Measurement of Electrode Potentials

To measure the electrode potential of a test electrodes, M, we usually use an electrochemical cell consisting of test electrode M and reference electrode both of which are coimected by a metal lead of A and A of the same metallic conductor to a potentiometer outside the cell as shown in Fig. 4-23. The difference in the electrode potential, E, measured between the test electrode and the reference electrode, conventionally called the electromotive force, equals the difference in the Fermi level of electrons between the two electrodes E = - 8j(M) - EjtM ) 

In the cell used for measuring the electrode potential, in which the two electrodes are immersed in a single phase of electrolyte solution, the outer potential, tps, ofthe test electrode-solution is equal to the outer potential, ips, of the reference electrode-solution as shown in Fig. 4—24. The difference in the Fermi level of electrons, CFtu)- between the test electrode M and the reference electrode M , then, is represented by the difference in the real potential of electrons, M/aw) - .(M0/ V). tuid hence by the difference in the electrode potential (absolute electrode potential), AE = E-E°, between the two electrodes. This difference also equals the difference in the work function, 4 no/3/v - 4 ji/s/v between the two electrodes. Thus, the potential E of the test electrode relative to the reference electrode is the difference in the electrode potential (absolute electrode potential) between the two electrodes as indicated in Eqn. 4-35 . 

Measurement of relative electrode potential by a potentiometer M = test electrode M° = reference electrode F = potentiometer = Fermi level of electron in electrodeM = Fermi level of electrons in terminal A E = relative electrode potential. 

The relative electrode potential nhe referred to the normal (or standard) hydrogen electrode (NHE) is used in general as a conventional scale of the electrode potential in electrochemistry. Since the electrode potential of the normal hydrogen electrode is 4.5 or 4.44 V, we obtain the relationship between the relative electrode potentiEd, Ema, and the absolute electrode potential, E, as shown in Eqn. 4-36  

A two-electrode configuration also can be used in a voltammetric or polar-ographic cell in which the current is measured as a function of the applied potential. In this case the working-electrode potential will be less than the applied potential because of the iR drop in the cell. In addition, the current passing through the reference electrode may cause its potential to deviate from its equilibrium (zero-current) value, due to changes in concentration of the electroactive species at the metal-solution interface. Both of these effects act to reduce the potential of the working electrode  

To avoid serious errors, the cell current and internal cell resistance must be kept as small as possible, and the reference electrode must be designed to have low internal resistance and a metal-solution interface of sufficient area to minimize internal polarization. Under ordinary polarographic conditions (10-/tA current and 1000-Q internal cell resistance) the error amounts to 10 mV. 

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This procedure of using a single measurement of electrode potential to determine the concentration of an ionic species in solution is referred to as direct potentiometry. The electrode whose potential is dependent upon the concentration of the ion to be determined is termed the indicator electrode, and when, as in the case above, the ion to be determined is directly involved in the electrode reaction, we are said to be dealing with an electrode of the first kind . 

An important step in measurements of electrode potentials is that of selecting a suitable reference electrode (RE). Reference electrodes with electrolytes of the same nature and same (or similar) composition as that at the working electrode are used... 

Potentiometry is the measurement of electrode potential in chemical analysis procedures for the purpose of obtaining qualitative and quantitative information about an analyte. The reference electrode is a half-cell that is designed such that its potential is a constant, making it useful as a reference point for potential measurements. Ground is the ultimate reference point in electronic measurements. 

Direct Potentiometry The procedure adopted of employing a single measurement of electrode potential to determine the concentration of an ionic species in a solution is usually termed as direct potentiometry. 

Fig. 6.2 Use of the method recommended by IUPAC for the measurement of electrode potentials the case of cyclic voltammetry, (a)...

Figure 6-3 Device for measurement of electrode potentials. The electrode reactions are indicated below each half-cell. The maximum electrical work that can be done by such a cell on its surroundings is - AG = nEF, where E = V2-V1as measured by a potentiometer. If A is reduced to AH2 by H2/ electrons will flow through an external circuit as indicated. A will be reduced in the right-hand cell. H2 will be oxidized to H+ in the left-hand cell. Protons will flow through the gel bridge from left to right as one of the current carriers in the internal circuit.

Measurement of electrode potentials. It is not practical to attempt to measure the absolute potential difference between each element and a... 

Figure 5.1 Cell and circuit elements for the measurement of electrode potentials. The upper system (a) illustrates the dilemma of attempts to measure single-electrode potentials.

Measurements of electrode potentials using reference electrodes are of two general types those that involve liquid junctions and those that do not. An example of a cell which does not have a liquid junction is ... 

There are many methods for determining stability constants but measurements of electrode potentials, pH titrations or the use of spectroscopic methods for determining the concentrations of species in solution are of particular importance. In the simplest... 

It might at first seem that the direct measurement of electrode potential would, by analogy to pH measurement, afford a simple means for determining pM during the course of EDTA titrations. Unfortunately, many metal electrodes do not behave reversibly, particularly at the extremely low metal ion concentrations involved near... 

We will use standard electrode potentials throughout the rest of this text to calculate cell potentials and equilibrium constants for redox reactions as well as to calculate data for redox titration curves. You should be aware that such calculations sometimes lead to results that are significantly different from those you would obtain in the laboratory. There are two main sources of these differences (1) the necessity of using concentrations in place of activities in the Nernst equation and (2) failure to take into account other equilibria such as dissociation, association, complex formation, and solvolysis. Measurement of electrode potentials can allow us to investigate these equilibria and determine their equilibrium constants, however. 

Fig. 6.6 Schematic arrangements for measurements of electrode potentials using an electrometer, (a) Circuit completed through a salt bridge between the reference electrode and the specimen electrolyte, (b) Circuit completed through the specimen electrolyte...

The inflexion points or (quasi-) current plateaus in Figures 2(a) and 3(a) of Section III.l also prove to originate from the phase transition, from the comparison between the cumulative charges during the CT experiments and during the measurement of electrode potential. Furthermore, the origin of the shoulders and of the more than one local maxima in Figures 2(b) and 3(b) is, undoubtedly, the phase transition. 

The scanning Kelvin probe, which measures the Volta potential difference between a specimen and the calibrated sensing probe, is introduced as the only electrochemical technique which allows nondestructive, real-time measurements of electrode potentials at adhesive/metal oxide interfaces in situ, even if they are covered with an adhesive layer. 

As the nature of the electrified interface dominates the kinetics of corrosive reactions, it is most desirable to measure, e.g., the drop in electrical potential across the interface, even where the interface is buried beneath a polymer layer and is therefore not accessible for conventional electrochemical techniques. The scanning Kelvin probe (SKP), which measures in principle the Volta potential difference (or contact potential difference) between the sample and a sensing probe (which may consist of a sharp wire composed of a conducting, stable phase such as graphite or gold) by the vibrating condenser method, is the only technique which allows the measurement of such data and therefore aU modern models which deal with electrochemical de-adhesion reactions are based on such techniques [1-8]. Recently, it has been apphed mainly for the measurement of electrode potentials at polymer/metal interfaces, especially polymer-coated metals such as iron, zinc, and aluminum alloys [9-15]. The principal features of a scanning Kelvin probe for corrosion studies are shown in Fig. 31.1. 

For carrying out the experimental measurements of electrode potentials, a system chosen as the reference electrode should be easy to fabricate, and also stable and reproducible. This means that any pair of reference electrodes of the same type fabricated in any laboratory should demonstrate stable zero potential difference within the limits of experimental error. Additionally, the potential differences between two reference electrodes of different type should remain constant for a long time. 

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