Measurement of single electrode potentials

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... 

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... 

Measurement of. Single Electrode Potentials.—In order to measure tire potential between an electrode and a solution, it is necessary to have another electrode and solution, the potential difference between which is known. As standard electrode we shall employ what is generally known as the... 

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

Kinetic Considerations Some of the essential principles involved in the measurement of an electrode potential can be described qualitatively by consideration of the behavior of a single electrode (platinum) immersed in an acidified Fe -Fe solution. To cause the passage of a finite current at this electrode, it is necessary to shift the potential from its equilibrium value. We thus obtain a curve depicting the resulting current as a function of the electrode potential (polarization curve). At the equilibrium potential, that is, at the point of zero applied current, the half-reaction... 

We note that it is possible to measure absolute single-electrode potentials by nonthermodynamic techniques. In principle, the singleelectrode potential can be determined from a measurement of quad-rupole radiation from the oscillating electrode. 

The measurement of an electrode potential requires the use of a reference electrode [2,28] to complete the electrical circuit (Fig. 4.1.6). The SHE is chosen as the primary reference electrode and its equilibrium potential is assigned the value of zero when the activities of H" " and H2(g) are unity. Thus, the equilibrium single electrode potential values in Table 4.1.2 are relative, as they are measured with respect to the SHE, and are not absolute. 

The electromotive series is a list of the elements in accordance with their electrode potentials. The measurement of what is commonly known as the "single electrode potential", the "half-reaction potential" or the "half-cell electromotive force" by means of a potentiometer requires a second electrode, a reference electrode, to complete the circuit. If the potential of the reference electrode is taken as zero, the measured E.M.P. will be equal to the potential of the unknown electrode on this scale. W. Ostwald prepared the first table of electrode potentials in 1887 with the dropping mercury electrode as a reference electrode. W. Nernst selected in 1889 the Normal Hydrogen Electrode as a reference electrode. G.N. Lewis and M. Randall published in 1923 their table of single electrode potentials with the Standard Hydrogen Electrode (SHE) as the reference electrode. The Commission of Electrochemistry of the I.U.P.A.C. meeting at Stockholm in 1953 defined the "electrode potential" of a half-cell with the SHE as the reference electrode. 

Measurements of the corrosion potential of a single metal corroding uniformly do not involve an IR drop, but similar considerations do not apply when the metal is polarised by an external e.m.f., and under these circumstances the IR drop must be minimised by using a Luggin capillary placed close to the surface of the electrode (see Fig. 1.22, Section 1.4). Even so, the IR drop is not completely eliminated by this method, and a further error is introduced by the capillary shielding the surface from the current flow... 

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. 

We call S R and S electrode potentials and S R and SJR standard electrode potentials or standard reduction potentials. Notwithstanding these names, these quantities really are not the potentials of single electrodes, but rather the measured potentials of cells XII and XIII. These measured potentials contain... 

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.

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. 

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... 

Potentiometry is the measurement of an electrical potential difference between two electrodes (half-ceUs) in an electrochemical cell (Figure 4-1) when the cell current is zero (galvanic cell). Such a cell consists of two electrodes (electron or metallic conductors) that are connected by an electrolyte solution (ion conductor). An electrode, or half-cell, consists of a single metallic conductor that is in contact with an electrolyte solution. The ion conductors can be composed of one or more phases that are either in direct contact with each other or separated by membranes permeable only to specific cations or anions (see Figure 4-1). One of the electrolyte solutions is the unknown or test solution this solution may be replaced by an appropriate reference solution for calibration purposes. By convention, the cell notation is shown so that the left electrode (Mi,) is the reference electrode the right electrode (Mr) is the indicator (measuring) electrode (see later equation 3). ... 

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