Potential of single electrode

Corresponding to the charge in the potential of single electrodes which is related to their different overpotentials, a shift in the overall cell voltage is observed. Moreover, an increasing cell temperature can be noticed. Besides Joule-effect heat losses Wj, caused by voltage drops due to the internal resistance Rt (electrolyte, contact to the electrodes, etc.) of the cell, thermal losses WK (related to overpotentials) are the reason for this phenomenon. 

We call S R and S electrode potentials and S R and SJR standard electrode potentials or standard reduction potentials. Notwithstanding these names, these quantities really are not the potentials of single electrodes, but rather the measured potentials of cells XII and XIII. These measured potentials contain... 

The absolute potentials of single electrodes have been a subject of interest since Ostwald, Nernst, and their contemporaries formulated the beginnings of modern electrochemistry in the nineteenth century. The twentieth century brought new methods of applying electrochemical measurements to analytical problems. For these, a knowledge of electrode potentials, or, alternatively, the activities of individual species of ions, could provide simplicity and accuracy not hitherto attainable. Nevertheless, ordinary thermodynamic procedures are incapable of measuring these quantities. 

When calculating the potential of single electrodes according to eqn. (6.55) it is important that we obey the sign convention used the following procedure will ensure this. 

On the basis of the charged capacitor, we will now discuss the changes induced by filling the space between both electrodes with a liquid electrolyte. Because of its experimental relevance, we will consider a single electrode/electrolyte interface only, where the electrostatic potential of the electrode will simply be designated as... 

Sensitive electrochemical techniques have also been developed to directly measure the release of oxidizable neurotransmitters such as catecholamines (CAs) and serotonin (5-hydroxytryptamine, 5-HT). Current flows in the circuit when the potential of the electrode is positive enough to withdraw electrons from, i.e. oxidize, the released neurotransmitter. The technique is very sensitive and readily detects the release of individual quanta of neuro transmitter resulting from the fusion of single secretory vesicles to the plasmalemma (Fig. 10-2). 

The corrosive electrochemistry parameters are listed in Table 7.4. From Fig. 7.30 and Table 7.4, it can be seen that, after adding xanthate 5x 10" mol/L, the corrosive potential of galena electrode decreases from -48 to -94 mV, the corrosive current of the galena eleetrode decreases from 3.45 to 0.99 pA/cm, the polarization resistance increases to 18.7 kG and the inhibiting eorrosive efficiency increases to 28.34. Figure 7.31 shows that the EIS of galena electrode appears to have single capacitive reactance loop characteristic and the radius of the capacitive reactance loop increases with the increase of collector concentration. 

What does this result mean It indicates that at the pzc most of the water molecules (i.e., 68%) are associated as dimers. The rest of the molecules, 32%, remain as monomers, most probably oriented parallel to the surface of the electrode (see Fig. 6.75). As the potential of the electrode changes to positive values, the dimers break into single water molecules and, together with the monomers, they stand with their oxygens close to the electrode, in the flip-up position (see Fig. 6.75). 

Finally, the measurement of single electrode potentials is of importance in itself for obtaining the depolarizing values, i.e., the potential differences of an electrolyte in connection with a certain electrode with or without a depolarizer. It is evident that these depolarizer values are characteristic quantities for the chemical nature of the depolarizer, and are very closely related to the constitution and configuration of the molecule. Introductory experiments on this question for nitro- and nitroso-bodies have been made by Panchaud de Bottens.3 Lob and Moore 4 have also measured the depolarizing values for nitrobenzene at different electrodes and current strengths. It was... 

Figure 5.46. Schematic diagram of a cell with a sandwich-type DE1E reference electrode [64], (Reprinted from Electrochimica Acta, 49, Li G, Pickup PG. Measurement of single electrode potentials and impedances in hydrogen and direct methanol PEM fuel cells, 4119— 26, 2004, with permission from Elsevier and the authors.)...

Li G, Pickup PG (2004) Measurement of single electrode potentials and impedances in hydrogen and direct methanol PEM fuel cells. Electrochim Acta 49 4119-26... 

Sign of the Electrode Potential.—The convention concerning the sign of the E.M.F. of a complete cell (p. 187), in conjunction with the interpretation of single electrode potentials just given, fixes the convention as to the sign of electrode potentials. The e.m.f. of the cell... 

Since kxjk is a constant at definite temperature, this equation is obviously of the same form as the electrode potential equations derived by thermodynamic methods, e.g., equation (85) for an electrode reversible with respect to positive ions. The first term on the right-hand side of equation (30) is clearly the absolute single standard potential of the electrode it is equal to the standard free energy of the conversion of solid metal to solvated ions in solution divided by and its physical significance has been already discussed. 

If the potential of an electrode deviates from the reversible or equilibrium value, a current flows in either the anodic or cathodic direction. The deviation of the potential from its equilibrium value is the anodic or cathodic overpotential of the electrode. The terms emf and voltage are used here to refer to a cell, whereas the term potential refers to a single electrode (Section 12-1). Overvoltage represents the additional voltage above the reversible cell emf required to permit the passage of a finite current, and overpotential refers to the deviation of the potential of a single electrode from its reversible value. In both cases the ohmic voltage drop iR is first subtracted, as seen below. 

Attention may be called to a nmtter of interest in connection with the B.K.F. of a cell. As seen above, the latter is the sum of an oxidation and a reduction potential, and this is equivalent to the difference of two oxidation piOentials. As a consequence, the b.m.f. of a cell is independent of the arbitrary zero chosen for the representation of single electrode potentials the actual value of the zero of the scale, whatever it may be, cancels out when taking the difference of two potentials on the same scale. 

As already discussed in Section 3.2 the potential across a single solid-liquid interface cannot be measured. One can only measure the potential of an electrode vs. a reference electrode. It has already been shown in Section 3.2 that a certain potential is produced at a metal or semiconductor electrode upon the addition of a redox system, because the redox system equilibriates with the electrons in the electrode, i.e. the Fermi level on both sides of the interface must be equal under equilibrium. It should be emphasized here that the potential caused upon addition of a redox couple to the solution occurs in addition to that already formed by the specific adsorption of, for instance, hydroxyl ions. A variation in the relative concentrations of the oxidized and reduced species of the redox system leads to a corresponding change of the potential across the outer Helmholtz layer, as required by Nernst s law (see Eq. 3.47), which can be detected by measuring the electrode potential vs, a reference electrode. However, there still exists a potential across the inner Helmholtz layer which remains unknown. 

The potentials of other electrodes are expressed in reference to the normal hydrogen electrode. To evaluate the potential for any other single electrode, it is necessary to couple it with a standard or normal hydrogen electrode and the e.m.f of the galvanic cell is measured potentiometrically. Since the e.mf of the cell is known and is equal to the algebraic sum of the two electrode potentials of which the potential of the other electrode is obtained. If... 

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