Advisory Panel
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Jonathan W . Amy Glenn L. Booman Robert L. Bowman
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Jack W . Frazer Howard V. Malmstadt William F. Ulrich
New Directions for Ion-Selective Electrodes Garry A. Rechnitz Department of Chemistry, State University of New York, Buffalo, Ν. Υ. 14214 Electrode development and applications mutually stimulate one another.
Outlook
for the future indicates that ion-selective electrodes will play an important part in measurement/science
' T ' H E IMPACT of ion-selective electrodes
on solution chemistry is comparable to that of the laser on optical physics. Since solutions are part of the make-up of all living organisms and cover a ma jority of the earth's surface, it is not surprising that a new experimental tool capable of measuring the ionic compo sition of solutions should be of major importance. Moreover, ion-selective electrodes are most effective for the measurement of exactly those ions— e.g., N a + , K + , Ca 2 + , F~, S 0 4 2 - , S 2 ~ , N 0 3 - , C10 4 ~—which are most difficult to measure by other techniques. Ion-selective electrodes measure the activities of ions in solution with con siderable sensitivity (often to below one part per billion) and selectivity (se lectivity ratios in excess of 10,000 are not uncommon). Such measurements are rapid, nondestructive, and can be carried out on a continuous, automated basis. Because of these desirable char acteristics ion-selective electrodes are being used widely for chemical studies, biomedical measurements, pollution and océanographie monitoring, and industrial control. About two dozen different ion-selective electrodes are already available, but more are being developed as the fundamentals of electrode operation and selectivity are elucidated (1). Despite this notable progress, the development and exploitation of ion-selective electrodes are still in a state of infancy—perhaps comparable to the state of polarography in the early 1930's. Entirely aside from the possibility of a major fundamental breakthrough, which cannot be predicted, several new directions for the future development of new types of electrodes
have become apparent this year. These are: Immobilized liquid membrane electrodes Mixed-crystal membrane electrodes Enzyme electrodes Antibiotic electrodes The first two types are useful and important extensions of contemporary electrodes while the latter two are radical departures arising from the impact of the biological sciences upon analytical chemistry and chemical instrumentation. Historically, the development of ionselective electrodes began with glass electrodes and a major forward step was taken just 10 years ago when researchers (2) succeeded in correlating glass compositions with electrode selectivity. Up to that time, selection of compositions had been made largely on an empirical basis. These correlations and accompanying theoretical advances immediately resulted in the development of practical cation-sensitive glass electrodes. I t was soon realized, however, that ion-selective electrodes consisting of glass membranes would be primarily limited to electrodes responsive to univalent cations and that electrodes for other ions would require new types of materials. During the last three or four years two classes of new membrane materials have been found to be highly useful— ionically conducting inorganic solids and organic liquid ion exchange resins (3). The first category includes salt crystals, such as LaF 3 , Ag2S, and the silver halides, which can be used directly as membranes or in combination
with some inert matrix material; the second consists of viscous solutions of exchangers such as organophosphorus compounds which require the use of support membranes to construct stable electrodes. In practice, the range of pure crystalline and other inorganic materials suitable as electrode membranes is limited but highly effective in specific, favorable cases. The liquid ion exchangers, on the other hand, offer a wide range of possibilities for almost any ion but usually yield electrodes which are only moderately selective for the ion of interest. At the present time, the available range and degree of effectiveness of ionselective electrodes still offer plenty of scope for improvement and innovation. The field is a highly competitive one, of course, with substantial commercial potential; thus, much valuable research may be going on in secrecy. Nevertheless, several possibilities of promise are discernible: Immobilized (Matrix) Liquid Membranes One of the disadvantages of the current types of liquid membrane electrodes is that the sample (itself a liquid) comes into contact with the liquid exchanger resin, which is restrained by some inert support material at a rather poorly defined interface. This liquid-liquid interface is subject to stirring and pressure effects, has poor mechanical stability, and can readily lead to mutual contamination of the two liquid phases. I t would be advantageous, indeed, if the fundamental advantages of liquid membrane electrodes
VOL. 4 1 , NO. 12, OCTOBER 1969 • 109 A
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110 A • ANALYTICAL CHEMISTRY
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INSTRUMENTATION could be retained with some improve ment in the mechanical and physical characteristics of these electrodes. Now, the main property of liquid membrane electrodes which makes them so useful as ion-selective electrodes is that their exchange sites are "mobile" (4, 5). Because of this property, liquid membrane electrodes can be made to respond to cations and anions of vary ing charge. However, the mobile charge carriers (sites, ions or ion-site pairs) are of molecular size and there is no reason why the bulk membrane phase must be mobile—i.e., a flowing, if viscous, liquid. This realization suggests that the ex changer liquid could be immobilized in a bulk matrix permeable to the mi croscopic charge carriers. One such matrix material is collodion and an early prototype electrode (β), selective for calcium, was indeed prepared by immobilizing a solution of the calcium salt of a dialkylphosphoric acid in col lodion. In this manner, a solid mem brane electrode is achieved which re tains the desirable characteristics of the liquid membrane system. Recently, an improved version has been described (7) which has important handling and selectivity advantages over the liquid membrane calcium electrode, especially for biochemical application. The principle involved here is per fectly general and it should be possible to prepare a wide variety of such ma trix electrodes. At the present time, several new anion- and cation-selective electrodes of this type are being eval uated in the author's laboratory. Mixed Crystal Membrane Electrodes
A useful extension of crystal mem brane electrodes to additional ions can be achieved through the employment of suitable salt mixtures as the membrane phase (8). The well-established (9) sulfide ion-selective electrode, respon sive to sulfide and silver ions, has as its active element a polycrystalline Ag2S membrane and functions by the trans port of silver ions in the membrane. If this membrane is altered from pure Ag2S to a CuS-Ag u S mixture (see Fig ure 1), a cupric ion-selective electrode results. Similarly, if the membrane is made of a CdS-Ag 2 S mixture or a PbSAg2S mixture, one obtains cadmium-se lective or lead-selective electrodes, re spectively. These electrodes still trans port charge by the movement of silver ions, but their potential is determined indirectly by the availability of S 2 ~ which, in turn, is fixed by the activity of the divalent metal in contact with
Silver-silver chloride reference electrode
/\s
•^Internal filling solution
CiiS Ag,S crystal
Figure 1. Cupric ion-selective elec trode
TETRAETHYLENEPENTAMINE / Cuill] COMPLEX
Figure 2. Potentiometric titration using Cu-Ag2 electrode A. B.
the membrane. Thus, an important re quirement is t h a t the solubility of the divalent metal sulfide be greater than t h a t of Ag 2 S. Practical electrodes need not have the configuration shown in Figure 1; a silver wire could be fast ened directly to the inside surface of the membrane to yield a completely solid-state electrode. Because they function well in aqueous and nonaqueous media, these mixed crystal electrodes are of considerable importance to analytical chemists. T h e CuS-Ag 2 S electrode has been success fully employed for potentiometric ti trations (10) in nonaqueous media (see Figure 2) while the PbS-Ag 2 S electrode has already been found useful for the determination of sulfate b y potentiometric titration with P b 2 + (11). I t should be kept in mind t h a t the mixed crystal concept is not limited to sulfides; indeed, there should be a va riety of two- and three-component crystal mixtures which meet the chem ical and electrical requirements of elec-
T a b l e 1."
ΚΆ
NH 4 + 0.4
K+ 1
mitochondria 30 liver cells 120
Acetone Methanol
trode membrane systems. A recent r e view {12) of ionic conduction and dif fusion in solids points the way toward new ion-selective electrodes of this type. Enzyme Electrodes Ion-selective electrodes need not be limited to inorganic substances or, even, to ionic species, provided a means can be found to provide charge transport and selectivity. Enzymes appear to be promising in this connection. The ac tion of enzymes is very highly selective and, furthermore, the product of enzy matic reactions is often a simple ion to which conventional ion-selective elec trodes respond. Thus, if the enzyme system can be interposed between the sample solution and the final sensing electrode, a highly selective measurement system should result. Guilbault and Montalvo {IS, 14) have accomplished just this by immobilizing an enzyme in a matrix which coats a conventional cation-sen-
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Na+ 150
Li+ 1780
Li+ 4700
H+ 18,000
" Courtesy Prof. W. Simon
T a b l e 11."
K££
Rb+ 0.52
" Courtesy Prof. W. Simon
K+ 1
I C I f o r V a l i n o m y c i n (25 °C) (Diphenyl ether) M Cs+ NH 4 + Na+ 2.6 84 3800
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VOL. 4 1 , NO. 12, OCTOBER 1969
•
111
A
No sacrifice in response or efficiency. Choose a Sargent combination pH electrode by size alone.
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INSTRUMENTATION
sitive glass electrode. The enzyme urease is fixed in a layer of aerylamide gel held in place around the glass electrode bulb by porous nylon netting or thin cellophane film. The urease acts specifically upon urea in the sample solution to yield ammonium ions which diffuse through the gel to give rise to a potential at the glass electrode proportional to the original urea concentration in the sample. The electrode could be used continuously for three weeks without loss of activity and, since the catalytic efficiency of enzymes is very high, the overall system is essentially a nondestructive, urea-specific sensor. The exciting possibilities of this approach must be obvious. There are literally thousands of enzymes with high activity and selectivity. Many of these yield products measurable with existing ion-selective electrodes; thus, it should be feasible to prepare electrodes of the immobilized enzyme type with selectivity for a variety of organic and biological substances such as glutamine, asparagine, and amino acids (15). Antibiotic Electrodes
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112 A • ANALYTICAL CHEMISTRY
Some recent studies on biological membranes are directly applicable to the development of ion-selective electrodes as analytical sensors. Indeed, there are formal similarities between ion electrodes and biological membrane systems which virtually ensure that progress in one of these areas will also advance the other. Recently, it has been shown (16) that certain antibiotics display notable selectivity in their interaction with alkali metal cations. Nonactin and valinomycin, for example, preferentially associate with K + rather than Na + . Since the construction of a successful potassium-selective electrode by conventional means has so far eluded workers in the field, this observation has important practical implications and led Simon (17) to the construction of workable antibiotic-based membrane electrodes. The antibiotics nonactin and valinomycin were suspended in solvents consisting of Nujol/2-octanol and diphenylether, respectively, and incorporated into liquid membrane electrodes similar to the conventional versions described above. The potentiometric selectivity constants for the resulting electrodes are given in Tables I and II and show most remarkable electrode characteristics. The valinomycin electrode, for example, displays a selectivity for K+ over Na+ of 3800:1. This
may be compared with a maximum K + to N a + selectivity of about 30:1 for the best available glass electrode. Fur thermore, the valinomycin electrode has an 18000:1 selectivity for K + with re spect to H + ; this means that the elec trode should be usable in strongly acidic media, where cation-sensitive glass electrodes lose their effectiveness. The nonactin electrode, on the other hand, shows an interesting selectivity for N H 4 + over hydrogen ion and the alkali metal ions and may be of con siderable practical value in this con nection. I t is too early to say whether anti biotic electrodes will be of general util ity and give rise to a broad new class of ion electrodes; however, there is no question but that electrodes based upon the synthetic organic analogs of such compounds are worthy of serious in vestigation. Pedersen (18) recently synthesized a whole series of cyclic polyethers, so-called "Crown" com pounds, which bind (19) alkali metal ions selectively. These compounds can be tailor-made to display desired ion binding and transport properties; thus, they should play a major role in the development of new ion-selective elec trodes. Two U. S. manufacturers have recently announced potassium ion-selec tive, liquid-membrane electrodes. A K + to Na+ selectivity of about 5000:1 is claimed for one of these. New directions for ion-selective elec trodes are not limited, of course, to the development of electrodes. Novel and imaginative applications are of equal importance. Electrode development and application mutually stimulate one another, however, so that the present vigorous pace of research in this area assures ion electrodes a major place in modern measurement science.
Literature Cited (1) G. A. Rechnitz, Chem. Eng. News, 43 (25), 146 (1967). (2) G. Eisenman, (Editor) "Glass Elec trodes for Hydrogen and other Cat ions," Marcel Dekker, New York, Ν . Υ., 1966. (3) R. A. Durst, (Editor) U. S. Bureau of Standards Monograph on Ion-Selec tive Electrodes, Government Printing Office, Washington, D . C., 1969.
a portable FURNACE
(4) G. Eisenman, ANAL. C H E M . , 40, 310
(1968). (5) M. J . Brand and G. A. Rechnitz, ibid., 41, 1185 (1969). (6) F . A. Schultz, A. J . Petersen, C. A. Mask, and R. P . Buck, Science, 162, 267 (1968). (7) G. A. Rechnitz and Τ . Μ . Hseu, ANAL. C H E M . , 41, 111 (1969).
(8) J. W. Ross, paper presented at meet ing of the Electrochemical Society, New York, M a y 1969. (9) T. M. Hseu and G. A. Rechnitz, ANAL. C H E M . , 40, 1054 and 1661 (1968).
(10) G. A. Rechnitz and N . C. Kenny, Anal. Letters, 2,395 (1969). (11) J. W. Ross and M. S. Frant, ANAL. C H E M . , 41, 967 (1969).
(12) J. Kummer and Μ. Ε. Milberg, Chem. Eng. News, 47 (20), 90 (1969). (13) G. G. Guilbault and J. G. Montalvo, J. Am. Chem. Soc, 91, 2164 (1969). (14) G. G. Guilbault and J. G. Montalvo, Anal. Letters, 2, 283 (1969). (15) G. G. Guilbault, R. K. Smith, and J. G. Montalvo, ANAL. C H E M . , 41, 600
(1969). (16) L. A. R. Pioda and W. Simon, Chimia, 23, 72 (1969). (17) W. Simon, paper presented at meet ing of the Electrochemical Society, New York, Ν . Υ., M a y 1969. (18) C J. Pederscn, J. Am. Chem. Soc, 89,7017 (1967). (19) R. M . Izatt, J. H . Rytting, D . P . Nelson, B. L. Haymore, and J. J. Christensen, Science, 164, 443 (1969).
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Τ \ κ . RECHNITZ'S ESSAY on this subject
-L-' is most stimulating and should convince analysts that continued re search and development of ion selective electrodes is a fertile field. What has been accomplished so far is impressive and very useful, but, as he has pointed out, the possibilities are almost unlimit ed. Dr. Rechnitz and his associates continue to contribute heavily to the subject and this discussion combines en thusiasm with extensive experience. It seems quite certain that a host of useful systems can be developed for in-
organic, organic, or biological systems. We are not too happy about the pres ent state of knowledge of the electrical behavior of selective ion electrodes. For example, what is the equivalent cir cuit of such systems? How are poten tial, current, capacitance, and resis tance related and how do they combine to account for the observed behavior? If this query reeks too much of the electrical engineer's "black box," it still seeks to get a practical answer. Knowledge about the attainment of equilibrium at the electrode is unsatis-
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VOL. 41, NO. 12, OCTOBER 1969 • 113 A
