Absolute electrode potentials, approaches

There have been a number of theoretical approaches to the determination of absolute electrode potentials (relative to the electric potential at a location infinitely distant from all charges). All of them require the use of nonthermodynamic theories. One work cites a value of —4.43 V (absolute) for the standard hydrogen electrode. Other workers have come up with values ranging from this value to —4.73 V. We will use only half-cell potentials relative to the standard hydrogen electrode. 

Once free energies of solvation are available, other solution properties can be determined, such as solute conformations, pKa values, electrode potentials, reaction energetics, etc.9 10 82 For example, Reynolds applied ab initio (HF and MP2) QM/MM approaches to computing the electrode potentials in water of a group of quinines 83 the average absolute deviations for the most stable conformations were 0.024 (HF) and 0.033 (MP2) volts, for a range of 0.322 volts. 

For a reversible process involving species in solution, the absolute value of the peak potential separation, - EpcI, approaches 59/n (mV at 298 K), whereas the half-sum of such potentials can, in principle, be equal to the formal electrode potential of the couple. Under the above conditions, the peak current is given by the Randles-Sevcik equation (Bard et ak, 2008) ... 

The predicted values from QC calculations of the absolute E absCM) need to be converted to the commonly used standard hydrogen electrode (SHE) or Li+/Li potential scale in order to compare them with experimental data. There are two main approaches for accomplishing this. In the first approach, well-established data for aqueous electrolytes are used to connect the absolute and electrochemical scales. This approach neglects the influence of the nature of a particular aprotic solvent on the lithium free energy of solvation. The International Union of Pure and Applied Chemistry (lUPAC)-recommended values [4] for the absolute SHE potential in... 

To this end, various computational approaches have proven to be important tools for making a priori predictions of the electrochemical stability of solvents and salts, as well as additives. More precisely the aim of the modelling is to access the electronic energy levels of the molecules/materials, which for many of the methods used are easily accessible, and then directly or indirectly correlate these with the observed experimental data or electrode potentials on an absolute or relative energy scale - to truly test the predictive power of electrochemical stability. 

Finally, let us point out that the absolute standard electrode potential value of the couple H+w/H2(g) is actually about 4.5 V. This value cannot be verified since we cannot measure an absolute potential. It was obtained by using thermodynamic cycles, taking into account some thermodynamic data such as the proton hydration enthalpy and entropy. These last ones have been approached by considering the quadrupole model of water (see Chap. 1). It is quite evident that the value of 4.5 V differs considerably from the conventional one (0.00 V). However, it does not change the redox phenomena provision since only the standard electrode potential differences are taken into account. 

This approach will not be practical for some time to come. The fundamental properties of surfactants (micelle formation, enrichment at interfaces) mean that the activity of a surfactant will usually differ from its absolute concentration (1). Just as serious is the technical problem that current surfactant-selective electrodes suffer from response which varies with their past and recent history they are also sensitive to the concentration of nonsurfactant ions. The result is that quantitative applications use electrodes not in direct measurements relating potential to concentration, but as indicators of the end point of a titration. In this latter application, it is not important that the electrode potential be exactly reproducible, but only that the potential change sharply as the surfactant concentration changes. For the titration of an anionic surfactant with a cationic surfactant, the electrode used for end point detection can be chosen to respond to either surfactant. Because of the drift in electrode potential, titrations must be conducted to an inflection in the titration curve rather than to a specific millivolt value. Details of the potentiometric titration methods can be found earlier in this chapter. The electrodes have also been demonstrated as detectors for flow injection analysis. 

It follows from the definition cited that the size of the zeta potential depends on the structure of the diffuse part of the ionic EDL. At the outer limit of the Helmholtz layer (at X = X2) the potential is j/2, in the notation adopted in Chapter 10. Beyond this point the potential asymptotically approaches zero with increasing distance from the surface. The slip plane in all likelihood is somewhat farther away from the electrode than the outer Helmholtz layer. Hence, the valne of agrees in sign with the value of /2 but is somewhat lower in absolute value. 

Another form of this definition [equation (3.6.15)] has sparked much debate in the scientific community [121-124]. In this approach Vapp (or Vbias) is taken as the absolute value of the difference between the potential at the working electrode measured with respect to a reference electrode (Vmeas) and the open circuit potential (Voc) measured with respect to the same reference electrode under identical conditions (in the same electrolyte solution and under the same illumination). In the case of a semiconductor photoanode where oxygen evolution takes place the efficiency is calculated as ... 

When the electrode resistance is very large, e.g., with an ion-selective microelectrode, the method described above does not work. An alternative approach consists of fitting the experimental concentration profile calculated from the tip potential to the theoretical profile predicted for the substrate geometry and activity. When the system investigated is at steady state, there is a unique relationship between tip potential and tip-substrate distance and an absolute distance scale can be determined. However, the procedure fails when the system is not at steady state because most ISE are unable to follow rapid changes in concentration. 

Indeed, there are no factors limiting the reversible operation of gas oxygen electrodes at high and low pO values. Theoretically, if the concentration of the potential-determining particle in the solution is equal to zero, then the absolute magnitude of the potential of the corresponding electrode approaches infinity ( oo). This conclusion has no physical sense it means that the Nemst equation is applicable to the description of the electrochemical processes in the solution, if their concentration exceeds the certain limit. In practice, the deviations from the Nernst equation arise because of the effect of the binary electric layer... 

PM-IRRAS exploits the different attenuation of s- and p-polarized light by adsorbed species at a reflective (electrode) surface to annul the unchanging contributions to the infrared signal at the detector from the solvent, window, and so on, and produces an absolute rather than difference spectrum at a particular potential. In this approach, a photo-elastic modulator is employed to modulate the polarization state of the incident infrared ray between s- and p-states. On the basis of Greenler s theory [81, 82], this polarization modulation gives rise to an AC signal at the detector, which is proportional (/p —7s)-the difference in intensity of the two polarizations. Since, in principle, /p is absorbed... 

Nearly a century has passed since Ostwald formally introduced the use of absolute potentials in his Lehrbuch der Allgem. Chemie. Although the Nernst forces carried the day and established the normal hydrogen electrode as the basis of the redox scale, Ostwald s elegant concept of an absolute potential as a base for the system has attracted attention of prominent scientists from time to time since then. New concepts and new experimental approaches have been tested, but in no case does there seem to be a system developed likely to supplant hydrogen. The chemistry of the time led both Nernst and Ostwald to believe that they were dealing with systems much less complex than the experience of a century of research has proved. 

One possible way to avoid some of the problems described above would be to use an electrode pair without a liquid junction, i.e., a cell without transference. In this way, uncertainties due to the liquid junction, such as alteration of the sample solution by electrolyte diffusion, streaming potentials, suspension effect, and the liquid junction potential itself, may be eliminated by using a pH or other ion-selective electrode as the reference electrode. The difficulty in this approach arises because, in order to assign an accurate emf value to the reference electrode, the activity of the reference ion in the sample solution must be accurately known and remain constant. Once again we are confronted by the necessity of a bootstrap operation. There is no way, at the present state-of-the-art, to accurately calculate the activity of an ion in such a complex mixture as a biologic fluid. If an activity is arbitrarily assigned to the reference ion and if it remains constant, then such an electrode system can be used for precise measurements of relative ion activities, but little can be said about the absolute activities. 

Absolute potential at the electrode surface Overall conversion related yield Absolute potential of the bulk solution phase Absolute potential at the plane of closest approach of cations Potential field strength Rotation rate... 

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