Conventions oxidation-reduction sign

The signs of the current refer to the American convention that reduction currents are positive, whereas oxidation currents are negative (remember that in electrochemistry the signs are conventional). 

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. 

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. 

Half reactions are by convention, always written as reductions, that is showing addition of electrons going from left to right. Take careful note of the signs. When balancing the two half reactions however, one of them must be written as an oxidation. 

The notation H+ (aq) represents the hydrated proton in aqueous solution without specifying the hydration sphere. It means that the species being oxidized is always the H2 molecule and is always related to a reduction. This is the reason why we speak of reduction potentials. In the opposite case, the numerical value of E would be the same but the sign would differ. It should be mentioned that in old books, for example, in Latimer s book [1], the other sign convention was used however, the International Union of Pure and Applied Chemistry (lUPAC) has introduced the unambiguous and authoritative usage in 1974 [2, 3]. 

W. M. Latimer, Oxidation Potentials. 2nd edn.. Prentice-Hall. New York, 1952. As its title implies, this book contains oxidation potentials (i.e. reduction potentials with their signs reversed, as was the convention at the time of its publication) for all the elements. This is a classic book, full of good chemistry, written by the inventor of Latimer diagrams and still available in libraries. 

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. 

One should carefully note how Convention 3 is applied / ° Pt Pr> Pt may be r a(i from Table 4.7.1. However, in writing the lower half reaction as a reduction equation, we need a sign reversal in converting f for an oxidative process to f° for a reductive process. The contributions from C cancel in the algebraic addition process because the activity is the same on both sides. 

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. 

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-... 

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. 

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