Electrode potential, absolute additivity

Since the absolute and the conventional electrode potentials differ only by an additive constant, the absolute potential depends on the concentration of the reactants through the familiar Nernst s equation. This dependence is implicitly contained in Eq. (2.6) the real Gibbs energies of solvation contain an entropic term, which depends on the concentration of the species in the solution. 

In practice, the value of k is never obtained as such, because the meter is adjusted so that the standard reads the correct value for its pX, the scale being Nernstian. As k contains in addition to the reference electrode potentials, a liquid-junction potential and an asymmetry potential, frequent standardization of the system is necessary. The uncertainty in the value of the junction potential, even when a salt bridge is used, is of the order of 0.5 mV. Consequently the absolute uncertainty in the measurement of pX is always at least 0.001/(0.059// ) or 0.02 if n = I, i.e. a relative precision of about 2% at best. For the most precise work a standard addition technique (p. 32) and close temperature control are desirable. All measurements should be made at constant ionic strength because of its effect on activities. Likewise,... 

Our inability lo measure absolute potenliaHfyr half-cell processes is not a serious problem because relative half-cell potentials, measured versus a common reference electrode, arc just as useful. These relative potentials can be combined to give real cell potentials. In addition, they can be used toealculale equilibrium constants of oxidation-reduction processes. 

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. 

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.,... 

Similarly to any chemical system, rate of electrochemical reaction can be changed by temperature, pressure and the concentration of reactants. However, additional control parameter for the rate of eleetroehemical reaction is the electrode potential (is). Its absolute value is not accessible to measurements, so the zero of electrode potential scale is set by introduction of hydrogen scale of electrode potentials. Let us consider an electrode reaction ... 

A water-alone monolayer potential above the pzc is in accordance with an absolute work function measurement for the water monolayer on Pt(lll) of 4.8 eV (29). Comparing this to the hydrogen electrode (4.7 eV below vacuum (30) for the normal hydrogen electrode NHE) corrected by 7x0.059 V for a nominaI pH 7 yields a water-alone mono-layer potential of +0.5 V vs. RHE at pH 7. This lies 0.3 V above our proposed pzc of 0.2 V RHE. This relatively high apparent potential of the water monolayer has been noted previously (Sass, J.K., private communication), and has raised concern about the relevance of the UHV monolayer to real electrochemical conditions, since most electrochemical measurements of the pzc of polycrystalline Pt have been closer to 0.2 V than to 0.5 V (31). By showing that the water monolayer lies above, not at, the pzc, the present H.+H-O data remove part of the apparent discrepancy between the electrochemical and UHV results. If future UHV work function data show a large ( 0.3 V) decrease in the water monolayer work function upon addition of small (<20X saturation) amounts of hydrogen, all of the apparent discrepancy could be quantitatively accounted for. 

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