Potentiometric redox electrodes

Potcntiomctric Titrations In Chapter 9 we noted that one method for determining the equivalence point of an acid-base titration is to follow the change in pH with a pH electrode. The potentiometric determination of equivalence points is feasible for acid-base, complexation, redox, and precipitation titrations, as well as for titrations in aqueous and nonaqueous solvents. Acid-base, complexation, and precipitation potentiometric titrations are usually monitored with an ion-selective electrode that is selective for the analyte, although an electrode that is selective for the titrant or a reaction product also can be used. A redox electrode, such as a Pt wire, and a reference electrode are used for potentiometric redox titrations. More details about potentiometric titrations are found in Chapter 9. 

C. Potentiometric methods. This is a procedure which depends upon measurement of the e.m.f. between a reference electrode and an indicator (redox) electrode at suitable intervals during the titration, i.e. a potentiometric titration is carried out. The procedure is discussed fully in Chapter 15 let it suffice at this stage to point out that the procedure is applicable not only to those cases where suitable indicators are available, but also to those cases, e.g. coloured or very dilute solutions, where the indicator method is inapplicable, or of limited accuracy. 

The indicator electrode employed in a potentiometric titration will, of course, be dependent upon the type of reaction which is under investigation. Thus, for an acid-base titration, the indicator electrode is usually a glass electrode (Section 15.6) for a precipitation titration (halide with silver nitrate, or silver with chloride) a silver electrode will be used, and for a redox titration [e.g. iron(II) with dichromate] a plain platinum wire is used as the redox electrode. 

Let us revisit the electrochemical cell shown earlier in Figure 3.1. In this figure, two redox electrodes are immersed in solutions of their respective ions, with the half cells being connected by a salt bridge. If we were to connect an infinite-resistance voltmeter between the cells, then it would be possible to perform potentiometric experiments such as those described in the previous chapter. One electrode would be positive with respect to the other, with the separation in potential between the two electrodes being the emf - but only if the measurement was performed at equilibrium. (As before, we take the word equilibrium to imply that no charge flows.)... 

L.B. WINGARD Jr. and J. CASTNER, "Potentiometric biosensors based on redox electrodes", in "Biosensors Fundamentals and Applications", Oxford University Press, Oxford, 1987, p. 153. 

A rapid potentiometric flow inject technique for the simultaneous determination of oxychlorine species (e g., CIO2 ) was developed by Ohura et al. (1999). The analytical method is based on the detection of a large transient potential change of the redox electrode due to chlorine generated via the reaction of the oxychlorine species (e.g., CIO2 ). The detection limit for C102 is 3.4 ppb. 

Potentiometric redox measurements are often performed in nonaqueous or mixed-solvent media. For such solvents various potentiometric sensors have been developed, which, under rigorously controlled conditions, give a Nemstian response over a wide ranges of activities, particularly in buffered solutions. There are some experimental limitations, such as with solvent purification and handling or use of a reference electrode without salt bridges, but there also ate important advantages. Solutes may be more soluble in such media, and redox... 

Potentiometric Methods. By far the most common endpoint system for titrations is a potentiometric indicating electrode because of its simplicity and its universal applicability. It only requires a redox couple that gives some... 

Redox potential is measured potentiometrically with electrodes made of noble metals (Pt, Au) (Fig. 12). The mechanical construction is similar to that of pH electrodes. Accordingly, the reference electrode must meet the same requirements. The use and control of redox potential has been reviewed by Kjaergaard [218]. Considerations of redox couples, e.g. in yeast metabolism [47], are often restricted to theoretical investigations because the measurement is too unspecific and experimental evidence for cause-effect chains cannot be given. Reports on the successful application of redox sensors, e.g. [26,191], are confined to a detailed description of observed phenomena rather than their interpretation. 

Owing to their specificity, sensitivity, and range of measurable concentrations, potentiometric sensors based on ionic or enzymatic selectivity have wide application in analytical determinations. Other potentiometric sensors using electrodes of the first kind (Mn+ M), used in precipation titrations, are not easy to manipulate and redox electrodes have a reduced application owing to their lack of selectivity, reacting to any oxidizable or reducible species. 

The simplest potentiometric technique is based on the concentration dependence of the potential, E, at reversible redox electrodes according to the Nemst equation ... 

An outgrowth of mediated potentiometric redox titrations has been the evolution of various models which attempt to explain why biological molecules do not readily communicate their redox states directly to potential-indicating electrodes. Insulation of the redox site from the electrode by the protein portion of the molecule, the lack of the requisite enzyme character on an electrode surface, and adsorption of the biological molecule on the electrode surface are among the features of models which have been proposed. 

Guilbault and Montalvo were the first, in 1969, to detail a potentiometric enzyme electrode. They described a urea biosensor based on urease immobilized at an ammonium-selective liquid membrane electrode. Since then, over hundreds of different applications have appeared in the literature, due to the significant development of ion-selective electrodes (ISEs) observed during the last 30 years. The electrodes used to assemble a potentiometric biosensor include glass electrodes for the measurement of pH or monovalent ions, ISEs sensitive to anions or cations, gas electrodes such as the CO2 or the NH3 probes, and metal electrodes able to detect redox species some of these electrodes useful in the construction of potentiometric enzyme electrodes are listed in Table 1. 

Table 4 Typical potentiometric redox titrations of inorganic and organic compounds (in most cases a R electrode was used as the indicator electrode)...

The variation of E with the ratio [Ox]/[Red] is represented graphically in Fig.2.6. As discussed in Sec.2.2.3, a titration of an oxidant against a reductant is possible if their E values are sufficiently different and if a suitable redox indicator is available. However, if the latter is not available or if the colour of the solutions prohibit the use of an indicator, potentiometric redox titration can be carried out by connecting the metal electrode and a reference electrode (dipping in the titration vessel through its salt bridge) to a pH meter. The plot of E against the volume of titrant will show an inflection point which indicates the end-point where the equivalence point of reductant and oxidant is reached. 

Potentiometric sensors, in which the potential of the indicator electrode (ion-selective electrode, redox electrode, metal/metal oxide electrode) is measured against a reference electrode. 

The detection of potentiometric redox potential involves the use of an inert electrode such as platinum that is measured against a suitable reference electrode. Since the measurement is carried out under zero current conditions, the rate of oxidation and reduction at the electrode must be identical. The current amplitude of... 

Electrochemical methods may be classified into two broad classes, namely potentiometric metiiods and voltannnetric methods. The fonner involves the measurement of the potential of a working electrode iimnersed in a solution containing a redox species of interest with respect to a reference electrode. These are equilibrium experiments involving no current flow and provide themiodynamic infomiation only. The potential of the working electrode responds in a Nemstian maimer to the activity of the redox species, whilst that of the reference electrode remains constant. In contrast, m voltannnetric methods the system is perturbed... 

Finding the End Point Potentiometrically Another method for locating the end point of a redox titration is to use an appropriate electrode to monitor the change in electrochemical potential as titrant is added to a solution of analyte. The end point can then be found from a visual inspection of the titration curve. The simplest experimental design (Figure 9.38) consists of a Pt indicator electrode whose potential is governed by the analyte s or titrant s redox half-reaction, and a reference electrode that has a fixed potential. A further discussion of potentiometry is found in Chapter 11. 

The following experiments may he used to illustrate the application of titrimetry to quantitative, qtmlitative, or characterization problems. Experiments are grouped into four categories based on the type of reaction (acid-base, complexation, redox, and precipitation). A brief description is included with each experiment providing details such as the type of sample analyzed, the method for locating end points, or the analysis of data. Additional experiments emphasizing potentiometric electrodes are found in Chapter 11. 

This experiment describes the use of coated graphite electrodes for the potentiometric monitoring of precipitation, acid-base, complexation, and redox titrations. 

Potentiometric Titrations. If one wishes to analyze electroactive analytes that are not ions or for which ion-selective electrodes are not available, two problems arise. First, the working electrodes, such as silver, platinum, mercury, etc, are not selective. Second, metallic electrodes may exhibit mixed potentials, which may arise from a variety of causes. For example, silver may exchange electrons with redox couples in solution, sense Ag" via electron exchange with the external circuit, or tarnish to produce pH-sensitive oxide sites or Ag2S sites that are sensitive to sulfide and haUde. On the other... 

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