Sign convention, electrode

Sign Conventions Since the reaction of interest occurs at the working electrode, the classification of current is based on this reaction. A current due to the analyte s reduction is called a cathodic current and, by convention, is considered positive. Anodic currents are due to oxidation reactions and carry a negative value. 

Besides the magnitude of the current that flows through the electrode, its direction or sign, for which some convention must be adopted, should be specified. The sign convention of positive anodic and negative cathodic currents is recommended by IUPAC and corresponds to regarding currents of positive electricity passing from the electrode into the electrolyte as positive currents [8, 9]. The opposite convention in found in the electro-analytical literature. 

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. 

The sign convention adopted for metals (but not for ionic conductors) is that 0 is positive if the hot electrode is negative, so that positive current flows from a to b at the hot junction. 

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

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... 

Some variations may be found in the sign convention used in different texts on electrode kinetics. In this book we shall consistently define anodic currents and anodic overpotentials as positive and the corresponding cathodic quantities as negative. The foregoing equations are consistent with this notation. Thus, if q > 0, the anodic... 

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 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 ... 

A clarification of nomenclature and sign convention, which may often be confusing, is called for in this context. It can be stated categorically that the cathode is always the electrode at which a reduction process (e.g., hydrogen evolution or metal deposition) takes place. Similarly, the anode is always the electrode at which oxidation (e.g., oxygen evolution or metal dissolution) takes place. But, which is the positive and which is the negative elec-... 

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 difficulties that are sometimes caused by the so-called electrochemical -sign conventions-have arisen-largely because the-term electrode-potential has been used to mean two distinct things ... 

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. 

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