Electrode potential absolute scale

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.] ... 

The implications of Equations (7.11) and (7.12) are quite significant. They allow for the establishment and straightforward measurement of a unique absolute electrode potential scale in solid state electrochemistry. 

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. ... 

In this chapter we introduce and discuss a number of concepts that are commonly used in the electrochemical literature and in the remainder of this book. In particular we will illuminate the relation of electrochemical concepts to those used in related disciplines. Electrochemistry has much in common with surface science, which is the study of solid surfaces in contact with a gas phase or, more commonly, with ultra-high vacuum (uhv). A number of surface science techniques has been applied to electrochemical interfaces with great success. Conversely, surface scientists have become attracted to electrochemistry because the electrode charge (or equivalently the potential) is a useful variable which cannot be well controlled for surfaces in uhv. This has led to a laudable attempt to use similar terminologies for these two related sciences, and to introduce the concepts of the absolute scale of electrochemical potentials and the Fermi level of a redox reaction into electrochemistry. Unfortunately, there is some confusion of these terms in the literature, even though they are quite simple. 

In studying interfacial electrochemical behavior, especially in aqueous electrolytes, a variation of the temperature is not a common means of experimentation. When a temperature dependence is investigated, the temperature range is usually limited to 0-80°C. This corresponds to a temperature variation on the absolute temperature scale of less than 30%, a value that compares poorly with other areas of interfacial studies such as surface science where the temperature can easily be changed by several hundred K. This "deficiency" in electrochemical studies is commonly believed to be compensated by the unique ability of electrochemistry to vary the electrode potential and thus, in case of a charge transfer controlled reaction, to vary the energy barrier at the interface. There exist, however, a number of examples where this situation is obviously not so. 

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

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 ... 

In aqueous alkaline solution the ground state of rhodamine takes up an electron from a metal electrode at —0.75 V (NHE) forming the reduced dye 2D- 13>. This corresponds to an electron affinity of about 3.75 eV in absolute scale 8> or ° d/d- = — 3.75 in our nomenclature. The reduced molecule is a semiquinone with pK = 5.5 for the uptake of a proton 2>. The neutral semiquinone has an absorption maximum at 425 nm whereas its protonated form is colourless 2>. The oxidation potential estimate, as pointed out before, can be obtained from the ionisation energy for crystalline rhodamine which is 5.3 eV 14>. This would mean ° d/d+ — 5.3 eV or °U = +0.8 V (NHE). The reduction and oxidation... 

Fig. 12. Position of the band edges of various semiconductors in absolute and conventional electrode potential scale at pH = 7 GaP (40). CdSe (41), CdS (42), ZnO (43), T1O2 (44, 45), SnC>2 (46) and the electron transfer terms of the excited rhodaminc in solution...

Fortunately not, but to measure the absolute potential at an interface, another reference state would have to be used, as well as the nature of the metal-electron interactions. Later, in Chapter 8, it will be shown that relevant calculations can be made of this difference of inner potentials (sometimes called the Gcdvani potential difference), but their accuracy is on the order of 0.1 V, which is not yet enough to compensate for our lack of ability to measure the quantity. In the next sections, some useful concepts will be described and in Section 6.7.2 we will return to the concept of absolute electrode potential and the possibility of creating a scale of practical absolute-electrode potentials. 

It is understood that it would not be necessary to determine values of Zs (abs) for all the existing electrode reactions. According to Eq. (6.48), if we could manage to determine at least one electrode system, say. ZsR(abs), then, by measuring V, it would be possible to obtain the values of other systems, i.e., other Zi abs). Since the hydrogen reaction has been already used to obtain the relative scale of electrode potentials (Section 6.3.4), it would be very convenient if the absolute value for this reaction could be determined. In this way we could simply add or subtract to the absolute potential of this reaction, /H+(abs), the appropriate values of a given... 

Then, an absolute electrode potential was defined, Z fabs). It was established that the absolute electrode potential for the reference hydrogen electrode has a value between -4.44 V and -4.78 V, and a scale of absolute potentials lor different reactions was obtained. This was an important step because knowledge of this scale allows one to predict the direction of electron flow when two electrodes are brought into electrical contact. 

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... 

However, some electrochemists prefer to call potentials calculated from Eq. (9.2) potentials on the "vacuum scale, rather than an absolute, potential. Thus, "absolute is a loaded word and requires detailed explanation. Some chemists are surprised when one identifies the word electrode potential with (9.1), for they tend, intuitively, to... 

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

Electron transfer is a fast reaction ( 10-12s) and obeys the Franck-Condon Principle of energy conservation. To describe the transfer of electron between an electrolyte in solution and a semiconductor electrode, the energy levels of both the systems at electrode-electrolyte interface must be described in terms of a common energy scale. The absolute scale of redox potential is defined with reference to free electron in vacuum where E=0. The energy levels of an electron donor and an electron acceptor are directly related to the gas phase electronic work function of the donor and to the electron affinity of the acceptor respectively. In solution, the energetics of donor-acceptor property can be described as in Figure 9.6. 

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. 

Absolute potential (also called single electrode potential) — is a hypothetic p. of an isolated - electrode without referring it to any reference electrode. Although it has long been known that only relative - electrode p. can be measured experimentally, numerous attempts were undertaken to determine such a value (see in [i-x]). The problem was also formulated as a search for the hypothetical reference state determined as reckoned from the ground state of - electron in vacuum (a physical scale of energy with the opposite sign). In... 

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]. 

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,... 

However, most of the time, the corrective term to E° is unfortunately unknown and beyond the error associated with potential measurements on an absolute (NHE) scale, mainly because of junction potential and reference electrode potential drifts. Thus in actual experiments Ej/2 and E° (or E°) are generally considered identical [94]. 

The redox potential is generally referred to the standard hydrogen potential (SHE), which has an exactly defined energy, E y, relative to the energy of the free electron in vacuum or at infinity. Thus, electrode potentials of redox couples can be expressed on the absolute energy scale according to... 

Therefore, by measuring the flatband potential at pzc, one can determine the energy level of the semiconductor band in an electrolyte relative to the absolute scale or the vacuum scale. The pzc of a silicon electrode in aqueous electrolyte is similar to that of SiOi, at about pH 2.2, since the silicon surface is generally covered with a thin layer... 

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