Electrode potential, absolute standard

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

Fig. 4-12. Electron energy levels in electron transfer from a standard gaseous electron throu an electrol3rte solution into an electrode a,(M/sn)) = real potential of electron in electrode E = electrode potential (absolute electrode potential).

Absolute single electrode potential is a characteristic property of a metal. All metals have characteristic electrode potentials. Absolute single electrode potentials cannot be measured directly. They can be measured with respect to a standard electrode such as standard hydrogen electrode (SHE). The standard hydrogen electrode is a widely used standard electrode. It is arbitrarily assumed to have zero potential at all temperatures by definition. Thus, it provides a zero reference point for electrode potentials. The construction of the standard hydrogen electrode is shown in Fig. 2.10. As shown in the figure. 

H. Reiss, and A. Heller, The absolute potential ofthe standard hydrogen electrode A new estimate, J. Phys. Chem. 89, 4207 (1985). 

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. ... 

Unfortunately, as shown by Trasatti, because of water adsorption both these values cannot escape a certain ambiguity. The absolute electrode potential of the standard hydrogen electrode is most often reported as 4.44 0.02 V. Recent studies showed, however, that values between 4.2 and 4.8 V can also be considered. 

The standard electrode potential [1] of an electrochemical reaction is commonly measured with respect to the standard hydrogen electrode (SHE) [2], and the corresponding values have been compiled in tables. The choice of this reference is completely arbitrary, and it is natural to look for an absolute standard such as the vacuum level, which is commonly used in other branches of physics and chemistry. To see how this can be done, let us first consider two metals, I and II, of different chemical composition and different work functions 4>i and 4>ii-When the two metals are brought into contact, their Fermi levels must become equal. Hence electrons flow from the metal with the lower work function to that with the higher one, so that a small dipole layer is established at the contact, which gives rise to a difference in the outer potentials of the two phases (see Fig. 2.2). No work is required to transfer an electron from metal I to metal II, since the two systems are in equilibrium. This enables us calculate the outer potential difference between the two metals in the following way. We first take an electron from the Fermi level Ep of metal I to a point in the vacuum just outside metal I. The work required for this is the work function i of metal I. 

Figure 9.9. Equations relating Eh, Eh°, and pH, where Eh° is the standard electrode potential R is the gas constant T is the absolute temperature n is the number of electrons F is the Faraday constant (Red), (Ox), and (H+) the concentration of the reduced and oxidized species and the hydrogen ion, respectively.

The relative electrode potential nhe referred to the normal (or standard) hydrogen electrode (NHE) is used in general as a conventional scale of the electrode potential in electrochemistry. Since the electrode potential of the normal hydrogen electrode is 4.5 or 4.44 V, we obtain the relationship between the relative electrode potentiEd, Ema, and the absolute electrode potential, E, as shown in Eqn. 4-36 ... 

Here cq and cr are the concentrations of the reactant and product, respectively, D is a common diffusion coefficient, Cq is the bulk concentration of the reactant, / is the current, n is the number of electrons, F is the Faraday constant, A is the electrode surface area, E is electrode potential, if is the standard potential, R is the gas constant, T is absolute temperature, x is the distance from the electrode surface and t is the time variable [45]. 

Thus, a more appropriate question to ask is Is it possible to measure the absolute potential of the hydrogen reaction, /H+(abs) Actually it is possible. Remembering the definition of a standard hydrogen electrode potential (see Section 6.3.4), this was defined as the potential obtained when a metal comes in contact with a solution containing H+ under thermodynamically reversible conditions at unit activity, and H2 at 1 atm, at 298 K. As to the identity of the metal base, it can in principle be any metal at which it is possible to observe the reaction H2 H+ + e taking place at equilibrium. In practice, the metals used as substrates can only be noble metals because most other metals enter into equilibria with their own species in solution. Usually platinum is the metal chosen. 

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 calculating this "absolute or "vacuum scale potential of the standard hydrogen electrode, the expression quoted as an electrode potential was... 

In practice, experimental redox potentials are reported relative to a standard electrode. If the standard is the NHE, one subtracts 4.36 V from the absolute reduction potential (the cost of the free electron) or adds 4.36 V to the absolute oxidation potential (the return from the removed electron) in order to determine the relative potential. Adjustment to other standard electrodes is straightforward, since their potentials relative to the NHE are well known. 

Any surface (typically a piece of metal) on which an electrochemical reaction takes place will produce an electrochemical potential when in contact with an electrolyte (typically water containing dissolved ions). The unit of the electrochemical potential is volt (TV = 1JC1 s 1 in SI units).The metal, or strictly speaking the metal-electrolyte interface, is called an electrode and the electrochemical reaction taking place is called the electrode reaction. The electrochemical potential of a metal in a solution, or the electrode potential, cannot be determined absolutely. It is referred to as a potential relative to a fixed and known electrode potential set up by a reference electrode in the same electrolyte. In other words, an electrode potential is the potential of an electrode measured against a reference electrode. The standard hydrogen electrode (SHE) is universally adopted as the primary standard reference electrode with which all other electrodes are compared. By definition, the SHE potential is OV, i.e. the zero-point on the electrochemical potential scale. Electrode potentials may be more positive or more negative than the SHE. 

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.] ... 

To express the absolute values of single potentials is made difficult by the fact that the absolute zero electrode is not known, in respect of which other elements could be measured. It is, therefore, necessary to be satisfied with comparative values. These will be obtained by referring each potential to an exactly defined arbitrary standard electrode the potential of which is conventionally taken as zero. Such comparative potential valuos, of course, do not prevent the calculation of the EMF s of cells composed of two elements because in such instance the zero electrode potential proper appears in the corresponding equation twice once with a positive, and once with the negative sign, so being annuled in the result. 

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]. 

Standard potential of an electrode reaction, abbreviated as standard electrode potential, is the value of the standard emf of a cell in which molecular hydrogen is oxidized to solvated protons at the left-hand electrode. For example, the standard potential of the Zn2+/Zn electrode, denoted (Zn2+/Zn), is the emf of the cell in which the reaction Zn2+(aq) + H2 ->2H+(aq) + Zn takes place under standard conditions (see p.61). The concept of an absolute electrode potential is discussed in reference [31]. 

---