Garry A. Rechnitz University of Pennsylvania Philadelphia
Cation-sensitive Glass Electrodes
Glass electrodes have gained nearly universal acceptance as versatile tools for the estimation of hydrogen ion concentrations under a great variety of circumstances. Although many details of their mode of operation are still only incompletely understood, glass electrodes come close to being ideal measuring devices because of their specificity and experimental simplicity, especially in situations where disturbances of the system to be studied must be minimized. I t would, therefore, be highly desirable to he able to devise glass electrodes capable of responding to chemical species other than hydrogen ion, particularly if the favorable characteristics of the classical glass electrode can be retained. Fortunately, owing to the efforts of Eisenman ( I ) , Schwabe (t), and many others, research in this area has progressed sufficiently that such electrodes are now commercially available.' Detailed directions for the preparation of electrodes havc also hren given in the literature (3). These electrodes, which may be the first step toward a whole range of specific ion electrodes, are particularly responsive to univalent cations such as Na+. K+, Li+, Rb+, Cs+, Ag+, NHn+, and H + and show negligible response to reasonable concentrations of other cations. Aside from conductivity measurements, which are limited to low concentration ranges, glass electrodes provide the first practical, nondestructive means of continuously monitoring alkali metal ion concentrations in solution. On this basis alone the acceptance of such electrodes is assured, but some of the less direct applications outlined here may prove to he of even greater usefulness in the futurp. Theory
The potential of cation-sensitive glass electrodes as a function of cation concentration can be most nearly described by means of the Xernst equation
where a,+ is the activity of the cation being measured, provided proper pre-treatment has been carried out and the univalent cation of interest is the only such ion present in substantial amounts. The potential developed in the presence of two types of univalent cations, A+ and B+, can he expressed by a more complicated relationship given by Eisenman, Rudin, and Casby (4)
where K is an em~irical~arametercalled the selectivitv 'From Beckmim Instruments, Inc., Fullerton, Calif., and Hlect,ronic Instruments, Iitd., Rirhmond, Snrrq-, (;re:~tBritain.
ratio. Equation (2) has been found to be generally valid for Na+, K+, Li+, Rb+, Cs+, Ag+, and NH4+at least in the pH range where the hydrogen ion concentration is small compared to that of the other cations. Some uncertainty still exists regarding the origin of glass electrode potentials. The two most popular mechanisms, involving diffusion and ion exchange phenomena, respectively, cannot as yet he unambignonsly distinguished by experimental means although actual penetration of the glass membrane by substantial amounts of hydrogen or metal ions has been ruled out by the conlometric tracer experiments of Schwabe and Dahms (5). An even more puzzling question concerns the origin of specificity in glass electrodes. Why is it that a change in glass composition alters the order of selectivity among alkali metal ions? The answer certainly involves the relative free energies of interaction of the ions with the solvent and the glass surface but our knowledge of this situation is, a t present,, cntirely empirical in nature and consists largely of correlations between glass properties and observed effects. A comprehensive discussion of this problem has been given by Eisenman ( I ) . Applications
Direct Potentimnetric Measurement. In solutions containing only a single type of univalent cation the electrodes behave as predicted by the Nernst equation over a concentration range of 4-5 orders of magnitude. .4t very low concentrations ( < l o - 4 M ) of univalent cations deviations from the expected Nernst slope are ilsually encountered owing to background contributions from solvent and electrolyte. Applications a t very high cation concentrations are limited by solubility and activity considerations. Because these electrodes also show a response to hydrogen ion, to a greater or lesser degree depending on glass composition, the pH of test solutions must be controlled so that the concentration of hydrogen ions is negligible compared to that of the cation of interest. Because of the nearly ideal electrode response to univalent cations over a moderately broad concentrat,ion range, direct potentiometric measurements have been used to determine alkali metal ion concentrations in a variety of simple and complex media. Bower (6) measured Na+ concentrations in saline solutions, Taulli (7) in acidic silica sol systems, and Mortland (8) estimated exchangeable K+ in aoils in this manner. The same technique was extended to the determination of silver ions by Budd (9) while H y m n (10) carried out fundamental activity measurements using such cation-sensitive glass electrodes. Obviously, the method is applicable, as ivell, to the determinat,ion of any material which can be converted to a suitable Volume 4 1 , Number 7, July 1964
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ration, as was shown by Katz and Rechnitz (11) in the estimation of urea after hydrolysis to ammonium iou by the action of urease. The precision and accuracy of such direct measurements is still limited primarily by the reliability of the glass electrodes whose measured potential in identical solutions may fluctuate by as much as i l millivolt despite careful pretreatment and random measurement,^ to eliminate time effects. Because of this uncertainty there is little benefit to be gained, a t present, by resorting to elaborate electrometer systems rather than commercially available pH meters for those measurements. Some workers have claimed relative accuracies of the order of 0.3% using specially constructed electrodes (3). Potentiometric Titration. The accuracy of univalent cation determinations can he improved considerably when the glass electrode is used to indicate the end point of a titration or other process involving the chemical generation or removal of the ion of interest. Following the preliminary studies of Geyer and Frank (I,%?),Rechnitz, et al. (IS) devised direct potentiometric titrations for the determination of K+, Cs+, Rb+, Ag+, and NH,+. Using a calcium tetraphenyl boron titrant, amounts of these ions ranging from 14 to 120 mg were easily determined with a relative error of a few tenths of a per cent. This technique should also be useful in the determination of Na+ and Li+ provided suitable titrants can be found. Equilibrium Studies. The most fruitful application of cation-sensitive electrodes may turn out to be their use in the study of ionic equilibria. If these electrodes respond only to the monovalent cations, then measure ments can be devised to yield information about chelat,ion, complexation, ion pairing, and other proccsses which involve changes in the equilibrium concentrations of the free (solvated) ions. Palaty (14) has already used this type of measurement to study sodium cbelates of ethylenediaminetetraacetic acid and has presented evidence for the existence of a previously unsuspected hydrogen chelate in neutral solution. Rechnitz and Brauner (16) have recently extended this technique to the study of weak complexes of the alkali metals, showing good agreement for formation constants calculated by direct measurement of sodium ion activity in the presence of malate ions with values determined by independent methods. Such studies essentially involve the direct measurement of cation activities under complexing and noncomplexing conditions and, therefore, are limited a t present by uncertainties in the electrode potentials developed. Improvements in electrode design and measuring techniques should facilitate precise studies in the future, however. Nonaqueous Systems. Information concerning the behavior of cation-sensitive glass electrodes in nonaqueous solvents is almost totally lacking. Recent
386 / Journal o f Chemicol Education
experiments (10) in ethanol-water and acetonewater mixtures, however, have shown promising indications that glass electrode potentials can be predicted by the Xernst equation (at least for KC1 solutions) in systems containing up to 90% of the organic constituent. Small deviations in the slopes of potential-log concentration plots from the "ideal" values can be largely accounted for on the basis of activity and solubility considerations. The surprising finding that differences in absolute potential values for different solvents and solvent-water ratios arise largely a t the reference electrode junction and not a t the glass electrode suggest that glass electrodes might, themselves, be useful as reference electrodes in nouaqueous solvents. Future Developments
A great deal of work needs to be done in all of these areas of investigation. From the fundamental viewpoint, it should be possible to learn much more about the mechanism of electrode response and selectivity using electrodes responsive to several ions than was possible with the pH type electrodes which lacked this extra variable except in highly alkaline regions. Little use has, as yet, been made of these electrodes to follow the rates of chemical reactions. Response time measurements (17) indicate that this application should be feasible for all but fast reactions. Finally, it seems likely that glass electrodes responsive to other cations will become available in the future; glasses have already been found which show significant response to such divalent cations as Ca++; even anion-sensitivc glass electrodes are not ruled out by basic considerations (1). Literature Cited
EISENMAN, G., H w p h y ~J., . 2,259 (1962). SCHW~BE, K., AND H. I>AHMS, Z . Ekktroeheul., 65, 518
- --
1 \ 10 ..fil,>.
PORTNOY, H. D., L. M. THOMAS, A N D E. P. GURDJIAN, Tahnta, 9,119 (1962). EISENMAN, G., RUDIN,D. O., AND CASBY, J. U., Science, 126.831 (1957).
AND DAHMS,H., finatsber. Ihul. ilkart. Wiss. Berlin., 1.2791195O). - , -~ -...,. BOWER,C. A,, Soil Sei. Soc. of Am. Proc., 23,2!l(1!159). TAULLI, T . A., Anal. Chem., 32,186 (1960). MORTIAND, M. M., Quart. X d l . , Mieh. Anr. E . ~ DSt~tlmn. . S C R W ~ E ,K.,
43,491 (1961).
BUDU,A. I.., J . Electmanal. Chenh.,5,35 (10633) HYMAN, E. S.,Anal. Chem.. 34.365 (19621. KATZ,5. A,, *KD RECHNIT~, G.'A.,i.An&. Chem., 196,XR (1963).
it. AND FRANK,H., Z.Anal. Chenl., 179,99 (1961). RECRNITZ, G. A,, KATZ,S. A., AND ZAMOCHNICK, S. R., Anal. Chem., 35,1322 (1963). PALATY, V., Can. J. Chem., 41,18 (1963). RECHNITZ, G. A., A X D I~RAUNER.J., Talania, 11,617 (1964). I~ECHXITZ, (;. A., .\VD Z.\MIICHNICK,S. 13.. Talanta, in preas. SAVAGE, J. A,, AX,, ISARD, 1'. O., Phr,.~. Chem. Glasses, 3, G~YEE,
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