Membrane-type indicator electrodes

Tissue electrodes [2, 3, 4, 5, 45,57], In these biosensors, a thin layer of tissue is attached to the internal sensor. The enzymic reactions taking place in the tissue liberate products sensed by the internal sensor. In the glutamine electrode [5, 45], a thick layer (about 0.05 mm) of porcine liver is used and in the adenosine-5 -monophosphate electrode [4], a layer of rabbit muscle tissue. In both cases, the ammonia gas probe is the indicator electrode. Various types of enzyme, bacterial and tissue electrodes were compared [2]. In an adenosine electrode a mixture of cells obtained from the outer (mucosal) side of a mouse small intestine was used [3j. The stability of all these electrodes increases in the presence of sodium azide in the solution that prevents bacterial decomposition of the tissue. In an electrode specific for the antidiuretic hormone [57], toad bladder is placed over the membrane of a sodium-sensitive glass electrode. In the presence of the antidiuretic hormone, sodium ions are transported through the bladder and the sodium electrode response depends on the hormone concentration. 

To measure the e.m.f. the electrode system must be connected to a potentiometer or to an electronic voltmeter if the indicator electrode is a membrane electrode (e.g. a glass electrode), then a simple potentiometer is unsuitable and either a pH meter or a selective-ion meter must be employed the meter readings may give directly the varying pH (or pM) values as titration proceeds, or the meter may be used in the millivoltmeter mode, so that e.m.f. values are recorded. Used as a millivoltmeter, such meters can be used with almost any electrode assembly to record the results of many different types of potentiometric titrations, and in many cases the instruments have provision for connection to a recorder so that a continuous record of the titration results can be obtained, i.e. a titration curve is produced. 

The cell consists of an indicator and a reference electrode, the latter usually being the calomel or silver-silver chloride type. The potential of the indicator electrode is related to the activities of one or more of the components of the solution and it therefore determines the overall cell potential. Ideally, its response to changes of activity should be rapid, reversible and governed by the Nernst equation. There are two types of indicator electrode which possess the desired characteristics - metallic and membrane. 

We will study two broad classes of indicator electrodes. Metal electrodes described in this section develop an electric potential in response to a redox reaction at the metal surface. Ion-selective electrodes, described later, are not based on redox processes. Instead, selective binding of one type of ion to a membrane generates an electric potential. 

An ideal indicator electrode responds rapidly and reproducibly to changes in the concentration of an analyte ion (or group of analyte ions). Although no indicator electrode is absolutely specific in its response, a few are now available that are remarkably selective. Indicator electrodes are of three types metallic, membrane, and ion-sensitive field effect transistors. 

Sohd state electrodes may he constructed as crystal membrane or precipitate-impregnated membrane type. The latter appeared on the scene first ca. 1950, but because of technical difficulties in their construction httle advance in use was apparent until 1966. Around this time, both forms of sohd state electrode started a climb to prominence as indicated by Buck [12] 1968, Eisenmann [13] 1969 and Ross [14] 1969. 

These examples clearly show how the EIS measurements allow the estimation of the membrane electrical parameters (R and C however, they also indicate the possibility of obtaining qualitative information on the membrane structure, which can also be of great interest. In any case, it should be pointed out that impedance is an extensive magnitude (it depends on the sample area), and for that reason, comparisons of the type of curves and the concentration dependence instead of particular values are usually made. In addition, IS measurements with the dry and wet membranes, but without an electrolyte solution between the electrode and the membrane surface, can also be performed and complementary information, mainly related to the membrane material itself or the interfacial (electrode/membrane) effects, can be obtained. 

Hgure 1 A simplified scheme of the CO2 membrane gas probe (Severinghaus type). 1, Detector body 2, indicator electrode (pH glass electrode) 3, reference electrode 4, internal electrolyte 5, gas-permeable membrane 6, medium analyzed and 7, voltmeter (pH-meter). (The components of the probe are not drawn to a real scale.)... 

Potentiometric detectors typically measure the potential difference (A ) across a membrane, which originates from the difference in analyte concentration in the eluent versus an internal reference solution. The most common potentiometric measuring device is a pH electrode, in which a glass membrane responds to hydronium ion concentration in the test solution. Other ion-selective, or indicator, electrodes are also available commercially. The attribute of an indicator electrode to be highly selective for a particular species is also its drawback, in that a different electrode is needed for each type of ion. Halides and sulfates can be monitored using silver/silver salt and lead/lead salt electrodes, respectively [50]. 

The essential component of a potentiometric measurement is an indicator electrode, the potential of which is a function of the activity of the target analyte. Many types of electrodes exist (see Table 9.1), but those based on membranes are by far the most useful analytical devices. The broader field of potentiometry has been reviewed recently (1). The potential of the indicator electrode cannot be determined in isolation, and another electrode (a reference electrode) is required to complete the electrochemical cell. Undoubtedly the best known of the potentiometric indicator electrodes is the glass pH electrode, the operation and use of which has been adequately discussed (2). Ion-selective electrodes (ISEs) are also commonplace, and have been the subject of several books (3-5) there is even a review journal for ISEs (6). Unfortunately, the simplicity of fabrication and use of ISEs has given rise to the idea that ISEs are chemical sensors. At the best this is a half-truth certainly, they can behave like chemical sensors under well-controlled laboratory conditions, but in the real world their performance leaves much to be desired. Moreover, from a manufacturing point of view important features of a sensor are that it can be fabricated in relatively large numbers, and that each device is identical to all the others. Although some ISEs can be mass-produced , many cannot, and even those that do lend themselves to this form of production invariably require calibration before use. Nonetheless, in spite of the limitations of ISEs, transducers based on potentiometric membrane electrodes have much to contribute to the field of chemical sensing. 

FIGURE 3.15. Volta-potential ( ) differences discussed in the text Me is the electrode metal or another conductor in electronic equilibrium with it 5 is the polarized electrolyte poly represents a membrane-type polymer film deposited on the electrode. The subscript ex indicates that the electrode was withdrawn from the electrolyte under potential control. 

Table 23-2 lists ihe various types of ion-selective membrane electrodes that have been developed. These differ in the physical or chemical composition of the membrane. The general mechanism by which an ion-sclective potential develops in these devices depend.s on the nature of the membrane and is entirely different from the source of potential in metallic indicator electrodes. We have seen that the potential of a metallic electrode arises from the tendency of an oxidation-reduction reaction to occur at an electrode surface. In membrane electrodes, in contrast. Ihe observed potential is a kind of junction potential that develops across a membrane thal separates the anidyte solution from a reference solution. 

Fig. 17.14 Hours of lifetime at 120°C (before catastrophic failure of the PEM) versus fluoride ion release rates (by IC) for NSTF and Pt/C catalyst-based membrane electrode assemblies (MEAs) having the same type PEM and GDL. Cells of 100 cm2 were operated at 0.4 A/cm2, 120°C, 300 kPa, 61/84% inlet relative humidity (RH). Electrochemical surface area and crossover were measured daily at 75°C. Total lifetimes were -- CSOO h for the NSTF MEAs due to diagnostic testing at 75°C. End-of-life criteria were severe falloff of cell voltage and corresponding ramp-up of F-ion release indicative of membrane pinhole formation. From reference [5]...

Kounaves and coworkers [70] have studied the Ag AgCl systems covered by membrane made of Nafion or polyurethane. These polymers were used to protect the studied solution from a NaCl leakage from the electrode solution. The studies on the stability of potentials of such electrodes have shown that the potentials of the electrodes protected by Nafion were significantly less stable than those covered by the polyurethane membrane. The drift of the potential, in the initial stage, could result from a slow equilibration between the Ag AgCl phase and KCl immobilized in the internal membrane. Further drift could be a consequence of the hydration of the external Nafion membrane. In the case of chloride solutions electrodes of both types exhibited decrease of the potential (about —41 ( 1) mV dec ) with the increase of chloride ions concentrations (from 10 mol dm to 1 mol dm ), showing a behavior similar to that of an indicator electrode. This change could result from the diffusion of more concentrated solution of chloride ions to the electrolyte immobilized in the poly(vinyl chloride) membrane situated under the external protective layer made of Nafion or polyurethane. 

Table 9.4 summarizes ion-selective electrodes of practical importance. They are classified according to analyte, with the membrane type and brief information on the major ingredient indicated in the secrnid and third colunm. The last colunm shows information on ion selectivity by giving logarithmic selectivity coefficients over the indicated potentially interfering ions. For references, see Table 9.4 and reference (15)... 

In potentiometry the potential signal is measured at zero current intensity (Scholz, 2010). The typical potentiometric analytical cell has two electrodes immersed in a solution containing the analyte, whose concentration is to be measured (Fig. 13.1). The reference electrode (RE) has a constant contribution to the signal, independently of the solution matrix. Usually it contains a metal electrode in contact with an insoluble salt of the same metal (second type of electrode) and its potential depends only on the solubility of the salt. The most used RE is the silver/silver chloride (Ag/AgQ). The second electrode is the indicator electrode (IE) that contains a membrane sensitive to the... 

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