Cell sign convention

In practice, applications requiring whole-cell calculations are rare in the Earth sciences however, it is essential to understand both half-cell and whole-cell sign conventions in designing sample and reference electrode combinations for laboratory or field measurements. This is also necessary to understand how pH, Eh, and specific ion electrodes really function. 

It is not appropriate in this chapter to tabulate quantities of electrochemical data since that required may be obtained from texts on electrodeposition.1 2 However, a brief mention of sign conventions must be made since, particularly in the early literature, confusion can arise. Two conventions have been used the European and the American .3 It is sometimes erroneously stated that the conventions differ only in sign however, the real difference lies in the distinction between the potential of an actual electrode and the EMF of a half cell reaction. 

In the past, different sign conventions were used in electrochemistry, which led to difficulty in interpretation of experiments and results. Consequently the electrochemical literature requires an understanding of this problem to avoid confusion. The approach followed in this book is summarised in this section. As pointed out in the previous section, all electrochemical cells are regarded as a combination of two half cells, with each of the latter represented by a half reaction written as a reduction ... 

The overall reaction of the electrolysis cell (Eq. 7) provides the required stoichiometric coefficients for the products and reactant used in Eq. 8. The sign convention is positive for products and negative for reactants with analogous definitions for AB, AC and AD. Data for the constants A, B, C and D are thermodynamic properties and are reproduced in Table 1 from,19... 

The potential of a cell such as that shown in Figure 18-4a is the difference between two half-cell or single-electrode potentials, one associated with the half-reaction at the right-hand electrode (f,the other associated with the half-reaction at the left-hand electrode ( eft)- According to the lUPAC sign convention, as long as the liquid-junction potential is negligible or there is no liquid junction, we may write the cell potential as... 

The lUPAC sign convention is based on the actual sign of the half-cell of interest when it is part of a cell containing the standard hydrogen electrode as the other half-cell. 

Any sign convention must be based on expressing half-cell processes in a single way—that is, either as oxidations or as reductions. According to the lUPAC convention, the term electrode potential (or, more exactly, relative electrode potential ) is reserved exclusively to describe half-reactions written as reductions. There is no objection to the use of the term oxidation potential to indicate a process written in the opposite sense, but it is not proper to refer to such a potential as an electrode potential. 

The sign convention for potentiometry is consistent with the convention described in Chapter 18 for standard electrode potential. In this convention, the indicator electrode is always treated as the right-hand electrode and the reference electrode as the left-hand electrode. For direct potentiometric measurements, the potential of a cell can then be expressed in terms of the potentials developed by the indicator electrode, the reference electrode, and a junction potential, as described in Section 21 A ... 

The dissociative form of O2 and NO2 adsorption is dominated for the NO sensor working at temperatures of 500-900°C. Desorption is accompanied by recombination of adsorbed atoms into molecules. The coordinate system and the sign convention for various fluxes in the electrochemical cell are illustrated in Figure 2.10 [40]. The species indices (1, 2, and 3) correspond respectively to NO2, O2, and H2O, respectively, in the SE (RE). 

FIGURE 2.10 Coordinate system and the sign convention for various fluxes in the YSZ-based electrochemical cell. (Reprinted from Zhuiykov, S., Mathematical modelling of YSZ-based potentiometric gas sensors with oxide sensing electrodes part I Model of interactions of measuring gas with sensor. Sensors and Actuators B, Ghent. 119 (2006) 456-465, with permission from Elsevier Science.)... 

The term R0I is called the ohmic drop.2 Figure 3.3 shows the schematic potential distribution in an electrochemical cell. At the anode a potential drop Ua occurs, which is given by the sum of the equilibrium potential U and the anodic over potential r]a. An analogous situation takes place at the cathode. Note that the electromotive force of the cell is U0 = Uejj a + Uej/ C. In practical applications of Equation (3.19), one has to pay attention to the sign convention attributed to the various quantities. 

For each surface of nonzero H, the sign convention for H reverses when the (orientable) surface is viewed from the other side. For the cases where the skeletal graph is self-dual, this corresponds to the two choices for the unit cell, one in which the centroid is a node of G and one where it is a node of G". Thus in these cases we need only substitute positive values for H in the computation, and use this transformation to display unit cells with H negative. In cases where G and G" are not congruent, both positive and negative values of H must be explicitly substituted. 

We make the sign convention that the curvature is with respect to the F graph, and for 77 0 we will display one L-F region, composed of 48 cells... 

Standard potentials for a number of metals are given in Table I the values given in many handbooks are oxidation potentials employing the opposite sign convention (the order of the electrochemical cell just shown is reversed). 

The sign conventions of electrochemistry have caused students and researchers a great deal of difficulty and misunderstanding over the years. All electrochemical cells are considered as a combination of two half-cells—one for the reduction reaction, one for the oxidation reaction. To have current flow in any electrochemical system, both an oxidation and a reduction reaction must occur electrons must have someplace to go, they simply do not appear and disappear. 

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