Conventional indicator electrodes

Potentiometric measurements are made using a potentiometer to determine the difference in potential between a working or, indicator, electrode and a counter electrode (see Figure 11.2). Since no significant current flows in potentiometry, the role of the counter electrode is reduced to that of supplying a reference potential thus, the counter electrode is usually called the reference electrode. In this section we introduce the conventions used in describing potentiometric electrochemical cells and the relationship between the measured potential and concentration. 

Also, by convention, potentiometric electrochemical cells are defined such that the indicator electrode is the cathode (right half-cell) and the reference electrode is the anode (left half-cell). 

The equipment required for direct potentiometric measurements includes an ion-selective electrode (ISE), a reference electrode, and a potential-measuring device (a pH/millivolt meter that can read 0.2mV or better) (Figure 5-1). Conventional voltmeters cannot be used because only very small currents are allowed to be drawn. The ion-selective electrode is an indicator electrode capable of selectively measuring the activity of a particular ionic species. Such electrodes exhibit a fast response and a wide linear range, are not affected by color or turbidity, are not... 

For simplicity, they treated a solution containing only one supporting electrolyte in addition to dissolved metal ions. Then, using a reference electrode (indicator electrode) reversible to one of the three kinds of ions (a dissolved metal ion, and an anion and cation of the supporting electrolyte), they examined the difference in potential between the working and indicator electrodes. Furthermore, the expressions derived were rewritten to correspond to the actual case when the potential of the working electrode was measured with respect to a conventional reference electrode as follows ... 

The above notation indicates, according to convention (cf., pp. 27-28), that the indicator electrode is cathodic, which is only so for pH < 5.8, whilst the reference electrode becomes cathodic for pH > 5.8 (cf., comparable situation in Fig. 2.13). We shall consider the latter situation (all potentials being indicated vs. solutions), and hence the emf is... 

In practice, it is very often necessary to determine the potential of a test (indicator) electrode connected in a cell with a well defined second electrode. This reference electrode is usually a suitable electrode of the second kind, as described in Section 3.2.2. The potentials of these electrodes are tabulated, so that Eq. (3.1.66) can be used to determine the potential of the test electrode from the measured EMF. The standard hydrogen electrode is a hydrogen electrode saturated with gaseous hydrogen with a partial pressure equal to the standard pressure and immersed in a solution with unit hydrogen ion activity. Its potential is set equal to zero by convention. Because of the relative difficulty involved in preparing this electrode and various other complications (see Section 3.2.1), it is not used as a reference electrode in practice. 

Note that some electrochemical cells use, instead of conventional reference electrodes, indicator electrodes. These are electrodes that are not thermodynamically reversible but which may hold then-potential constant 1 mV for some minutes—enough to make some nonsteady-state measurements (see Chapter 8). Such electrodes can simply be wires of inert materials, e.g.. smooth platinum without the conditions necessary to make it a standard electrode exhibiting a thermodynamically reversible potential. However, many different electrode materials may serve m this relatively undemanding role. 

The conventional voltammetric indicator electrodes are 0.5-5 mm in diameter. However, ultramicroelectrodes (UME) [8] that have dimensions of 1-20 pm, are also used as indicator electrodes. The tiny electrodes have some definite advantages over conventional ones ... 

If the three-electrode instrument is equipped with an iR-drop compensator, most of the iT-drop caused by the solution resistance can be eliminated. However, in order to minimize the effect of the iT-drop, a Fuggin capillary can be attached to the reference electrode with its tip placed close to the indicator electrode. Moreover, for a solution of extremely high resistance, it is effective to use a quasi-reference electrode of a platinum wire (Fig. 8.1(a)) or a dual-reference electrode (Fig. 8.1(b)), instead of the conventional reference electrode [12]. 

The electrode potential (reduction potential) for a redox couple is defined as the couple s potential measured with respect to the standard hydrogen electrode, which is set equal to zero (see hydrogen electrode later). This potential, by convention, is the electromotive force of a cell, where the standard hydrogen electrode is the reference electrode (left electrode) and the given half-cell is the indicator electrode (right electrode). The reduction potential for a given redox couple is given by the Nernst equation ... 

The sign convention for potentiometry is consistent with the convention described in Chapter 18 for standard electrode potential. In this convention, the indicator electrode is always treated as the right-hand electrode and the reference electrode as the left-hand electrode. For direct potentiometric measurements, the potential of a cell can then be expressed in terms of the potentials developed by the indicator electrode, the reference electrode, and a junction potential, as described in Section 21 A ... 

Compare this equation with Eqs. (15.7) and (15.15). By convention, the reference electrode is connected to the negative terminal of the potentiometer (the readout device). The common reference electrodes used in potentiometry are the SCE and the silver/silver chloride electrode, which have been described. Their potentials are fixed and known over a wide temperature range. Some values for these electrode potentials are given in Table 15.3. The total cell potential is measured experimentally, the reference potential is known, and therefore the variable indicator electrode potential can be calculated and related to the concentration of the analyte through the Nemst equation. In practice, the concentration of the unknown analyte is determined after calibration of the potentiometer with suitable standard solutions. The choice of reference electrode depends on the application. For example, the Ag/AgCl electrode cannot be used in solutions containing species such as halides or sulfides that will precipitate or otherwise react with silver. 

Smiechowski and Lvovich monitored the levels of acidity and basicity in industrial lubricant. The sensor was based on electrochemical impedance methodology. An iridium oxide potentiometric sensor was developed in both a conventional and MEMS configuration. Tests of the sensors in diesel lubricant showed good correlation between TAN, TEN, and the voltage output of each sensor [7]. Widera et al. used a potentiometric iridium oxide electrode as an indicating electrode with a silver/silver chloride reference electrode for the off-line monitoring of fuel acidity. The data showed that the iridium oxide sensor responds to compounds present in fuel that have acid-base character and it is possible to determine the acidity of different fuels and discriminate between unstressed and thermally stressed fuels. Experimental results indicated the ability to correlate the response of the iridium oxide sensor with the total acid numbers of different fuels [20]. 

Figure 5.24 shows a device intended to facilitate the use of conventional electrodes as flow sensors. The potentiometric circuit is closed by using a reference electrode in some cases, the electrode must be of the double salt bridge type to ensure compatibility with the analyte, e.g. chloride. A simple solution involves connecting the waste tube to the output of the indicator electrode in a beaker and inserting the reference electrode in the waste liquid. Alternatively, one can use a methacrylate well to screw the output of the selective electrode on one side and to evacuate the liquid excess via an overflow trough on the other. 

Un effect, the sign convention for elec(r<.>de potentials described in Section 22C-.i also designates the indicator electrode as the right-hand electrode by stipulating that half-reactions must alvays he written as reductions the standard hydre n ele rode. which is the reference electrode in this Case, is then the left-hand electrode. 

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