Standard potentials, absolute scale

Knowledge of the value of ij (abs) makes it possible to convert all relative values of electrode potential to an absolute scale. For instance, the standard electrode potentials of the oxygen electrode, the zero charge of mercury, and the hydrated electron, in the absolute scale are equal to -5.67,. 25, and 1.57 V, recpectively. ... 

The standard Gibbs energy of electrolyte transfer is then obtained as the difference AG° x ° = AG° ° - AG° x. To estabfish the absolute scale of the standard Gibbs energies of ion transfer or ion transfer potentials, an extrathermodynamic hypothesis must be introduced. For example, for the salt tetraphenylarsonium tetraphenyl-borate (TPAs TPB ) it is assumed that the standard Gibbs energies of transfer of its ions are equal. 

Potential of zero charge Electrode potential on absolute scale Electrode potential at standard conditions Electrode potential at equilibrium Galvani potential... 

I d/d+ standard redox potential for oxidation in absolute scale... 

Knowledge of the numerical value of the entity represented by Eq. (9.1) allows one to make up cells that give the potential of an electrode "on the absolute scale, just as the Celsius scale was later shown to be expressed on the absolute or Kelvin scale of temperatures, in which there is a rationally based zero at -273 °C. Thus, to find the absolute value, VM>abs of an electrode potential expressed on the standard hydrogen scale, one writes... 

In electrochemistry we have customarily employed, instead of the absolute electrode potential / abs scale, a relative scale of the electrode potential, E yila scale, referred to the standard or normal hydrogen electrode potential E m at which the hydrogen electrode reaction, 2H + 2e dox = H2(gas), is at equilibrium in the standard state unit activity of the hydrated proton, the standard pressure of 101.3 kPa for hydrogen gas, and room temperature of 298 K. Since Eniie is + 4.44 V (or + 4.5 V) in the absolute electrode potential scale, we obtain Eq. 9.9 for the relation between abs scile and [Refs. 4 and 5.] ... 

As stated in the previous chapter, to determine the reversible potential of any electrode in an arbitrary state, it is first of all necessary to know its standard potential. The required values of these potentials, stated in terms of the hydrogen scale and valid for a temperature of 25 °C, arc tabulated. Such data do not express the absolute potentials but the electromotive force of the combination of the given half coll and the standard hydrogen electrode. This fact must be remembered, when making calculations based on these potentials. 

In electrochemistry, it is usual to measure potentials with respect to a stable and reproducible system, known as - reference electrode. For the vast majority of practical electrochemical problems there is no need to determine - absolute potentials. However, this is necessary in cases where one wants to connect the relative electrode potential with the absolute physical quantities of the system, like electronic energies, as is the case of the work function. It is possible to convert all relative values of electrode potential to absolute-scale values and to electronic energies. For aqueous systems the - standard hydrogen electrode potential corresponds to -4.44 V in the physical scale taking electrons at rest in vacuum as reference and the absolute potential is given by the relation T(abs) = T(SHE) + 4.44 [vii]. 

Although the thermodynamic analysis has given results with clear contributions from each of the electrodes, the observed EMF cannot be separated into these contributions by experiment. As was seen in the previous section, the solution to this problem has been to choose the SHE as a reference and to quote the standard potentials of all other half-reactions with respect to this point on the redox potential scale. In order to illustrate the application of this concept using absolute electrode potentials, the following cell is considered ... 

The above analysis is easily extended to other half-cell reactions. Much of the data required to do the necessary calculations has been collected for cells involving aqueous solutions [1] and can also be found in thermodynamic tables published by the National Bureau of Standards in Washington [G3]. In practice, standard potentials are always used on the conventional scale because no extra-thermodynamic assumptions are involved in their calculation. Any of these quantities can be converted to the absolute scale by adding the estimate of the absolute potential of the SHE, that is, 4.43 V, to the conventional value of the standard potential. 

For most purposes in electrochemistry, it is sufficient to reference the potentials of electrodes (and half-cell emfs) arbitrarily to the NHE, but it is sometimes of interest to have an estimate of the absolute or single electrode potential (i.e., the potential of a free electron in vacuum). This interest arises, for example, if one would like to estimate relative potentials of metals or semiconductors based on their work functions. The absolute potential of the NHE can be estimated as 4.5 0.1 V, based on certain extrather-modynamic assumptions, such as about the energy involved in moving a proton from the gas phase into an aqueous solution (10, 29). Thus, the amount of energy needed to remove an electron from Pt/H2/H ( = 1) to vacuum is about 4.5 eV or 434 kJ. With this value, the standard potentials of other couples and reference electrodes can be expressed on the absolute scale (Figure 2.1.1). 

Only if E°in = 0—i.eif the standard hydrogen electrode is zero on an absolute scale—can one deduce from Baxendale s considerations that the potential neecssary to produce e aq in its standard state from a platinum black electrode is —2.6 volts. In fact in our work we used total potential differences across the cell much less than 2.6 volts. (One can of course make E°= 0 on an absolute basis simply by selecting the appropriate standard state of e iPt) for this to be so, even if it turns out to be 1040. However, if one arbitrarily takes some reasonable standard state for e Ft) then E°i0 0 on an absolute basis). 

Reaclion (-il.23) could be used to establish an absolute scale for standard reduction potentials, bnl Ihere have been no moves to do this so iar... 

Figure 31. Fermi levels (standard potentials) of a redox system in its ground and excited states in the absolute and conventional scale.

The overall activity of a transporter is influenced by numerous parameters, which include buffer and membrane composition, membrane polarization, and osmotic stress, to name only a few. The comparison of the intrinsic activity of different transporters on an absolute scale is nearly impossible for this reason. This is not further problematic because absolute activities are probably the least interesting aspect of synthetic transport systems and arguably deserve little priority. What really matters is responsiveness to specific chemical or physical stimuh. This includes sensitivity toward membrane composition, membrane potential, pH, anions, cations, molecular recognition, molecular transformation (catalysis), or light. These stimuli-responsive, multifunctional, or smart transport systems are attractive for use in biological, medicinal, and materials sciences. Standard techniques to identify such unique characteristics rather than absolute activities or mechanistic details are outlined in this section. 

The connection between this "absolute scale of redox potentials and the conventional scale is given by a linear shift of the zero points (shifted by the energy level of the electrons in the standard hydrogen electrode versus the vacuum level) and by an inversion of the sign of this scale. For photoelectrochemical reactions it is convenient to use the absolute scale even if one measures the redox potentials as usual in the conventional scale versus an arbitrarily chosen reference electrode. 

Figure 11.17 (a) Standard redox potentials in absolute scale at... 

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