Absolute electrode potentials, approaches measurements

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 ... 

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. 

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. 

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