Electrode potential, definition

Stanski, electrodeposition, 1301,1303 Standard hydrogen electrode, 1108 see also hydrogen electrode potential, definition, 840, 1060, 1061 Steady state, 1147, 1212 current, 1248... 

The most widely used reference electrode, due to its ease of preparation and constancy of potential, is the calomel electrode. A calomel half-cell is one in which mercury and calomel [mercury(I) chloride] are covered with potassium chloride solution of definite concentration this may be 0.1 M, 1M, or saturated. These electrodes are referred to as the decimolar, the molar and the saturated calomel electrode (S.C.E.) and have the potentials, relative to the standard hydrogen electrode at 25 °C, of 0.3358,0.2824 and 0.2444 volt. Of these electrodes the S.C.E. is most commonly used, largely because of the suppressive effect of saturated potassium chloride solution on liquid junction potentials. However, this electrode suffers from the drawback that its potential varies rapidly with alteration in temperature owing to changes in the solubility of potassium chloride, and restoration of a stable potential may be slow owing to the disturbance of the calomel-potassium chloride equilibrium. The potentials of the decimolar and molar electrodes are less affected by change in temperature and are to be preferred in cases where accurate values of electrode potentials are required. The electrode reaction is... 

The values of Hn and E are zero for water, by virtue of the constants 1.74 and 2.60. In these definitions, pKa refers to the acid ionization constant of the conjugate acid of the nucleophile, and E° to the standard electrode potential for the two-electron half-reaction ... 

Equation (22) shows that since electrode potentials measure electronic energies, their zero level is the same as that for electronic energy. Equation (22) expresses the possibility of a comparison between electrochemical and UHV quantities. Since the definition of 0 is6 the minimum work to extract an electron from the Fermi level of a metal in a vacuum, the definition of electrode potential in the UHV scale is the minimum work to extract an electron from the Fermi level of a metal covered by a (macroscopic) layer of solvent. ... 

For reasons which will become apparent below, such experimental problems are minimized in solid state electrochemistry so that both the definition and the direct measurement of absolute electrode potentials is rather straightforward. 

At a definite value of the electrode potential E, the charge of the electrode s surface and hence the value of drop to zero. This potential is called the point of zero charge (PZC). The metal surface is positively charged at potentials more positive than the PZC and is negatively charged at potentials more negative than the PZC. The point of zero charge is a characteristic parameter for any electrode-electrolyte interface. The concept of PZC is of exceptional importance in electrochemistry. 

The faradaic yield of CO formation on group II metals strongly depends on the value of the electrode potential. On silver and gold at definite potentials, yields up to 90 to 100% can be achieved. On zinc also, high yields (80%) were reported. 

Several descriptions of electrode reaction rates discussed on the preceding pages and the difficulty to standardize electrode potential scales with respect to different temperatures imply several definitions of activation energies of electrode reactions. The easiest way to determine this quantity, for example, for an irreversible cathodic process, employs Eqs (5.2.9), (5.2.10) and (5.2.12) at a constant electrode potential,... 

The case of the prescribed material flux at the phase boundary, described in Section 2.5.1, corresponds to the constant current density at the electrode. The concentration of the oxidized form is given directly by Eq. (2.5.11), where K = —j/nF. The concentration of the reduced form at the electrode surface can be calculated from Eq. (5.4.6). The expressions for the concentration are then substituted into Eq. (5.2.24) or (5.4.5), yielding the equation for the dependence of the electrode potential on time (a chronopotentiometric curve). For a reversible electrode process, it follows from the definition of the transition time r (Eq. 2.5.13) for identical diffusion coefficients of the oxidized and reduced forms that... 

This potential is related to the electrode potential, E (apart from an additive constant depending on the definition of the reference potential). The first term in Eq. (4.5.6), Af(j> characterizes the band bending... 

Reference Electrodes By definition, the normal hydrogen electrode (N H E) is the reference for electrode potentials (see Sect. 2.3.2.1), but practically it is scarcely usable. A reference electrode (RE) has to provide a well-defined potential between the electrolyte and its electric connector, joined with the input of the measuring instrument. Usually, a metal and a slightly soluble salt of this metal is applied (secondary electrode) [76, 77]. The electrolyte in the RE is connected to the electrolyte in the electrochemical cell via a diaphragm, which has to separate both electrolytes, as far as possible without a potential difference (see below). 

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