Potentiometry
Ion and Bio-Selective Membrane Electrodes Garry A. Rechnitz University of Delaware, Newark. DE 19711
In view of the great importance of ion-selective and other potentiometric membrane electrodes to chemistry, biology, and medicine, it is remarkable how little attention is given to this suhiect in the averaee undergraduate curriculum. The fault may lie with the av&ahle teaching sources; almost no general chemistry text even mentions membrane electrodes. Analytical chemistry textbooks, especially those oriented toward instrumental analysis (1-4), generally cover the membrane electrode field in a single chapter, whereas, the newest textbook on "Electrochemical Methods" devotes less than nine pages to the entire spectrum of potentiometric membrane electrodes (5). Almost all textbook treatments of the subject are derived from one another or from a 1967 review article. As a result, the scone and level of the treatment tend to be similar and outhated. While there are numerous monographs (6-7) and current review articles (8) available, these tend to overwhelm the typical student with too much detail. I t is clear, therefore, that there exists a gap between the standard textbooks and the research literature; it is this gap which challenges the instructor in anv effort to wrovide an effective treatment of potentiometriE membrane electrodes for undergraduate students. Teaching Strategies Two approaches are commonly used in teaching the principles of ion-selective memhrane electrodes:
of membrane potentials.
While both of these approaches have merit, a synthesis of their key elements is desirable. Such a unification is possible through the concept of electrode "selectivity" and involves a discussion of the principal types of membrane materials in terms of the various bulk and interfacial processes which operate to give rise to electrode selectivitv. The obvious similarities, which become apparent as particular membrane electrode materials are considered. lend continuitv and conceptual unity to the treatment. One word of caution is in order, however. Most undergraduate students do not readily grasp the distinction between metallic and membrane electrodes. As a result, it is absolutely
282
Journal of Chemical Education
essential to explain in detail both the physical construction and experimental arrangement for membrane electrodes. The key point which needs to be established is that membranes have two sides (or interfaces) and that the processes occurring at one side have to be known or controlled if meaningful measurements are to be made with membrane electrodes. This point very naturally leads to a discussion of reference electrodes and the special requirements imwosed when dealine mathematical fashion, e&eciall; if the lectu& is not coordinated to a laboratory experiment.
types of commercial electrodes can be passed hand to l&d among the members of the class. Such actual handling of memhrane electrodes tends to "de-mystify" the topic very effectively. Another point to he stressed in introducing membrane electrodes is that such devices are strictly a c t i h y sensors. Although students will be familiar with the concewts of accourses involve concentration measurements. ~ o r e o i e r , confusion can arise from the operational nature of the pH scale. Indeed, it is desirable to perform a lecture demonstration, perhaps using a sodium ion-sensitive glass electrode to show that the potentiometric data obtained in a series of NaCl solutions clearly follow the activity and not the concentration plot. The concept is then readily extended to ion-selective memhrane electrodes in general.
Figure 1. Enlarged view of glass membrane electrode showing internal and external reference elements.
Table 1.
Glass Electrode Selectivity Na20- 4 % AI203 - 69% SiOd
Students are always surprised that the contributions of ex-
(Composition of Membrane: 27%
Kt to Naf Selectivity
Method Potentiometric Calculated from K,
'K ,
8.5 8.9
= 102: W ~ + l ! . & ~ +=l 0.088
Table 2. I,
(&K+/$LN.+)~
Response of the Ag,S Membrane Electrode
Ionic conductor for Ag+, thus
but [As+] = (K,ol[S=])"2 and,
Ii.
What is the effect of pH? Consider two-step hydrolysis
and
E = const. -slope (pH) but, at
lower pH [SS] a K , (K2l[H+l2)
and
E = const. = 2X slope (pH)
it &sv to e x ~ l a i nwhv " elass electrodes for multivalent cations (poor mobility) and for anions (glasses are generally cation exchangers) are not likely to he developed. It remains only for the instructor to connect ionic processes with electrochemical behavior, perhaps using radiotracer data, to complete the ground work for a satisfying, even if semi-quantitative, understanding of glass memhrane electrodes. Some class exercises involving numerical manipulation of the Nernst equation, selectivity ratios, etc., will be helpful at this stage. Liquid Membrane Electrodes
Once the key principles of the response of the glass electrode are established, liquid membrane electrodes can he explained readily by focusing attention on the key difference; e.g., the sites are mobile and are able to travel to the ion! Of course, a distinction must be made between liouid memhrane electrodes which function via ion exchange or extraction and those which emnlov . . ion carriers. Fortunatelv," . the transport consideration entering into the selectivity properties are not too different. exceot with regard to charee. The older type of demountable liquid membrane electrodes, such as the Orion kits. are particularly convenient as a teaching tool in explaining theeonstruction and components of this t w e of membrane electrode. Special care must be taken to distinguish between the active liq&d membrane phase and any support membranes or matrices; it should be pointed out that the latter primarily functions to maintain a coherent liquid phase whether in a free-flowing or immobilized form. The formal similarity between liquid-membrane and glass membrane electrodes should be stressed. iustifv No new concents are eenerallv needed to . " the fact that the form of the Nernst equation for combinations of ions remains the same as for the glass electrodes. Students readily accept the fact that partition and association equilibria can replace exchange constants and mobilities in the selectivity expressions, hut the mobile versus fixed site situation should he made exulicit. Neutral carrier membrane electrodes lend themselves especially well to visualization of mohile ion-accepting sites and, moreover, offer a splendid opportunity to discuss the effect of molecular architecture on selectivity.
-
Glass Membrane Electrodes Crystal Membrane Electrodes
Although the pH type glass electrode is the single most widely used membrane electrode and predates almost all other such probes, its extremely high selectivity for the hydrated proton over other ions and its extraordinary response range Hctually limit its usefulness in demonstratingthe concepts &d properties of ion-selective membrane electrodes. This is not to that pH electrodes cannot be used as a point of departure in a discussion of glasses (e.g., electrical properties, amorphous structure, liquid nature, etc.), hut it is only under the limiting conditions of the "alkaline error" that the concept of selectivitv becomes meanineful. A straightforward, perhap;graphical, discussion of the relationship between elass composition and observed ion selectivity is-always appreciated-by the student. If a simple, three-component glass forming mixture such as Na20A1203-SiO2 is chosen for discussion, then all the major types of glass based ion electrodes can he illustrated and discussed on the molecular level in a single diagram. When this is combined with a cross-sectional representation of the various phases and interfaces that make up the operative portion of glass membrane electrodes, the relationship between the ion exchange and diffusion processes and electrode selectivity becomes readily apparent. The data in Table 1for a typical fixed site membrane show that the observed uotentiometric
Gas-Sensing Membrane Electrodes
ionic mobilities in the hydrated layer at the-glass surface.
The concept of a "building-block" approach, also useful for bioselective membrane electrodes, helps students to understand the nature and operation of potentiometric gas sensors.
say
The mechanisms of crystal memhrane electrodes can be discussed at almost any depth desired by the instructor. At the undergraduate levd, it & generally adequate to make the following principal points: The membrane may be made of single crystals, polycrystalline pellets, or mixed crystals. 2) Since the crystals used are generally sparingly soluble salts,their selectivity patterns can be predicted (except far solid solution formation,etc.) on the basis of solubility product constraints. 3) Some electrodes, e.g., the LaF3 type, are limited by surface film formation. 4) Stable uotentials can be obtained with either electrolvtic or solid-state internals, but the redox properties and temperature effectsare different. 1)
Further elaboration of these principles might include consideration of chemical steps in the solution phase, such as the effect of pH on electrode response (Table 2). Examples of applications in environmental and clinical measurements are plentiful and may be fruitfully cited to provide realistic limits about electrode sensitivity, resuonse. and .. selectivitv. .. dvnamic . useful lifetime.
Volume 60
Number 4
April 1983
283
only necessary to connect the m e a s ~ r e b b u a n t iwith t ~ the gas to he determined; e.g.,
'
+
[H
1 1
11 / NH
.....---d r m e
- KI[HZCO~I - KIKa[CO?I - [HC03-] [HCOn-]
and
. .....-cap
E = const.
RT
+ln[C02] F
for the case of the COa electrode. There are three elements of the gas electrode assembly which tend to confuse students and, therefore, require discussion:
...~..-. gas membrane Top View
.
is necessary since only the pH of the electrolyte layer is to be
mr~ni+nma ...,,...
If time permits, the discussion of gas electrodes can he extended to systems where the pH electrode is replaced with an ion-selective membrane electrode. The possibilities of making H F or H2S gas sensing electrodes using the F and S 2 crystal memhrane electrodes, respectively, adequately illustrate this point. Applications to clinical blood gas measurements, environmental analysis, and industrial process control are well received by undergraduate students.
enclosed bacteria
bacterial sandwcn
-
:'
sealed
held on by external cap
edge
S d e Viy:'
Figure 2. Typical bioselective membrane electrodes using bacterial cells as mediators.
which relates the various hiocatalvsts used to date in order of increasing organizational complexity. Care should he taken to explain the various advantages and disadvantages of each type of mediator and to establish some understanding of the factors which govern the choice of mediator for a particular situation. Considerations of activiiv levels. ~" -~ --., -interferenrea -. ~.--. .. . .... co.factors, activators, and lifetime are appropriate in connection. Since this is one of the most rapidly developing sub areas of the membrane electrode field, special care must manner and he taken to treat such material in an ooen-ended ---~---~.not leave students with a mere historical appreciation of the topic. ~
Enzyme and Other Bioselective Membrane Electrodes The building block concept is particularly useful in any discussion of hioselective memhrane electrodes. Here we have added another phase component to the ion or gas sensing electrodes previously discussed. That added phase is a hiocatalyst or, more generally, a mediator which converts the biochemical substance to he measured to a s ~ e c i e detected s by the memhrane electrode sensing element. For students who have had some exposure to hiochemistrv. the behavior of such enzyme electrodes can he convenientl; discussed in terms of such familiar Darameters as K ~, and V,,,. A wide range - of potentiometric memhrane electrode systems involving immohilized enzvmes as biocatalvsts is available in a review of the literaturei8) for possihle "se as illustrations of applications, immobilization techniques, miniaturization, etc. The newer types of bioselective memhrane electrodes using mitochondria, bacterial cells, or plant and animal tissue as 21, can be treated as loeical extensions of hiocatalvsts (Fie. u " earlier electrodes based on isolated enzymes (9).Indeed, i t is possihle to present a hierarchy of mediators: Enzymes
bacler~al smawicn
..~..~. ~-
1) The hydrophobic, gas-permeablemembrane which serves no net chemical or electrochemical function hut secures a reproducible
space lor gas/eleetrolyte separation. 2 ) The layer of electrolyte solution (NH4Clfor the NHBelectrode and NaHCOs for the Con electrode) in contact with the pH sensing membrane. These control the equilibria necessary for the mass action between dissolved eas and OH.
...~.
cap
~
~~~
~~
Conclusions The material outlined above represents a bare-bones treatment of memhrane electrodes corresnondine to anproximately six hours of lecture time. If the course structure permits a more extended discussion, the followine additional topics are recommended:
-
.
CHEMFETS and ISFETS Mechanistic treatment of ion translocation in membranes Immunoelectrodes Theory of steadystate and dynamic response Coated wire, flow-through,and other special electrode configurations.
There are, of course, numerous general and specific application areas which could he explored in connection with membrane electrodes. Selection of application examples is best tailored to the nature and level of the class; premedical students might profit from a clinical orientation while engineering students may prefer application examples drawn from environmental and process control systems. Literature Cited
i Enzyme sequences I
i Organelles (Mitochondria)
J. 1 Plant or animal tissue Whole cells
284
Journal of Chemical Education
(1) Willard. H. H., Me~ritt,L. L.. Dean, J. A , Settle. F. A.. "Instrumental Methods of Analysis."D. Van Nustrand Cu.,New Ymk, 6th ed., 1981. (2) Skmg, D. A,. West. D. M., "Principles of Indrumental Analysis," Seunders Cullege, Philadelphia, 2nd ed., ,980. (31 Bauer, H. H.. Christian, G. D.. O'RoiUy, J. E.. "Instrumental Analpis," Allw and Bacon. Inc.. Boston. 1978. (4) Pease, B. F.. "Basic Instrumental Analysis," D. Van Nostrand Co.. New Ymk, 1980. (51 Bard, A. J., Faulkner, L. R.. "Elartruchemicd Methods: duhn Wiley & Sons, i n c , New Yuck 1980. (6) Reiasr,H.,(Ed~lor~,"Ion~SalediveRlectrdesinAnd~calChemistiy,"I~lenumPrers, New Yurk, Vol. 1, 1878,and Vui. 2 , 1980. (7) Lakshminaraysnaiah, N.. -Membrane Electrodes," Academic preaa, 1nr. New York,
