Internal indicator electrodes

Neither the usual membrane ISEs nor the gas-sensing electrodes, in which their internal indicator electrode functions as a zero-current potentiometric half-cell, are under consideration here. 

An example of amperometric methods used for analytical purposes is the sensor proposed in 1953 by Leland C. Clark, Jr. for determining the concentration of dissolved molecular oxygen in aqueous solutions (chiefly biological fluids). A schematic of the sensor is shown in Fig. 23.1. A cylindrical cap (1) houses the platinum or other indicator electrode (2), the cylindrical auxiliary electrode (3), and an electrolyte (e.g., KCl) solution (4). The internal solution is separated by the polymer... 

Fig. 18a.l. Schematic diagram of a potentiometric cell with an ion-selective electrode (ISE) as the indicator electrode. EM is the electrical potential of the sensing membrane and IFS the internal filling solution. 

In short, therefore, the potential of the cell comprising of the Ag/AgCI reference electrode (E) i.e., the internal reference and the specific ion electrode (B) i.e., the indicator electrode is normally determined by the C02 concentration of the extemal solution containing dissolved gaseous analyte. 

Epoxy-based membrane of 2-[(4-chloro-phenylimino)-methyl]-phenol reveals a far Nemstian slope of 43 mV per decade for Pb+2 over a wide concentration range CIO 6 to 10 1 mol dm-3). The response time of the electrode is quite low (< 10 sec) and could be used for a period of 2 months with a good reproducibility. The proposed electrode reveals very high selectivity for Pb(II) in the presence of transition metal ions such as Cu2+, Ni2+, Cr and Cd2+at concentrations l.()xl() 3 M and 1.0><10 4 M. Effect of internal solution concentration was also studied. The proposed sensor can be used in the pH range of 2.50 - 9.0. It was used as an indicator electrode in the potentiometric titration of Pb+2 ion against EDTA. 

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. 

Any electrochemical device using a low molecular weight redox couple to shuttle electrons from the redox center of an enzyme to the surface of an indicator electrode, thereby increasing the effectiveness of amperometry in the detection of a substrate for the particular enzyme. The internal cavities of six-, seven-, and eight-membered cyclodextrins are trapezoids of revolution with larger open mouths dimensions (/. c., respective diameters of... 

Figure 4.17 — (A) Exploded view of a tubular flow-cell integrated microconduit system. I Ag/AgCl inner reference electrode M sensitive membrane S internal reference solution. (B) Detail of the integrated microconduit shown within the dotted lines in C. (C) Integrated-microconduit FI manifold for potentiometric measurements C carrier stream R reference electrode solution P pump V injection valve I indicator electrode R reference electrode I pulse inhibitor G ground W waste. (Reproduced from [140] with permission of Pergamon Press).

An example of an electrochemical cell for potentiometry is shown schematically in Fig. 1. The cell consists of an external reference electrode and an indicator electrode immersed in a test solution of analyte with some activity, (aOsampie- The indicator electrode is constructed of a reference electrode contained within a membrane and an internal reference electrolyte of fixed activity, (aOmtemai- The potential (F ceii) is measured by a pH/mV meter, and is equal to the sum of the potential between the internal (Fref.int) and external ( ref,ext) reference electrodes, the membrane potential (i memb), plus the liquid junction potential (Ey) that exists at the junction between the external reference electrode and the sample solution. 

The potential of a glass indicator electrode has three components (1) the boundary potential, given by Equation 21-9 (2) the potential of the internal... 

An ion-selective electrode (ISE) is a sensor with a membrane which is designed so that its potential indicates the activity of a specific ion in an electrolyte solution. The membrane may be a solid, either crystalline or glassy, or a liquid. The best known ISE is the glass electrode used to determine pH. The membrane not only develops a potential difference which responds to the unknown ionic activity in the test solution, but it also separates completely the test solution from the internal reference solution of the ISE. Three different ISEs involving glass, crystalline, and liquid membranes are shown in fig. 9.6. All of these systems involve an internal reference electrode. In each case, the membrane is at the bottom of the ISE, which is dipped into the test solution. More details about each of these electrodes are given in the following discussion. 

The potentiometric technique involves the use of glass, ISE and platinum electrodes, the latter used in connection with nearly all oxidation-reduction titrations. These electrodes use external or internal reference electrodes. In the main, the reference is an Ag/AgCl (3M KCl) unit with an outer compartment capable of being filled with an electrolyte of choice and changeable. For chloride titrations, for example, the indicator electrode is often a silver billet coated with AgCl, with a Ag/AgCl reference 3M KNO3 filled. 

The indicator electrode and the reference electrode are joined externally through a voltmeter (potentiometer) that is of the type which draws very little current because it has near-infinite internal resistance. Because there is little current the reaction at the indicator electrode does not shift perceptibly from equilibrium and there is no significant consumption of the species of interest at the electrode. The internal contact between the indicator and reference electrodes is through a salt bridge or liquid junction that allows the passage of ions but does not permit significant mixing of solutions. The potential of the cell is... 

Figure 15.7 (a) A schematic glass electrode for pH measurement, (b) A complete pH measurement cell, with the glass indicator electrode and an external saturated calomel reference electrode, (c) A commercial combination pH electrode, with built-in internal Ag/AgCl reference electrode. 

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.)... 

Gas Principal component of the internal electrolyte Equilibrium reaction Indicator electrode... 

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 standard hydrogen electrode is the universal and internationally agreed lUPAC (International Union of Pure and Applied Chemistry) reference for reporting relative half-cell potentials. It is a type of gas electrode and was widely used in early studies as a reference electrode, and as an indicator electrode (Chapter 18) for the determination of pH values. It is an arbitrary reference and in principle any metal/metal ion system could be used as reference electrode. To use an analogy, heights are measured using sea level as an arbitrary zero. Any level above or below sea level could be used as the reference point. 

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. 

In many cases, only the first two eonditions ean be fulfilled, with the result that the measured potential of an indicator electrode cannot be placed upon a viable thermodynamic scale (e.g., the Standard Hydrogen Seale, SHE, see below). In this case, the reference electrode is referred to as a pseudo reference electrode or PREs, which might be eapable of internal calibration against some potential-determining proeess that occurs at the indicator electrode. PREs have been used extensively in corrosion studies in high suberitical and supercritieal aqueous systems, as described below, and are generally servieeable provided that limited accuracy is acceptable. 

The experimental value for Agl is 1.97 FT cirT1 [16, 3], which indicates that the silver ions in Agl are mobile with nearly a thermal velocity. Considerably higher ionic transport rates are even possible in electrodes, by chemical diffusion under the influence of internal electric fields. For Ag2S at 200 °C, a chemical diffusion coefficient of 0.4cm2s, which is as high as in gases, has been measured... 

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