Electrodes, oxidation-reduction sign, convention

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. 

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. 

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-... 

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. 

A reversible electrode consists of an oxidized and a reduced state, and the reaction which occurs at such an electrode, when it forms part of an operating cell, is either oxidation (i.e. reduced state —> oxidized state "I" electrons) or reduction (i.e. oxidized state "I" electrons reduced state). It can be readily seen, therefore, that in a reversible cell consisting of two reversible electrodes, a flow of electrons, and hence a flow of current, can be maintained if oxidation occurs at one electrode and reduction at the other. According to the convention widely adopted, the e.m.f. of the cell is positive when in its normal operation oxidation takes place at the left-hand electrode of the cell as written and reduction occurs at the right-hand electrode. If the reverse is the case, so that reduction is taking place at the left-hand electrode, the e.m.f. of the cell, by convention, will have a negative sign. 

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. 

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. 

The American convention would assign a positive value to E° for the Zn Zn2+(aq) half cell written as an oxidation, but a negative sign if written as a reduction. It is seen that the European convention refers to the invariant electrostatic potential of the electrode with respect to the SHE, whereas the American convention relates to the thermodynamic Gibbs free energy which is sensitive to the direction of the cell reaction. 

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. 

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. 

It is to be remembered that reduction potential of an electrode is same as its oxidation potential with the sign changed. Usually anode of a cell is written in the left and cathode in the right. It is also a common convention that current in external circuit flows from cathode to anode although the electrons are flowing in the opposite direction through the wire. 

It should be noted that iex has been consistently defined as iex = iox,M - red,M, where iox M and ired M are always positive quantities. Therefore, the sign of iex will reveal whether the net reaction is oxidation (iex > 0) or reduction (iex < 0). This convention is consistent with external current measurements, wherein positive values reflect net oxidation at the working electrode and negative values net reduction. 

Not all texts use the lUPAC convention so that caution must be exercised in using values of E° taken from tables. Notably, the extensive presentation of half-reaction potentials in Oxidation Potentials, 2nd ed., by W. M. Latimer (Prentice-Hall, Inc., Englewood Cliffs, N.J., 1952) employs a convention that is exactly opposite to that used here i.e., the half-reaction is written as an oxidation rather than a reduction, and the E value has a sign opposite to the standard reduction potential. In other textbooks, the value for the reduction half-reaction has a sign opposite to the standard electrode potential so that AG - nFE. 

We shall follow the practice of denoting all standard potentials in the reduction direction, called the lUPAC, or Stockholm convention. This has the advantage, over the oxidation potential method, of giving the potentials the electrostatic sign of the electrode when connected to standard hydrogen, and also the mathematical sign indicating spontaneity (-I-) or nonspontaneity ( —) in the Nernst equation. The methods are evaluated in detail by Bockris and Reddy. ... 

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