Potentials of Selected Half-Reactions at

TABLE 8.28 Potentials of Selected Half-Reactions at 25°C Continued)... 

Table 4.2. Electrode potentials (reduction potentials) of selected half-reactions at 25° C. (The dashed lines show the limits of electrode potential in aqueous systems).

Dean, J.A. Potentials of selected half-reactions at 25 EC Potentials of the elements and their compounds at 25 EC. Lange s Handbook of Chemistry 13th Ed. McGraw-Hill New York, 1985. Weast, R.C. Astle, M.J. Electrochemical series. CRC Handbook of Chemistry and Physics 63rd Ed. Boca Raton, FL D162-D167. 

TABLE 5.2. Reduction Potentials of Selected Half-Cell Reactions in Soil-Water Systems at 25°C... 

By convention, the electrode potential of any half-reaction is expressed relative to that of a standard hydrogen electrode (half-reaction 2H+ -p 2e -H2) and is called the standard electrode potential, E . Table 34.1 shows the values of E" for selected half-reactions. With any pair of half-reactions from this series, electrons will flow from that having the lowest electrode potential to that of the highest. " is determined at pH = 0. It is often more appropriate to express standard electrode potentials at pH 7 for biological systems, and the symbol is used in all circumstances, it is important that the pH is clearly stated. 

Table 4.7 Standard reduction potentials (in V) of selected half-reactions in the aqueous H Oy system (Milazzo and Carol 1978, Bard et al. 1985, Stanbury 1989, Wardman 1989, Holze 2007), at 25 °C. e electron transferred from electrode, hydrated electron, (g) gase-ons, aq) dissolved.

Proceeding in this way, you can obtain the electrode potential for a series of halfcell reactions. Table 20.1 lists standard electrode potentials for selected half-cells at 25°C. 

Since in experiments such as the one we have just discussed, it is only possible to determine potential differences between two electrodes (and not the absolute potential of each half cell), it is now useful to choose a reference system to which all measured potential differences may be related. In accord with the IUPAC 1953 Stockholm convention, the standard hydrogen electrode (SHE) is commonly selected as the reference electrode to which we arbitrarily assign a zero value of electrical potential. This is equivalent to assigning (arbitrarily) a standard free energy change, ArG°, of zero at all temperatures to the half reaction ... 

The electromotive series is a list of the elements in accordance with their electrode potentials. The measurement of what is commonly known as the "single electrode potential", the "half-reaction potential" or the "half-cell electromotive force" by means of a potentiometer requires a second electrode, a reference electrode, to complete the circuit. If the potential of the reference electrode is taken as zero, the measured E.M.P. will be equal to the potential of the unknown electrode on this scale. W. Ostwald prepared the first table of electrode potentials in 1887 with the dropping mercury electrode as a reference electrode. W. Nernst selected in 1889 the Normal Hydrogen Electrode as a reference electrode. G.N. Lewis and M. Randall published in 1923 their table of single electrode potentials with the Standard Hydrogen Electrode (SHE) as the reference electrode. The Commission of Electrochemistry of the I.U.P.A.C. meeting at Stockholm in 1953 defined the "electrode potential" of a half-cell with the SHE as the reference electrode. 

Figure 3. Half-reaction reduction potentials of selected organic redox couples (left side), iron(III)/iron(II) couples (middle), and of some biogeochemically important redox couples (right side). Indicated are standard reduction potentials, EJ(w), at environmentally relevant conditions, i.e., T = 25°C, pH = 7.0, [Cl ] = [HCO3] = lO M, [Br ] = 10 M. Note that the standard free-energy change, AG°(w), for a given redox reaction is obtained from the difference between the EJ(w) values of the corresponding half-reactions (see also example given by Eqs. 3-1 and 3-2) AG°(w) = - n F AEj(w) where n is the number of electrons transferred, and F = 96.48 kJ mol V is the Faraday s constant. Data from Stumm (72), Schwarzenbach et al. (63), and references cited therein (am = amorphous aq = aqueous phen = phenanthroline sal = salicylate s = solid porph = porphyrin).

The Status of the Hydrogen Electrode. Probably no area of electrochemistry is more greatly neglected in current texts than the history of the choice of the hydrogen electrode as the reference standard for electromotive force measurements. Since all tables of potentials of oxidation-reduction half-reactions are based on the half-cell reaction 35H2=H +e , it would seem that the selection of this reaction as the standard should warrant more attention. If the selection is treated at all, it is usually dismissed as an arbitrary choice, which it is, with no reference made to the people and events involved in establishing this fundamental reference point for the EMF scale. One possible exception may be noted ( ). The referenced edition of this work is perhaps the best previously existing source on this topic. However, the subsequent edition omits the subject entirely. 

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