D Indicator Electrodes

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. 

It is convenient to classify metallic indicator electrodes as electrodes of the first kind, electrodes of the second kind, or inert redox electrodes. 

An electrode of the first kind is a pure metal electrode that is in direct equilibrium with its cation in the solution. A single reaction is involved. For example, the equilibrium between a metal X and its cation X is 

We often express the electrode potential of the indicator electrode in terms of the p-function of the cation (pX = —log x )- Thus, substituting this definition of pX into Equation 21 -2 gives 

Electrode systems of the first kind are not widely used for potentiometric determinations for several reasons. For one, metallic indicator electrodes are not vety selective and respond not only to their own cations but also to Other more easily reduced cations. For example, a copper electrode cannot be used for the determination of coppeifll) ions in the presence of silverfl) ions because the electrode poten- 

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Glass electrodes are now available as combination electrodes which contain the indicator electrode (a thin glass bulb) and a reference electrode (silver-silver chloride) combined in a single unit as depicted in Fig. 15.2(h). The thin glass bulb A and the narrow tube B to which it is attached are filled with hydrochloric acid and carry a silver-silver chloride electrode C. The wide tube D is fused to the lower end of tube B and contains saturated potassium chloride solution which is also saturated with silver chloride it carries a silver-silver chloride electrode E. The assembly is sealed with an insulating cap. 

Fig. 17. Cyclic voltammogram of the water-soluble Rieske fragment from the bci complex of Paracoccus denitrificans (ISFpd) at the nitric acid modified glassy carbon electrode. Protein concentration, 1 mg/ml in 50 mM NaCl, 10 mM MOPS, 5 mM EPPS, pH 7.3 T, 25°C scan rate, 10 mV/s. The cathodic (reducing branch, 7 < 0) and anodic (oxidizing branch, 7 > 0) peak potentisds Emd the resulting midpoint potential are indicated. SHE, standEU d hydrogen electrode.

This method is sometimes abbreviated to LSV. In this method, a static indicator electrode (A cm2 in area) is used and its potential is scanned at constant rate v (V s-1) from an initial value ( ) in the positive or negative direction (Fig. 5.18). A typical linear sweep voltammogram is shown in Fig. 5.19. In contrast to DC polar-ography, there is no limiting current region. After reaching a peak, the current decreases again.9 For a reversible reduction process, the peak current ip (A) is expressed by Eq. (5.26), where D and C are the diffusion coefficient (cm2 s 1) and the concentration (mol cm-3) of the electroactive species ... 

As for the reoxidation of reduced heteropoly compounds in the solid state, few reliable studies have been reported. It was reported that the reoxidizability increases with an increase in standard electrode potentials of countercations (108). In the case of reoxidation by O2 of le -reduced CsxHj - PMo 12O40, the rates divided by the surface area show a monotonic variation (Fig. 53e) as in Figs. 53c and d, indicating a surface reaction. A similar variation was observed for the Na and K salts. The presence of water vapor sometimes accelerates the migration of oxide ion, probably in the form of OH- or H20, and makes surface-type reactions more like bulk type II reactions (266). 

Figure 17 FTIR spectra obtained ex situ from gold electrodes treated in MF/LiAsF6 0.5 M solution. The potential was swept from open circuit voltage (OCV) to predetermined potentials as indicated in spectra (a)-(d). The electrodes were held at this potential for 15 min, followed by washing (pure solvent) and drying (vacuum) [33]. (With copyrights from Langmuir.)...

Biosensors. Sensors are required to adequately monitor bioreactor performance. Ideally, one would like to have online sensors to minimize the number of samples to be taken from the bioreactor and to automate the bioreactor process. Most bioreactors have autoclavable pH and dissolved oxygen (D.O.) electrodes as online sensors, and use offline detectors to measure other critical parameters such as glucose and glutamine concentration, cell density, and carbon dioxide partial pressure (pC02). An online fiber-optic-based pC02 sensor is commercially available and appears to be robust.37 Probes are also commercially available that determine viable cell density by measuring the capacitance of a cell suspension. Data from perfusion and batch cultures indicate that these probes are reasonably accurate at cell concentrations greater than 0.5 X 106 cells/mL.38,39... 

Figure 6.3. Titration of H30 and Cu aq with ammonia (a) and with tetramine (trien) (b). Equilibrium diagrams for the distribution of NH3-NH4 (c) of the amino coppeifll) complexes (d) and of Cu ", Cu-trien (e). The similarity of titrating with a base and titrating a metal ion with a base (Lewis acid-base interaction) is obvious. Both neutralization reactions are used analytically for the determination of acids and metal ions. A pH or pMe indicator electrode (glass electrode for and copper electrode for Cu " ) can be used for the end point indication.

The subscript d indicates that the surface concentration of substrate is dependent only on diffusion of material to the electrode m is the mass (mg) of mercury flowing out of the capillary per second, and t is the drop lifetime (s). Polarographic current data are fre-... 

To determine the acid number by the potentiometric titration method (ASTM D-664, IP 177), the sample is dissolved in a mixture of toluene and isopropyl alcohol containing a small amount of water and titrated poten-tiometrically with alcoholic potassium hydroxide using a glass indicating electrode and a calomel reference electrode. The meter readings are plotted... 

Figure 46. Difference diffractograms (obtained at +0.10 and + 0.40 V vs SCE) for the nickel hydroxide/oxyhydroxide system for aged (A) and unaged (C) electrodes. (B) and (D) indicate the assignment of the features in A and C respectively. (From Fleishmann, M., Oliver, A., and Robinson, J., Electrochimica Acta 31, 899 (1986), with permission.)...

Figure 11.5.4 Current-potential curves at a platinum electrode during titration of Fe with Ce at different fractions titrated,/, illustrating the currents attained (d) for an indicator electrode at potential E ( ) ...

Nicholson proposed a differential potentiometric tltrator involving two indicator electrodes for the automatic control of processes in industrial plants [35]. As can be seen from Fig. 7.20, the sample and reagent streams are split and led to two half-cells via capillary tubes adjusted to provide slightly different titrated fractions. The potential difference (AE) between the two indicator electrodes Is transmitted to a control and detection system (D) which regulates the flow of titrant in an automatic fashion by means of valve V, thereby maintaining the preselected AE between the two ends of the cell. The speed of titrant addition, reflected by the flow meter (M), is a measure of the sample composition. An evaluation of the instrument carried out by the titration of dichromate with iron(II) revealed that the conditions to be used must be carefully selected. Thus, stable electrode responses are only obtained in the zone where Fe(II) prevails, and not in that where dichromate prevails over the former as the process determining the potential obtained in such a zone is irreversible. This method therefore has limited application in the control of slow reactions. 

Figure 3.20. Theoretical current-versus-voltage curves at a platinum electro during an amperometric titration of Fe with Ce with two polarized or indicator electrodes. AE is the constant voltage applied to the two indicator electrodes. A At the start of the titration. B At the midpoint of the titration. C At the equivalence point. D After the equivalence point.

Figure 16.5 Open-circuit potential (OCP) of the steel/epoxy-hole electrode vs. time in aerated 0.1 M UCIO4 solution, r and d indicate the times at which the steel electrode is connected and disconnected, respectively, from the PMT/Pt electrode. (Reprinted with permission from Electrochimica Acta, Corrosion protection by ultrathin films of conducting polymers by U. Rammelt, P.T. Nguyen and W. Plieth, 48, 9, 1257-1262. Copyright (2003) Elsevier Ltd)...

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