experimental evidence showed that lifetime is a function of the partition coefficients of the membrane carrier and plasticizer between the organic phase and aqueous sample solution: the higher the Kpartthe longer the lifetime. In addition, it was also shown that very little carrier is actually required within the membrane to make functioning electrodes. A t the Dortmund conference, Ammann et al. ( 3 4 reported on development of other carrier based ISEs applicable for the determination of sodium and potassium in undiluted urine samples. Simon has also patented an electrode based on neutral organic tin compounds which reportedly had excellent selectivity for bicarbonate over chloride (107d). The use of crown ethers as neutral carriers in liquid membrane ISEs was Also the subject of several investigations. Bis crown ethers were proven to be valuable in the preparation of potassium and cesium selective PVC membrane electrodes (374 74d, 1154 which had improved selectivities over electrodes prepared with the corresponding monomer ethers. The potassium selective electrodes were made with bis-benzo-15crown-5, while the cesium electrodes employed bis-benzo18-crown-6or ap ester derivative. A potassium electrode based on dibenzo-24-crown-8 was also reported (92d). In none of the above instances did the resulting potassium electrodes have better selectivities or response properties when compared to the conventional valinomycin based potassium electrode. Maeda and co-workers (84d) found that a PVC membrane based on dibenzo-18-crown-6actually exhibited selective responses to cationic surfactants (e.g., 1-dodec lpyridinium) over common inorganic cations by a factor of 10K. The use of PVC membrane electrode measurements to evaluate the enantiomer selectivities of chiral crown ethers was also reported by two groups (13d, 1 2 0 4 although it was rightfully pointed out by Simon (108d) that his group first used membrane electrodes for such studies back in 1975. A new synthetic macrocyclic polyether diamide (7,19-dibenzyl-2,3-dimethyl-7,19-diaza1,4,10,13,1~-pentaoxaheneicosane-6,20-dione) apparently acts as a highly selective calcium ionophore in nitrobenzene (59d) and a report of its use in a liquid calcium selective electrode
ION-SELECTIVE ELECTRODES
has appeared (954. Similarly, a cadmium selective ionophore (N,N,N’,N’-tetrabutyl-3,6~-dioxaoctanedithioamide) was reported (98d) but not as ylet thoroughly studied as a potential cadmium selective electrode. The potassium selective electrode based on the antibiotic valinomycin was also the focus of some fundamental studies. The interference of certaiin lipophilic anions on the electrode was studied by Yurinskaya et al. (121d) who found that the picrate anion response of a valinomycin-based membrane was dependent on the valinornycin content of the membrane and the potassium level in the test solution. They concluded that picrate response is not due to high mobility of the picrate ion alone in the membrane but rather on specific interactions of the picrate ion with the valinomycin-potassium complex. Stefanova and Suglobova further examined the properties of PVC membranes containing different amounts of valinomycin (1134. Their results concur with Oesch and Simon’s findings that very little Valinomycin is actually required to make functioning electrodes (tu3 little as 0.001 wt %). However, electrodes with this veny low valinomycin content did not function well in mixed solutions containing sodium. In addition, the transient response properties of valinomycin membranes to potassium were studied ( 2 4 with the conclusion being that time response is predominantly determined by stirring rate and liquid injection speed. Theoretical treatment of the selectivities and absolute membrane potentials observed with neutral carrier based ISEs in general were considered in several papers (70d, 110d, 1124 while the effect of plasticizer used on the selectivity of neutral carrier based PVC ISEs were addressed in a Chinese article (154. Ammonium responsive polymer membrane based on the antibiotic nonactin or its derivatives were also prepared and studied (224 52d) and it was found that such membranes could be used to also sense thallium(1) ( I l l d ) . The latter study utilized divinylbenzene as the polymer support material rather than PVC. Finally, an init,eresting new approach to the analytical use of liquid membrane electrodes in general was summarized by Koryta (78d) in a review of the technique known as electrolysis at the interface of two immiscible electrolyte solutions (ITIES). By this method, analogous to electrolysis a t metal electrode/electrolyte interfaces, Koryta suggests that cyclic voltarnmetry ( 5 7 4 using ISE membranes as working electrodes may be used for ion determinations, neutral carrier determinations (antibiotic assays), and selectivity evaluations of certain ISE membranes. Ion-Exchanger Based ISEs. For starters, several articles appeared dealing in general with new electrode designs and fundamental aspects of ion-exchanger based membrane electrodes. These include a new simplified design for liquid ISEs ( 4 5 4 , a study on the dynamic response relationship for ion-exchanger liquid membrane electrodes ( 4 1 4 , studies relating ion-exchange constants of membrane ion-exchangers to ultimate electrode selectivity properties (40d), investigations concerning the transport of counterions in ion-exchanger based membranes ( 1 2 4 , and the detection limits and selectivities of general anion responsive ion-exchanger based electrodes which utilize capriquat in PVC (714. With regard to new ion-exchanger ISEs of significance or improvement studies or1 existing membrane systems, the calcium selective electrode was the most extensively studied. Craggs et al. (19d) studied the effect of anionic surfactants on calcium electrodes based on a bis(Coctylpheny1)phosphate ion exchanger and found1 that changing from a dioctyl phenylphosphonate solvent mediator to a decanol solvent diminishes negative surfactant interference, but at the expense of poorer selectivity over magnesium. The negative interference itself was attributed not to the ionic binding of calcium in solution by the surfactant but rather to surfactant interactions at the electrode’s surface. The same group also found that by changing solvent mediators from 1-decanol to di-noctyl phenylphosphonata! for PVC calcium electrodes based on bis(di-n-decyl phosphate) or bis(di(4-(1,1,3,3-tetramethylbuty1)phenyl) phosphate), dramatic improvements in selectivity over magnesium could be achieved (17d). A divalent electrode (with equal response to Ca and Mg) was prepared by using bis(didecy1 phosphate), didecylphosphoric acid, and decanol in a polly(methy1acrylate) (PMA) support matrix, although the resultin electrode proved inaccurate in determining total water l~ar&-~ess in real samples ( 5 1 4 . An elaborate study was reported by Lakshminarayanaiah (82d)
on the use of 4-(1,1,3,3-tetramethylbutyl)phenyl mono- and diesters of phosphoric acids as calcium selective membrane materials (with PVC). Optimum detection limits (down to mol/L) could be realized with the diester based 7.9 x membranes while the monoester electrodes proved unsatisfactory. Only Iron(II1) and lanthanum(II1) significantly interfere with the diester probe. A Japanese group (1174 studied the memory effect of conventional calcium ISEs and found it to be significant when rapidly going from high to low concentrations. Bipolar pulse conductance measurements with a commercial calcium electrode proved useful for detecting calcium at IO4 mol/L and above ( 9 7 4 with faster response times (10 ms) than normal potentiometric measurements (4-5 8).
Numerous papers have appeared concerning the development of ion-exchange electrodes sensitive for large organic cations, particularly tri- and tetralkyl ammonium ions. It has already been mentioned in section I1 of this review that Selig has found that many commercial polymer membrane ISEs (advertised for inorganic cations) actually have rather large responses to a series of alkylammonium ions. Freiser and co-workers at Arizona have attempted to prepare electrodes specifically designed to detect large organic cations in the presence of small inorganic ions. As a result, PVC membranes containing dinonylnaphthalenesulfonic acid (DNNS) were studied and appear to work well for this purpose (85d). In addition, these offer interesting possibilities in their applications for the detection of drugs of abuse (e.g., opiate alkaloids, cocaine, etc.), as exemplified by the development of a phencycline (“angel dust”) selective electrode (86d, 87d). Similarly, Hassan and Elsayes (50d) used a strychnine picrolonate ion pair in a liquid nitrobenzene based electrode to detect the pharmaceutical alkaloid strychnine. Tetraphenylborate-novacaine and dipicrylaminate-novacaine complexes dissolved in nitrobenzene were employed by Negoiu et al. (91d) to prepare a novacaine responsive membrane electrode capable of determining the novacaine content of pharmaceutical preparations. A similar electrode for codeine was also described (60d). Diamandis et al. (23d) also used tetraphenylborate complexes to prepare electrodes sensitive to the drugs atropine and novatropine. It was pointed out in that paper that other alkaloids interfere considerably. Indeed, for most of the above mentioned organic cation responsive electrodes, selectivity over other organic ions is generally poor, making real applications limited to simple samples with fairly well-defined compositions (pharmaceutical preparations, etc.) or to specific fundamental studies involving the pure compound of interest. The latter principle was elegantly demonstrated in a report ( I l d ) describing the development of an ethidium ISE and its application to studying the binding of ethidium to DNA. The electrode was prepared using an ethidium-tetraphenylborate complex dissolved in 3-nitro-o-xylene. Tetraphenylborate--barium-nonylphenoxypoly(ethyleneoxy)ethanol dissolved in 2-nitrophenyl phenyl ether formed the basis of a PVC ISE which was fairly selective for barium (69d). The electrode was shown to suffer interferences from a wide range of cations and anions when performing sulfate determinations by titration with barium chloride. It was suggested that to determine sulfate accurately in complex solutions, cations should first be removed with ion-exchange resins. A uranyl ion (U022+)selective membrane was formulated ( 3 9 4 by incorporating uranyl-or anic phosphite complexes into PVC membranes. Markedly ietter electrodes were obtained with phosphites vs. using organic phosphates. Indeed, the resulting sensor had excellent selectivity for UOZz+ over most common cations except iron(II1) and cerium(1V). With regard to anion responsive ion-exchange ISEs, Hadjiioannous and Diamandis continue to make use of their picrate selective electrode originally described in 1977. The electrode, based on tetrapentylammonium picrate dissolved in 2-nitrotoluene, has been utilized to determine some common alkaloids via titrations with picrate (25d), to determine selenium by its catalytic effect on the picrate-sulfide reaction (24d), and for the titrimetric determination of iodide, thiourea, hexacyanoferrate, and cationic surfactants (27d). A slightly different picrate selective ISE was prepared by Homolka (56d) in which a PVC membrane containing cetyltrimethylammonium and n-hexyl o-nitrophenyl ether was used as the active phase. Only perchlorate ions presented a major inANALYTICAL CHEMISTRY, VOL. 54, NO. 5, APRIL 1982
* 33R
ION-SELECTIVE ELECTRODES
Table 111. Additional New Ion-Exchanger Based Liquid and Polymer Membrane ISEs no.
membrane components tri-n-octylmethylammoniuin benzoate in
1
2 3
4 5 6 7 8
9
o-dichlorobenzene with p-t-octylphenol: liquid aspartate-trioctylamine complex in decane: liquid hexadecyltrimethylammonium acetate or oxalate in decanol: liquid Victoria Blue-dodecylbenzene sulfonate in nitrobenzene: liquid bis( 2-ethylhexy1)sulfosuccinate-zephiraminecomplex in nitrobenzene: liquid hexadecyltrimethylammonium dodecylsulfate and 4-t-octylphenol in 1,2-dichlorobenzene: liquid naphthylazoxine-zephiramine complex in nitrobenzene: liquid tributyl(octadecy1)ammonium cotylsulfate or tributyl( octadecy1)ammonium dodecylsulfate in toluene-nitrobenzene: liquid 2,4,6-triphenylthiopyrylium-tetrachloroferrate
complex in dichloroethane: liquid 10
benzyldimethyltetradecylammonium-hydrogen
chromate in dinonyl phthalate: PVC membrane 11 trioctylmethylammonium chromate or similar complexes: in PVC 12 quaternary or phosphonium salts, nickelphenanthroline complex and dibutyl phthalate: in PVC 13 tris( 1,lO-phenanthroline)ruthenium(11) iodide in dichlorobenzene: liquid 14 copper( I)-bis(neocupro1ine nitrate), dichloroethylene, and plasticizers: in PVC 1 5 didodecyltin arsenate in a chloroform/decanol mixture: liquid 1 6 didodecyltin phosphate in a chloroform/decanol mixture: liquid 17 chloroniobiate-tetraphenylarsonium complex, dichloroethane, and dibutyl phthalate: in PVC 18 (alkyl),Sn(NO,), in chloroform and high molecular wt alcohol: liquid 19 histamine-tetraphenylborate in nitrobenzene: liquid 20 21 22 23
24 25 26 27
creatininium-tetraphenylborate in nitrotoluene:
liquid tris( 2-nitroso-4-chlorophenyl)iron(111)in nitrobenzene: liquid myristate-heavy metal complexes in benzene/ butanol mixtures: liquid ammonium tetranitrodiamminecobaltate and gold thiourea complex in chlorobenzene : liquid thallium(I)-o,o '-didecyl dithiophosphate in chlorocy clohexane tris-o-(benzy1acetoanido)-1,l,l-trimethylolpropane, sodium thiocyanate, and plasticizers: in PVC 2,4,6-trinitrophenol and silver thiourea in various organic solvents: liquid polyketo long chain alkyl compound and calcium tetraphenylborate in nitrophenyl octyl ether: in PVC
terference to this new electrode. In addition, the construction of a fairly selective periodate liquid membrane ISE containing the ion pair of tri-n-octylmethylammonium periodate in nitrobenzene was reported ( 8 1 4 and utilized in the titration of diols and amino alcohols. The complexes of 3,5-dinitrosalicylate with tetraphosphonium or dimethyldioctadecylammonium ions in decanol or p-nitrocumene can serve as selective membrane materials for a 3,5-dinitrosalicylate responsive electrode (474. The resulting sensor was used by Hadjiioannous and Gritzapis to titrate iron(II1) and copper(II1) with EDTA and in the titration of large quaternary ammonium ions with dinitrosalicylate. Several Japanese groups have come up with rather unusual types of ion-exchanger based electrodes which could also be easily classified as fixed site solid-state devices. Imato et al. (64d) utilized oleophilic anion exchange resin materials based on cross-linked polystyrene to develop anion responsive 34R
ANALYTICAL CHEMISTRY, VOL. 54, NO. 5, APRIL 1982
principal species sensed benzoate aspartate oxalate, acetate dodecylbenzene sulfate bis( 2-ethylhexyl)sulfosuccinate anionic surfactants, dodecyl sulfate naph thylazoxine octylsulfate, dodecylsulfate
interferences small organic acids
48d
most anions
lOOd 14d
dodecylsulfate, perchlorate, periodate
75d 72d 49d
anionic surfactants carboxylic acids, I'
tetrachloroferrate (FeC1,-) hydrogen chromate (HCr0,-)
ref
73d 109d 61d
Cl-, NO,-,
so:
63d
-
chromate (CrO>-)
perchlorate
67d
tetrafluoroborate (BF,-)
c10,-, 1-
42d
iodide (I-)
4d
nitrate (NO,-)
Cl-, F-, HCO;
116d
arsenate ions (H,AsO,- or HAsO,Z-) phosphate (HPO,,- or H,PO,-) niobiate (NbO,-)
c1-, I-
123d
arsenate, I-,
122d
c1-
105d 124d
histamine creat ininium ammonium (NH,') heavy metals (Cuz+,Cuz+, CdZ+,Zn2+,etc.) gold (Au+) thallium (Tl+)
quaternary amines creatine, K'
80d
K', organic cations
7 6d
26d
8d 102d mono- and bivalent cations
114d
sodium (Na')
54d
silver ( Ag+)
lOld
calcium (Caz+)
Na+,Mg2+
21d
membranes. Selectivity of such membranes was a problem but it could be controlled to some extent by wetting the membrane with different organic solvents. Oka et al. ( 9 4 4 prepared a chloride responsive membrane based on a plasticized polystyrene film covalently containing appropriate quaternary ammonium groups. Acting as a fixed site ion exchanger (similar to a glass membrane), a Donnan potential develops at the membrane interface as determined by the diffusion of ions at the phase boundary. Impregnation of the membrane with sulfonic acid residues improves selectivity enough so that chloride measurements could be made directly in whole blood. Similarly, a hydrogen ion selective membrane was prepared by using a strongly acidic cation exchange resin material (65d) for measurements of low pHs. A great many other liquid or polymer membrane ion-exchanger based ISEs have been fabricated and some of these are listed in Table 111. Most have only limited applications,
ION-SELECTIVE ELECTRODES
Table IV. Additional !Rudies with and Applications of Ion-Exchanger Based ISEs study or application no. electrode used or studied 1 calcium 2
3 4 5 6 7 8
9 10
divalent (Ca and Mg) nitrate
11
12 13
14 15
tetraalkylphosphonium responsive 16 crystal violet rlesponsive 17 18 1 9 quaternary ammonium responsive 20 21 22 23 24 25 26 27 28
nicotine respoinsive tetraphenylboirate responsive mercury( 11) responsive thiocyanate responsive tetrafluoroborate anionic surfactant responsive barium
determ of calcium in silicate rocks parotid siliva fruits measurement of calcium effluxes from or uptake by cells measurements in the presence of complexing agents as a tool in dental research stability constant of MgSO, in solution direct potentiometric water hardness determination dynamic response and behavior determ of nitrate in meat products in natural waters in high halide concentrated solutions in thorium nitrate solutions as indicator electrode in titration of hydrogen molybdate determination of bacterial cell membrane potentials two phase titrations of aromatic carboxylic acids titration of basic dyes determ of gold in electroplating solns by titra with crystal violet titration of alkyl aromatic sulfonates determ of activity of quaternary ammonium compd in formulated products determ of nicotine in tobacco determ of organic bases by titration determ of quaternary ammonium salts determ of sulfa drugs determ of thiocyanate in waters determ of micromolar levels of boron determ of anionic-active matter in detergents studies of response to nonionic surfactants (alkoxylates)
once again due to poor selectivity characteristics, although these electrodes could be used in highly specialized situations (e.g., binding studies etc.). ome additional basic studies and novel applications of commercial as well a1 “home-made” ion-exchanger based liquid and polymer membrane electrodes are surveyed in Table IV.
MICROELECTRODES, COATED WIRE ELECTR,ODES AND ISFET’S Efforts to make analytical measurements in very small volumes of sample or even single cells, and the desire to utilize ISEs for in vivo monitoring of important blood electrolytes, continue to prompt considerable research into the design of miniature and microsized ISE devices. Conventional micropipet type ISEs, coatesd wire electrodes (CWE), and ion-selective field effect transistors (ISFET) offer hope in these directions. Comprehensive reviews on each of these electrode types were noted in the opening section of this review or were given in Table I. Many of the miniature and microdevices developed thus far have their origin with earlier liquid or polymer membrane systems. Thus, the utilization of ionexchange or neutral carrier based microelectrode membranes constitute the bulk of what will be covered here. (Note CWEs and ISFETs may be considered CSSDs type devices.) Micro-pH electrodes are of essential importance to physiological studies and efforts to improve the design of such electrodes have been made. Interestingly, Savinell et al. (34e) found that the use of a nonaqueous internal electrolyte in a miniature glass pH electrode results in less swelling of the tip (due to nonhydration of the glass) and thus, the electrode becomes more suitable for micropuncture type applications. Nonglass micro-pH electrodes offer obvious advantages and a micro antimony type?pH sensor with a tip size of 4-10 Mm behaved favorably when compared to a conventional glass electrode (20e). A major step in the design of nonglass membrane pH microelectrodes has been realized with the development of ion-exchanger (7e) and neutral carrier (2e) based micro-liquid meimbrane electrodes. The latter utilized Simon’s earlier tridodecylamine pH sensing membrane com-
ref 79d 36d Id, 53d 103d, 104d, 118d, 34d 18d 10d 31d, 32d 88d 7d 16d 6d, 106d 29d 119d 9d 62d, 83d, 89d, 90d 44d 35d 3 8d 55d 20d 30d 46d 96d 66d 17d 43d 2 8d 68d
position and the former was based on the ion-exchanger p octadecycloxy-n-chlorophenylhydrazonemesoxalonitrile. Reducing noise problems associated with the use of conventional micropipet-based microelectrodes has been the focus of several research efforts. Ujec et al. (42e,43e) proposed low impedance double-barrel micro ISEs for potassium and calcium by introducing, coaxially, a thin micropipet into the ISE barrel. This effectively reduces the longitudinal distance between the sample solution and the internal electrolyte (Le., effectively a thinner ISE membrane at the electrode tip) yielding reduced resistance and less noise. Reduced noise was also observed for a potassium selective microelectrode (tip diameter
