Potential, single electrode

Since it is not possible to measure a single electrode potential, one electrode system must be taken as a standard and all others measured relative to it. By international agreement the hydrogen electrode has been chosen as the reference ... 

The physical concept of a single electrode potential has been also discussed in terms of the energy levels of ions in electrode systems. This concept may be usefirl in the cases where the system has no electronic energy levels in a range of practical interest, such as in ionic solid crystalline and electronically nonconductive membrane electrodes. "... 

If the unknown cell in the Cu-Zn cell is connected to the circuit, the emf measured is the combined potentials of two single electrode potentials for the two metals (zinc and copper) making up the cell, and it is impossible to state from the value of the emf measured what proportion is due either to the zinc, or to the copper. 

In the case of a solution with a previously known aH+ (see below), we could determine 2°H+-.H2(iatm)> provided that a reference electrode of zero potential is available however, experiments, especially with the capillary electrometer of Lippmann, did not yield the required confirmation about the realization of such a zero reference electrode16. Later attempts to determine a single electrode potential on the basis of a thermodynamic treatment also were not successful17. For this reason, the original and most practical proposal by Nernst of assigning to the standard 1 atm hydrogen potential a value of zero at any temperature has been adopted. Thus, for F2H+ H2(iatm) we can write... 

The potential of a mixed electrode at which a coupled reaction of charge transfer proceeds is called the mixed electrode potential , this mixed electrode potential is obviously different from the single electrode potential at which a single reaction of charge transfer is at equilibrium. For corroding metal electrodes, as shown in Fig. 11—2, the mixed potential is often called the corrosion potential, E . At this corrosion potential Eemt the anodic transfer current of metallic ions i, which corresponds to the corrosion rate (the corrosion current ), is exactly balanced with the cathodic transfer current of electrons for reduction of oxidants (e.g. hydrogen ions) i as shown in Eqn. 11-4 ... 

While this potential cannot he determined for a single electrode, a potential can be derived if the potential of the other electrode in a cell is defined, i.e. the potential of the standard hydrogen electrode (SHE) is arbitrarily taken as 0.(XXX)V. In this way. a potential scale can then be devised for single electrode potentials - see Section 3.2. t The abbreviation emf , in upright script, is often used in other lextNmks as a direct , i.e. non-variable, acronym for the electromotive force. Note, however, that in this present text it is used to represent a variable (cell potential) and is therefore. shown in italic script. 

The applied condition represents a relatively large positive deviation of the single-electrode potential for a cathode from the oxidation potential of the redox couple [R]/[0], For a single-electron reaction at room temperature, the above criterion for the deviation Ec — E corresponds to RJInF = 0.026 V, and one would therefore expect the simplification that leads to eq 13 to hold true for most of the overcharge situations encountered in practical applications. 

R. Parsons, The Single Electrode Potential Its Significance and Calculation and Standard Electrode Potentials Units, Conventions and Methods of Determination, in Standard Potentials in Aqueous Solution, A. J. Bard, R. Parsons, and J. Jordan, eds. Chs. 1 and 2, Marcel Dekker, New York (1985). 

Thus, one uses V for the single electrode potential and E for a cell potential. 

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.

The direct potentiometric measurement of a junction potential is not possible because of the impossibility of directly measuring a single-electrode potential. 

Interfacial potential differences are not directly observable. The usual way of measuring a potential difference between two points is to bring the two leads of a voltmeter into contact with them. It s simple enough to touch one lead of the meter to a metallic electrode, but there is no way you can connect the other lead to the solution side of the interfacial region without introducing a second second electrode with its own interfacial potential, so you would be measuring the sum of two potential differences. Thus single electrode potentials, as they are commonly known, are not directly observable. 

Finally, the measurement of single electrode potentials is of importance in itself for obtaining the depolarizing values, i.e., the potential differences of an electrolyte in connection with a certain electrode with or without a depolarizer. It is evident that these depolarizer values are characteristic quantities for the chemical nature of the depolarizer, and are very closely related to the constitution and configuration of the molecule. Introductory experiments on this question for nitro- and nitroso-bodies have been made by Panchaud de Bottens.3 Lob and Moore 4 have also measured the depolarizing values for nitrobenzene at different electrodes and current strengths. It was... 

A single electrode potential, if defined as the difference in electrostatic potential between the spaces just outside the metal and the solution, is definite, but it cannot be measured by merely connecting up the phases with wires, and adjusting a potentiometer, until no current flows for this connexion introduces more than one phase boundary. Practically all electrolytic cells consist of at least three phase boundaries and the terminals at which the electromotive force of the cell is measured are, finally, of the same metal. There may, of course, be any. greater number of phase boundaries. A simple type of cell consists of two metals, M and M, dipping into a solution 8 containing the ions of each metal. 

Absolute potential (also called single electrode potential) — is a hypothetic p. of an isolated - electrode without referring it to any reference electrode. Although it has long been known that only relative - electrode p. can be measured experimentally, numerous attempts were undertaken to determine such a value (see in [i-x]). The problem was also formulated as a search for the hypothetical reference state determined as reckoned from the ground state of - electron in vacuum (a physical scale of energy with the opposite sign). In... 

Simultaneous application of spectroscopy in voltammetry — Numerous spectroelectrochemical methods employ spectrometers fast enough to acquire complete spectra within seconds or fractions of a second. Acquisition of spectra during a slow electrode potential scan as employed in -> cyclic voltammetry or during single scan voltammetry can thus be related to a single electrode potential (within a range of a few millivolts or even more narrow). Thus interfacial phenomena as detected with the applied spectroelectrochemical technique can be related to the electrode potential directly even in case of unstable intermediates. 

Figure 5.46. Schematic diagram of a cell with a sandwich-type DE1E reference electrode [64], (Reprinted from Electrochimica Acta, 49, Li G, Pickup PG. Measurement of single electrode potentials and impedances in hydrogen and direct methanol PEM fuel cells, 4119— 26, 2004, with permission from Elsevier and the authors.)...

Li G, Pickup PG (2004) Measurement of single electrode potentials and impedances in hydrogen and direct methanol PEM fuel cells. Electrochim Acta 49 4119-26... 

Oppenheim, L, Single electrode potentials. J. Phys. Chem. 68, 2959-2961 (1964). Paddison, S. J., Pratt, L. R., Zawodzinski, T., and Reagor, D. W., Molecular modeling of trifluoromethanesulfonic acid for solvation theory. Fluid Phase Equilibria 151, 235-243 (1998). 

Arbitrary Potential Zero The Hydrogen Scale.—Since the single electrode potential [cf. equation (10)] involves the activity of an individual ionic species, it has no strict thermodynamic significance the use of such potentials is often convenient, however, and so the difficulty is overcome by defining an arbitrary zero of potential. The definition widely adopted, following on the original proposal by Nernst, is as follows ... 

Sign of the Electrode Potential.—The convention concerning the sign of the E.M.F. of a complete cell (p. 187), in conjunction with the interpretation of single electrode potentials just given, fixes the convention as to the sign of electrode potentials. The e.m.f. of the cell... 

Absolute Single Electrode Potentials.— The electrode potentials discussed hitherto are actually the e.m.f. s of cells resulting from the combination of the electrode with a standard hydrogen electrode. A single electrode potential, as already seen, involves individual ion activities and hence has no thermod3mamic significance the absolute potential difference at an electrode is nevertheless a quantity of theoretical interest. Many attempts have been made to set up so-called null electrodes ... 

Attention may be called to a nmtter of interest in connection with the B.K.F. of a cell. As seen above, the latter is the sum of an oxidation and a reduction potential, and this is equivalent to the difference of two oxidation piOentials. As a consequence, the b.m.f. of a cell is independent of the arbitrary zero chosen for the representation of single electrode potentials the actual value of the zero of the scale, whatever it may be, cancels out when taking the difference of two potentials on the same scale. 

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