Electrode potential sign conventions

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... 

In consulting tables of standard electrode potentials (see Table 7.23), it is neces-saiy to be aware of an (unfortunate) difference in conventions for the sign of the E° values. Consider, e.g., the E° values for zinc electrodes dipped in Zn2+ solutions of unit activity and for copper electrodes dipped in Cu2+ solutions of unit activity. Tables... 

Fig. 7.175. The IUPAC convention ascribes to the standard electrode potential the same sign as that experimentally observed when the electrode in question is connected to a cell with the SHE.

Note also that if infinitesimally small currents are allowed to flow (see Fig. 7.175) through the galvanometer G in one or another direction, (i.e., when the electrode reactions occurring at both electrodes are reversed from their spontaneous direction), the polarity of the electrodes remains unchanged. Thus the sign of the electrode potential remains in this convention invariant, irrespective of whether the electrode processes proceed in the spontaneous or reverse direction, and thus are written as... 

Thus, the IUPAC decision supports the zinc-minus-copper-plus table of standard electrode potentials. The first thing to do, therefore, when consulting a table of standard electrode potentials is to examine the E° values of the zinc and copper electrodes. If the values are -0.76 and +0.34 V, respectively, the table can be used. If, however, the values are +0.76 and -0.34 V, the convention contravenes the IUPAC decision. To use such a table, one can retain all the magnitudes of the E° values, but change all the signs of the E° values the table will then be in accord with the international convention (Table 7.23). 

In the past the electrostatic convention has often been called the European convention and the thermodynamic convention popularized by Luitmer (The Oxidation Potentials of the Elements and Their Values in Aqueous Solution Prenlicc-HBlI Englewood Cliffs. NJ, (952) the American convention. In an effort to reduce confusion, the IUPAC adopted the "Stockholm convention" in which electrode potentials refer to the electrostatic potential and end s refer to the thermodynamic quantity. Furthermore, the recommendation is that standard reduction potentials be listed as electrode potentials" to avoid the possibility of confusion over signs. 

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. 

Historically, various sign conventions have been adopted for charge flow, electrode potential, and reaction direction. Benjamin Franklin arbitrarily called the charge... 

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. 

Many physical chemists in Europe and practical eiectrochemists in America use a convention as to the sign of e.m.f. and electrode potential which is the opposite of that employed here. 

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... 

In contrast to the silver electrode, the cadmium electrode is negative with respect to the standard hydrogen electrode. Consequently, the standard electrode potential of the Cd/Cd " couple is by convention given a negative sign, and Ecd icd — —0.403 V. Because the cell potential is negative, the spontaneous cell reaction is not the reaction as written (that is, oxidation on the left and reduction on the right). Rather, the spontaneous reaction is in the opposite direction. 

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. 

To convert these oxidation potentials to electrode potentials as defined by the lUPAC convention, one must mentally (1) express the half-reactions as reductions and (2) change the signs of the potentials. 

The sign convention used in a tabulation of electrode potentials may not be explicitly stated. This information can be readily deduced, however, by noting the direction and sign of the potential for a half-reaction with which one is familiar. If the sign agrees with the lUPAC convention, the table can be used as is if not, the signs of all of the data must be reversed. For example, the reaction... 

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 ... 

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