Selecting constant potential

The EIS technique is very convenient for characterizing different electrodeelectrolyte interfacial phenomena [189]. It measures the effects that govern physical and chemical properties of a substance present at the interface using a broad range of frequencies at a selected constant potential applied [190, 191]. The capacitance and impedance of the interface can be measured with EIS based on the following relation [117] ... 

Selecting a Constant Potential In controlled-potential coulometry, the potential is selected so that the desired oxidation or reduction reaction goes to completion without interference from redox reactions involving other components of the sample matrix. To see how an appropriate potential for the working electrode is selected, let s develop a constant-potential coulometric method for Cu + based on its reduction to copper metal at a Pt cathode working electrode. 

Concentration of Electrolyte Myer and Sievers"" applied the Donnan equilibrium to charged membranes and developed a quantitative theory of membrane selectivity. They expressed this selectivity in terms of a selectivity constant, which they defined as the concentration of fixed ions attached to the polymer network. They determined the selectivity constant of a number of membranes by the measurement of diffusion potentials. Nasini etal and Kumins"" extended the measurements to paint and varnish films. 

The determination of polarisation curves of metals by means of constant potential devices has contributed greatly to the knowledge of corrosion processes and passivity. In addition to the use of the potentiostat in studying a variety of mechanisms involved in corrosion and passivity, it has been applied to alloy development, since it is an important tool in the accelerated testing of corrosion resistance. Dissolution under controlled potentials can also be a precise method for metallographic etching or in studies of the selective corrosion of various phases. The technique can be used for establishing optimum conditions of anodic and cathodic protection. Two of the more recent papers have touched on limitations in its application and differences between potentiostatic tests and exposure to chemical solutions. ... 

The ratio of the rate constant as observed with the modified electrode at a selected electrode potential is divided by the rate constant observed rmder the same conditions with an unmodified electrode. 

Figure 1 Electrochemical detection of catechol, acetaminophen, and 4-methyl catechol, demonstrating the selectivity of differential pulse detection vs. constant potential detection. (A) Catechol, (B) acetaminophen, and (C) 4-methylcatechol were separated by reversed phase liquid chromatography and detected by amperometry on a carbon fiber electrode. In the upper trace, a constant potential of +0.6 V was used. In the lower trace, a base potential of +425 mV and a pulse amplitude of +50 mV were used. An Ag/AgCl reference electrode was employed. Note that acetaminophen responds much more strongly than catechol or 4-methylcatechol under the differential pulse conditions, allowing highly selective detection. (Reproduced with permission from St. Claire, III, R. L. and Jorgenson, J. W., J. Chromatogr. Sci. 23, 186, 1985. Preston Publications, A Division of Preston Industries, Inc.)...

In the above derivation we may assume that aA-(S) = aA- and aB (S) = aB, because by analogy with the build-up of an electrode potential (see pp. 26-27) the build-up of the ion-exchange potential will not significantly alter the original concentrations of A- and B in the solution under test. Hence in eqn. 2.80 the ratio aB-(n)/aA- n), which reflects the exchange competition of B versus A, still depicts the interference ratio of B in more straightforward manner than does the so-called selectivity constant (k), usually mentioned by ISE suppliers. 

Controlled potential methods have been successfully applied to ion-selective electrodes. The term voltammetric ion-selective electrode (VISE) was suggested by Cammann [60], Senda and coworkers called electrodes placed under constant potential conditions amperometric ion-selective electrodes (AISE) [61, 62], Similarly to controlled current methods potentiostatic techniques help to overcome two major drawbacks of classic potentiometry. First, ISEs have a logarithmic response function, which makes them less sensitive to the small change in activity of the detected analyte. Second, an increased charge of the detected ions leads to the reduction of the response slope and, therefore, to the loss of sensitivity, especially in the case of large polyionic molecules. Due to the underlying response mechanism voltammetric ISEs yield a linear response function that is not as sensitive to the charge of the ion. 

Source A. J. Bard, R. Parsons, and J. Jordan (eds.), Standard Potentials in Aqueous Solution (prepared under the auspices of the International Union of Pure and Applied Chemistry), Marcel Dekker, New York, 1985 G. Chariot etal. .Selected Constants Oxidation-Reduction Potentials of Inorganic Substances in Aqueous Solution, Butterworths, London, 1971. 

As discussed in Sect. 2.3.2.1, electroor-ganic reactions can often be selectively controlled by a constant potential of the working electrode, even at decreasing reactant concentrations (see Fig. 3). A precondition of this operation mode is a suitable potential-measuring equipment in the cell (special practical aspects of potential measurement are discussed in Sect. 2.5.1.6). The optimal potential can be chosen using a current density-potential curve (see Fig. 1), available by cyclovoltammetry with a very low scan rate. 

This deficiency of the theory can be explained as follows The electrically neutral complexes IS and JS do not influence the diffusion potential in the same manner as do the charged complexes, for which the theory works so nicely. In practice, as neither of the two limiting cases defined by Eqs. (13) and (14) will be realistic (compare with the stipulations in Table 6), it seems more sensible to formulate the selectivity constant as follows ... 

It is used in IC systems when the amperometric process confers selectivity to the determination of the analytes. The operative modes employed in the amperometric techniques for detection in flow systems include those at (1) constant potential, where the current is measured in continuous mode, (2) at pulsed potential with sampling of the current at dehned periods of time (pulsed amperometry, PAD), or (3) at pulsed potential with integration of the current at defined periods of time (integrated pulsed amperometry, IPAD). Amperometric techniques are successfully employed for the determination of carbohydrates, catecholamines, phenols, cyanide, iodide, amines, etc., even if, for optimal detection, it is often required to change the mobile-phase conditions. This is the case of the detection of biogenic amines separated by cation-exchange in acidic eluent and detected by IPAD at the Au electrode after the post-column addition of a pH modiher (NaOH) [262]. 

Potentiometric measurements are based on the determination of a voltage difference between two electrodes plunged into a sample solution under null current conditions. Each of these electrodes constitutes a half-cell. The external reference electrode (ERE) is the electrochemical reference half-cell, which has a constant potential relative to that of the solution. The other electrode is the ion selective electrode (ISE) which is used for measurement (Fig. 18.1). The ISE is composed of an internal reference electrode (IRE) bathed in a reference solution that is physically separated from the sample by a membrane. The ion selective electrode can be represented in the following way ... 

In potentiometric measurements, the indicator electrode responds to changes in the activity of analyte, and the reference electrode is a self-contained half-cell with a constant potential. The most common reference electrodes are calomel and silver-silver chloride. Common indicator electrodes include (1) the inert Pt electrode, (2) a silver electrode responsive to Ag+, halides, and other ions that react with Ag+, and (3) ion-selective electrodes. Unknown junction potentials at liquid-liquid interfaces limit the accuracy of most potentiometric measurements. 

Sinusoidal voltammetry (SV) is an EC detection technique that is very similar to fast-scan cyclic voltammetry, differing only in the use of a large-amplitude sine wave as the excitation waveform and analysis performed in the frequency domain. Selectivity is then improved by using not only the applied potential window but also the frequency spectrum generated [28]. Brazill s group has performed a comparison between both constant potential amperometry and sinusoidal voltammetry [98]. 

Amperometric detection is based on applying a constant potential to the working electrode and measuring the resulting current. The selection of the optimum detection potential relies on the construction of HDYs. Since the detection potential can shift slightly depending on the electrode material and the separation voltage, HDYs must be recorded under the same separation conditions rather than those of the final sample analysis (Fig. 34.7). 

A transducer is selected with respect to the features of the biochemical reaction. In amme-tering transducers, constant potential applied to the reference electrode and the current generated in the redox transformation of the electrochemically active compound present on the enzymatic electrode surface is measured. Electron transfer rate is controlled by increasing or reducing the potential drop between electrodes. 

Chariot, G. Collumeau, A. Marchon, M. J. C., Selected Constants. Oxidation-reduction Potentials of Inorganic Substances in Aqueous Solution, Butterworths, London, 1971. 

By contrast, a battery-like electrode is characterized by an almost constant potential during charging and discharging. Thus, in order to get the largest cell voltage in an asymmetric capacitor, where the capacitive electrode is replaced by a battery-like one, the selected battery-electrode potential must be close to the low or high limit of the potential window. 

The solution of the linear system (24), which obeys the boundary conditions (23a,b) and has xj> and dy>/d , continuous at z is determined up to an arbitrary constant potential, which is irrelevant for the problem. It should be, however, noted that eqs 22 imply that the ion concentrations reach the values Ci, c% and oN/s 2L at the point where y> = 0. Selecting this point to be at = L... 

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