Electrode potentials, reversible sign convention

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... 

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... 

While the electromotive (driving) force of a half-reaction obviously changes sign vhen the half-reaction is written in the reverse direction, it is considered desirable to define electrode potentials so that they are insensitive to the reaction direction being onsidered. In this manner the standard electrode potential of the Zn +—Zn couple s given as —0.76 volt, using the convention of writing the half-reaction as a reduc-. lon. 

In order for the overall corrosion reaction in Eq. (2.50) to proceed as written, one should substitute the potential of the anodic reaction in Eq. (2.39), which as shown in Eq. (2.51) is oxidation of tin to tin oxide. According to lUPAC convention, the sign of the half-ceU electrode potential must be reversed from cathodic e° = —0.138 V in Table 2.2 to e° = +0.138 V. The potential of the anodic reaction has the same magnitude but the opposite sign of the cathode half-ceU reaction in Table 2.2. Because the potential of the SHE is set to zero, the ceU potential for reaction (2.50) is ... 

The reverse method, oxidation potentials (the American convention) is seen in many books and is used by some practicing chemists in their publications. Vigilance is required tn deduce what is meant by electrode potential without the qualifying terms, reduction, oxidation, or lUPAC convention, etc. Careful attention to directions and signs, holding to one consistent form of the Nernst equation, is recommended to avoid sign mistakes in calculations. 

The potential of an electrode measured relative to a standard, usually the SHE. It is a measure of the driving force of the electrode reaction and is temperature and activity dependent (p. 230). By convention, the half-cell reaction must be written as a reduction and the potential designated positive if the reduction proceeds spontaneously with respect to the SHE, otherwise it is negative. If the sign of the potential is reversed, it must be referred to as an oxidation potential. 

We conventionally record the e.m.f.s of cells as the potential difference between the right and the left electrodes. Thus if we had reversed the cell, so that Zn was on the right, the sign of would be reversed. 

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