Electrochemical potential indicator electrodes

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. 

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. 

The potential of the indicator electrode in a potentiometric electrochemical cell is proportional to the concentration of analyte. Two classes of indicator electrodes are used in potentiometry metallic electrodes, which are the subject of this section, and ion-selective electrodes, which are covered in the next section. 

Indicator electrodes are used both for analytical purposes (in determining the concentrations of different substances from values of the open-circuit potential or from characteristic features of the polarization curves) and for the detection and quantitative characterization of various phenomena and processes (as electrochemical sensors or signal transducers). One variety of indicator electrode are the reference electrodes, which have stable and reproducible values of potential and thus can be used to measure the potentials of other electrodes. 

Potentiometry is suitabie for the analysis of substances for which electrochemical equilibrium is established at a suitable indicator electrode at zero current. According to the Nemst equation (3.31), the potential of such an electrode depends on the activities of the potential-determining substances (i.e., this method determines activities rather than concentrations). 

Thus, an unambiguous correlation exists between the values of electrode potential (electrochemical scale) and the Fermi levels or values of electrochemical potential of the electrons defined as indicated (physical scale) see the symbols at the vertical axes in Fig. 29.2. 

Potentiometric methods are based on the measurement of the potential of an electrochemical cell consisting of two electrodes immersed in a solution. Since the cell potential is measured under the condition of zero cmrent, usually with a pH/mV meter, potentiometry is an equilibrium method. One electrode, the indicator electrode, is chosen to respond to a particular species in solution whose activity or concentration is to be measured. The other electrode is a reference electrode whose half-cell potential is invariant. 

Another interesting method of amperometric detection for LC is dualelectrode electrochemical detection. Instead of a single WE, one can place two WEs in series, parallel to or opposite each other. The series configuration is mostly used, mainly in the collection mode, i.e., the electroactive substance entering the detector is converted at the upstream (generator) electrode into a product that either is or is not detected at the downstream (indicator) electrode, depending on the potential of the latter. Hoogvliet et al.137,162 were easily able... 

In electrochemical kinetics, the concept of the electrode potential is employed in a more general sense, and designates the electrical potential difference between two identical metal leads, the first of which is connected to the electrode under study (test, working or indicator electrode) and the second to the reference electrode which is in a currentless state. Electric current flows, of course, between the test electrode and the third, auxiliary, electrode. The electric potential difference between these two electrodes includes the ohmic potential difference as discussed in Section 5.5.2. 

Janetski et al. (1977) also studied the behavior of a pyrite electrode in a solution of cyanide concentration in the absence and presence of xanthate using voltammetric technique. They reported that on increasing the concentration of cyanide at constant pH and xanthate concentration, the oxidation wave of xanthate is shifted to more anodic potential indicating that the presence of cyanide, which may react with the mineral surface to form an insoluble iron cyanide complex will result in the inhibition of the electrochemical oxidation of xanthate and the depression of pyrite. 

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. 

It is usual in electrochemical measurements to control the potential of the working (or indicator) electrode or the electrolytic current that flows through the cell. A potentiostat is used to control electrode potential and a galvanostat is used to control electrolytic current. Operational amplifiers play important roles in both of these. 

A potentiometric electrochemical cell consisting of a reference electrode, solid-state electrolyte(s), and an indicator electrode can provide information about the partial pressure of gas in the same way as the cells utilizing ion-selective electrodes and liquid electrolytes can. The general mechanism is as follows. A sample gas, which is part of a redox couple, permeates into the solid-state structure usually through the porous metal electrode and sets up a reversible potential difference at that interface according to the reaction... 

Derivative Electrochemical findings First reduction potential (V) vs see First oxidation potential (V) vs. indicated electrode... 

Fig. 14.10. Transmembrane electron movement and redox reactions. Also shown schematically are electrodes and circuit diagram for cyclic voltammetry. WE, working electrode SCE, saturated calomel electrode AE, auxiliary electrode, p, and /7 are chemical and electrochemical potentials, respectively. Bulk concentrations of reduced (RED) and oxidized (OX) species on either side of the membrane as indicated by subscripts 1 and 2 interface concentrations are designated by a superscripts (Reprinted from H. T. Tien, Aspects of Membrane Chemistry,...

Figure 26 Typical Nyquist plots obtained from impedance spectroscopic measurements of nickel electrodes polarized to different potentials in PC/LiAsF6 1 M solutions. The spectra were measured at the potentials indicated near each plot after the film formation was completed (the current reached a steady low value of Ca. 1 pA/cm2) [34]. (With copyright from The Electrochemical Society Inc.)...

Potentiometric sensors -> Electrochemical sensors for which the - potential of the -> indicator electrode is measured against a -> reference electrode. The commonly used - pH-sensitive electrodes and -> ion-selective electrodes belong to the group of potentiometric sensors. Se also potentiometry. 

The e.m.f. of an electrochemical cell can be regarded as the absolute value of the difference of the electrode potentials of the two electrodes. The two electrodes applied in building the electrochemical cell have different roles in the measurement, and must be chosen adequately. One of the electrodes, termed the indicator electrode acquires a potential which depends on the pH of the solution. In practice the glass electrode is used as the indicator electrode. The second electrode, on the other hand, has to have a constant potential, independent of the pH of the solution, to which the potential of the indicator electrode therefore can be compared in various solutions, hence the term reference electrode is applied for this second electrode. In pH measurements the (saturated) calomel electrode is applied as an indicator electrode. 

Similarly In the emulsion system the potentials are grouped around the oxidation potential of Hyamlne indicating a chemical oxidation of the compounds by the electrolytlcally oxidized %amlne. However, In the micelle system the oxidations are spread over a wide range of potentials Indicating direct electrochemical oxidation of the compounds. This Is very similar to the results obtained In nonaqueous solutions, once more showing the hydrophobic nature of the electrode interface. 

There are, however, many different types of electrochemical oxidations of phenol derivatives possible, the results of which largely depend on the methods used as well as the structure of the different phenols. Secondary chemical reactions of factors including the primary or secondary oxidation products can also occur. The various electrochemical methods used are dependent on solvents, pH values, electrode materials or absorption effects at the electrodes. These all influence the measured potentials. Moreover, the liquid/liquid potentials and the various indicator electrodes can give results, which cannot be safely compared with the general E scala of redox potentials in aqueous solutions. In this review we cannot go into the many details obtained by these methods. For some examples see Ref. 203 . 

In considering RTILs as an electrolyte for these applications, it is important to know the electrochemical stability of the RTILs toward a particular electrode. For this purpose the electrochemical potential range, starting from the point where no electrochemical reaction is observed, has been estimated using the electrochemical method. The electrochemical window (denoted EW) of RTILs is a term commonly used to indicate both the potential range and the potential difference it is... 

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