may be compared with a maximum K+ to N a + selectivity of about 30:l for the best available glass electrode. Furthermore, the valinomycin electrode has an 18OOO:l selectivity for K + with respect to H+ ; this means that the electrode 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 NH4+ over hydrogen ion and the alkali metal ions and may be of considerable practical value in this connection. It is too early to say whether antibiotic electrodes will be of general utility 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 investigation. Pedersen (18) recently synthesized a whole series of cyclic polyethers, so-called “Crown” compounds, which bind (19) alkali metal ions selectively. These oompounds 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 electrodes. Two U. S. manufacturers have recently announced potassium ion-selective, liquid-membrane electrodes. A K + to N a + selectivity of about 5000:l is claimed for one of these. New directions for ion-selective electrodes 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 (251, 146 (1967). (2) G. Eisenman, (Editor) “Glass Elec-
trodes for Hydrogen and other Cations,” Marcel Dekker, Xew York, N. Y.,
a Portable
1966. (3) R. A . Durst, (Editor) U. S. Bureau
of Standards Monograph on Ion-Selective Electrodes, Government Printing Office, Washington, D. C., 1969. (4) G. Eisenman, ANAL.CHEM.,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 T. M. Hseu, ANAL.CHEM.,41, 111 (1969). (8) J. W. Ross, paper presented at meet-
ing of the Electrochemical Society, New York. Mav 1969. (9) T. ‘ M . -Hseu and G. A. Rechnitz, ANAL.CHEM.,40, 1054 and 1661 (1968). (10) G. A . Rechnitz and N. C. Kenny,
Anal. Letters, 2,395 (1969). W.Ross and M. S. Frant, ANAL.
(11) J.
CHEM.,41,967 (1969). (12) J. Kummer and M. E. 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, ~ ~ N A LCHEM., . 41, 600 (1969). (16) L. A . R . Pioda and W. Simon, Chimia, 23, 72 (1969). (17) W. Simon, paper presented at meeting of the Electrochemical Society, New York, N.Y., May 1969. (18) C. J. Pedersen, J . Am. Chem. Soc., 89, 7017 (1967). (19) R. M. Izatt, J. H. Rytting, D. P.
K’elson, B. L. Haymore, and J. J. Christensen, Science, 164,443 (1969).
BASEDon a lecture presented at the Analytical Summer Symposium, Athens, Ga., June 1969.
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D
R.
RECHNITZ’S ESSAY on this subject
is most stimulating and should convince analysts that continued research and development of ion selective electrodes is a fertile field. What has been accomplished so far is impressive and very useful, but, as lie has pointed out, the possibilities are almost unlimited. D r . Rechnitz and his associates continue to contribute heavily to the subject and this discussion combines enthusiasm with extensive experience. I t 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 present state of knowledge of the electrical behavior of selective ion electrodes. For example, what is the equivalent circuit of such systems? How are potential, current, capacitance, and resistance 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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Circle No. 70 on Readers’ Service Card
VOL. 41, NO. 12, OCTOBER 1969
113A
11NSTR uMEN TAT ION^
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114 A
ANALYTICAL CHEMISTRY
See
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factory. In practically all cases, no data are given and it is difficult to analyze the emf vs. time curves in any precise fashion. Dr. Rechnitz has made repeated pleas for more information (data), to which we would add, “always give the final value of the equilibrium potential, even if you have to wait hours or days for the answer.” There is the temptation to quote the half-time ( tIl2)-i.e., the time required for the emf to attain half the final equilibrium value. I n the usual analytical techniques, this is an illusory constant, because the measurement, in the early stages, is a fruity melange of mixing time and electrode equilibration. It is not unusual to encounter in the literature a statement that “the electrode had fast response with a half time of 1 second” and then followed by “the potentials were recorded after an interval of 2.5 minutes.” This still gives no indication of the true potential equilibrium or what fraction thereof is attained in 2.5 minutes. There is an empirical approach which affords an accurate prediction of the equilibrium value, obtainable from a few values taken a t moderately short times. We speak of this briefly because of its utility, particularly in kinetic studies. Basically, the object is not to delineate the entire emf vs. time curve, but to get an accurate prediction of the equilibrium value. But in simpler terms, “one is interested in where he is going rather than where he has been.” For some time, here a t Baton Rouge, we have looked into this matter in collaboration with Dr. Doris Muller and Dr. Philip W. West. Measurements of emf to 20.1 mV as a function of elapsed time were made with an Orion Ag,S membrane electrode for “jump” increments of Ag+ ion. The data were accurately represented b y the equation: E = $ / ( a b t ) where E is the increase in potential caused by the sudden increment of Ag+ concentration. The curve is a hyperbola and a plot of t / E us. t yields an excellent straight line, the slope of which is b and the intercept is a. The maximum value of E (for t = m ) is equal to l l b , the reciprocal slope. As expected, there is a small but progressive deviation from linearity for the lower values of t in which range the experimental conditions are ill-defined. The prediction of the equilibrium potential E, does not involve any dubious extrapolation procedure because it is given by the reciprocal slope of a straight line and the degree of linearity of this line is a reliable criterion of the confidence which can be placed in the calculation of E,.
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116 A
ANALYTICAL CHEMISTRY
For a given value of consbant 6 , which defines E,, an infinite number of hyperbolas can be drawn, all converging to E, a t t = m , depending upon the choice of constant a. These would range from almost instantaneous response to almost infinitely slow or sluggish response. The ratio of these const'ants a (intercept) and b (slope) is a useful quantity; for example, the half-time, t i / , = a / b . The time required to attain any desired fraction f of the equilibrium value E , is simply: t, = [//(I - f ) ] x a / b . Beyond this, const'ant a is a reliable response speed measure of the electrode, but more useful when used in the ratio a l b . Some doubt may be expressed that E, is attained only a t infinite time. The apparent stabilization a t finite and relatively short times is a function of the sensitivit'y of the detecting system, and simple "eyeballing" of a recorder trace is not reliable. At very long time intervals after apparent equilibrium had been attained, we could detect increments in potential of the order of 10-50 pV. We have examined several curves reported in the literature, as far as our patience in disinterring the data permit,ted, and they were found to yield well t'o this treatment. The empirical approach to this problem is useful because the calculated values agree with the observations with the same precision t,hat the data are known. For want of better information, such results are preferable to plausible theoretical derivations yielding results in which the agreement between calculated and observed is called "encouraging." The hyperbolic relationship is not without significance for a n ultimate understanding of the time-response phenomenon. I t bears a formal, if not accidentai, similarit,y to the Langmuir adsorption isotherm and is, at least circuitwise, akin to the equivalent resistance of two resistors in parallel, one of which is changing uniformly with time. I n the latter case, it is conceivable that such an arrangement could be set u p as a compensator to turn out an emf value corresponding to the true Eo value over a reasonable period of elapsed time. One might even drive it with a n alarm clock. Dat,a supporting these conclusions are being submitted to this Journal. We obtrude these few generalizations upon this commentary merely in the sense that there are other questions relating t o selective ion electrodes wort,hy of detailed inquiry. They should not detract from the larger design and vista which Dr. Rechnitz has outlined for us.
