Single-electrode quantities

As an interesting side effect of reconsidered thermoelectrochemistry, some older uncertainties and errors of electrochemical theory can be corrected in a plausible way since modem thermoelectrochemistry is directing attention to single-electrode quantities. 

Thus the EMF has been separated into two terms, each containing a quantity related to a single electrode. If the surface potential of the electrolyte x(S) is added to each of the two expressions in brackets in Eq. (3.1.73), then the expression for the EMF contains the difference in the absolute electrode potentials for the absolute electrode potential of metal M we have... 

Metallic corrosion can be characterized by two electrochemical quantities, current and potential. The current associated with a single electrode reaction on a metal surface is related to the potential of the metal by ... 

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... 

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... 

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 ... 

The two bracketed expressions in eq. 1 A.6 do not contain quantities relating to other interfaces and are termed single electrode potentials. We can turn them into absolute potentials expressed with respect to the local solution vacuum level by adding to each the surface potential / of the solution, which is the electric potential difference between a point just inside the solution phase and just outside it (points P4 and Pi in Fig. lA.l)... 

Because the partition equilibria observed at the liquid liquid interface are relevant to interfacial phenomena in specific ion electrodes and biological membranes, there is an interest in determining single ion quantities associated with transfer of an ion from water to the non-aqueous phase. This quantity can only be estimated from experimental data after making an extrathermodynamic assumption. One common assumption discussed earlier in section 4.8 is the so-called TATB assumption, according to which... 

Gold or platinum are used in SPEs fabrication, avoiding the use of a great quantity of these expensive materials. In this sense, sometimes a narrow single-electrode sensor is used to replace metal electrodes. 

It would make sense to use overvoltage for the whole cell and overpotential for a single electrode. However, the authors mostly use both terms as synonyms and not as a means of discriminating two different quantities. In the following work, we will keep the term overpotential. ... 

The absolute potentials of single electrodes have been a subject of interest since Ostwald, Nernst, and their contemporaries formulated the beginnings of modern electrochemistry in the nineteenth century. The twentieth century brought new methods of applying electrochemical measurements to analytical problems. For these, a knowledge of electrode potentials, or, alternatively, the activities of individual species of ions, could provide simplicity and accuracy not hitherto attainable. Nevertheless, ordinary thermodynamic procedures are incapable of measuring these quantities. 

The potentials of Equations 13.20 and 13.21 depend on the properties of one electrode only and can be regarded as single-electrode potentials. A similar claim could already be made for the quantities between brackets in Equation 13.19. The potentials of Equations 13.20 and 13.21, however, are work functions and can be interpreted as absolute electrode potentials, as anticipated by the notation. This is why the surface potential was added in. With this additional term, we can replace p + by the real potential a° + combining Equation 13.21 with Equation 13.12, i.e.,... 

Chemoresistors for hquid phase (impedimetric sensors) have a design similar to that of gas sensors (Fig. 5.7). In contact with electrolytic solution, a specific electrochemical cell is estabhshed. With this cell, the measming set-up cannot be arranged to respond to effects of a single electrode alone, as was possible with classical electrochemical impedance experiments (Sect. 2.2.6). Hence, with chemoresistors the equivalent circuit must consider both electrodes. For a sensitive layer with some intrinsic conductivity, for the low frequency range the conditions can be symbolized approximately by Fig. 5.9. Cf and R symbolize the film s capacity and resistance, respectively. Q and R are the corresponding quantities of the sensor interface. 

Single-electrode potentials are important for some fundamental but unmeasurable quantities. The problem has been discussed in literature [2, 3]. Relative potential values (not absolute values which refer to an imaginary point in the universe ) can be calculated. Also single-electrode entropy values can be calculated by means of non-isothermal cells, but it is necessary to make use of some non-thermodynamic assumptions. 

The charge density on the electrode a(m) is mostly found from Eq. (4.2.24) or (4.2.26) or measured directly (see Section 4.4). The differential capacity of the compact layer Cc can be calculated from Eq. (4.3.1) for known values of C and Cd. It follows from experiments that the quantity Cc for surface inactive electrolytes is a function of the potential applied to the electrode, but is not a function of the concentration of the electrolyte. Thus, if the value of Cc is known for a single concentration, it can be used to calculate the total differential capacity C at an arbitrary concentration of the surface-inactive electrolyte and the calculated values can be compared with experiment. This comparison is a test of the validity of the diffuse layer theory. Figure 4.5 provides examples of theoretical and experimental capacity curves for the non-adsorbing electrolyte NaF. Even at a concentration of 0.916 mol dm-3, the Cd value is not sufficient to permit us to set C Cc. 

If the single-electron mechanism has not been demonstrated to be the rate-controlling process by an independent method, then, in the publication of the experimental results, it is preferable to replace the assumed quantity ax by the conventional value cm, provided that the charge number of the overall reaction is known (e.g. in an overall two-electron reaction it is preferable to replace = 0.5 by or = 0.25). If the independence of the charge transfer coefficient on the potential has not been demonstrated for the given potential range, then it is useful to determine it for the given potential from the relation for a cathodic electrode reaction (cf. Eq. 5.2.37) ... 

One mole of electrons carries a charge o/ L x e, where e is the charge on a single electron and L is the Avogadro constant. This quantity of charge (L X ej has a value of 96 487 C, and is known as the Faraday constant F, and so we talk in terms of faradays of charge . The basic reaction at an electrode is electron transfer to effect reduction (the analyte gains electrons) or oxidation (the analyte loses electrons), as follows ... 

Shetty and Fernando investigated the polarographic behavior of Ni(ethyl-dtp)2 at the dropping mercury electrode in ethanol and ethanol-water media. The nickel ion was catalytically reduced in the presence of small quantities of ethyl-dtp at more positive potentials than in the absence of ethyl-dtp. In ethanol a single wave that is almost completely controlled by diffusion was obtained whereas in ethanol-water mixtures, in which the water content was less than 40% by volume, two waves were obtained. The first wave is the... 

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