Standard Reduction Potentials at

Obviously the only variable on which the potential depends is [ CF]. The saturated KC1 present provides the [CF] for the reaction, and, since it is a saturated solution, [CF] is a constant at a given temperature represented by the solubility of KC1 at that temperature. If [CF] is constant, the potential of this halfcell, dependent only on the [CF], is therefore also a constant. As long as KC1 is kept saturated and the temperature kept constant, the SCE is useful as a reference against which all other potential measurements can be made. Its standard reduction potential at 25°C (see Table 14.1) is +0.2412 V. 

A variety of materials of pyrotechnic interest, and their standard reduction potentials at 25°C are listed in Table 2.5. Note the large positive values associated with certain oxygen-rich negative ions, such as the chlorate ion (CIO 3 ), and the large negative values associated with certain active metals such as aluminum (Al). 

Careful potentiometric titrations by Draper and Ingraham have given values (standard reduction potentials at pH 7.0) of —238 mV for E 02 and —172 mV for E i for FMN and corresponding values of —231 mV and —167 mV for riboflavin. Values of potential as a function of pH are tabulated in this paper and shown in a graphical presentation in Fig. 5. 

Many half-reactions of interest to biochemists involve protons. As in the definition of AG °, biochemists define the standard state for oxidation-reduction reactions as pH 7 and express reduction potential as E °, the standard reduction potential at pH 7. The standard reduction potentials given in Table 13-7 and used throughout this book are values for E ° and are therefore valid only for systems at neutral pH Each value represents the potential difference when the conjugate redox pair, at 1 m concentrations and pH 7, is connected with the standard (pH 0) hydrogen electrode. Notice in Table 13-7 that when the conjugate pair 2ET/H2 at pH 7 is connected with the standard hydrogen electrode (pH 0), electrons tend to flow from the pH 7 cell to the standard (pH 0) cell the measured E ° for the 2ET/H2 pair is -0.414 V... 

Table II. Standard Reduction Potentials (at 25°C.) for Various Plutonium Hydroxides in Equilibrium with Pu(OH)4 (21)...

E° Effective standard reduction potential at pH 7 (or at some other specified conditions). 

Given the following standard reduction potentials at 25°C, (a) balance the equation for the reaction of H2M0O4 with elemental arsenic in acidic solution to give Mo3+ and H3As04, and (b) calculate E° for this reaction. 

Table A5.6 gives selected values for standard reduction potentials at T = 298.15 K. The values were taken from W. M. Latimer, The Oxidation States of the Elements and their Potentials in Aqueous Solutions, Second Edition, Prentice-Hall, Inc., Engelwood Cliffs, N.J. (1952).

Standard potentials (reactants and products at unit activity, hydrogen ion activity of 1, i.e. pH = 0), of reduction, oxidation, net reaction, the ith redox-active species, and any half-reaction Standard reduction potential at hydrogen ion activity of 10 7 (pH = 7.0)... 

The ability of the fight actinides to access multiple oxidation states leads to rich, and, sometimes, complex electrochemistry. The standard reduction potentials at pH = 0 for each of the... 

The Tl -Tl relationship is therefore a dominant feature of thallium chemistry. The standard reduction potentials at 25 °C and unit activity of H+ are TIVtI = —0.336 V, T1 /T1 = +0.72 V, and Tl /Tli = +1.25V. Estimates have also been made for the couples T1 /T1 = +0.33 V and Tl /Tl = 2.22 V. The generally valid limitations concerning the use of standard electrode potentials to predict the redox chemistry of real systems are especially important in the case of thallium factors such as complex formation in the presence of coordinating anions or neutral ligands and pH dependence due to hydrolysis do affect the actual or formal redox potentials. For example, redox potentials have been measmed for TICI/TICI3 =+0.77 V in IM HCl and T10H/T1(0H)3 = —0.05 V in alkaline soluhon. These formal potentials differ from the standard value for Tiin/Tii = +1.25 V. The difference can be attributed to the substanhal difference between the complex forming abilities of Tl and Tl , which will be discussed in detail later. The... 

TABLE 11.1 Standard Reduction Potentials at 25°C (298 K) for Many Common Half-reactions... 

EcDs are used for quantitation of compounds which can be easily oxidized or reduced by an applied potential. The standard reduction potential at the electrode is measured... 

A galvanic cell is prepared with solutions of Mg2+ and Al i+ ions separated by a salt bridge. A potentiometer reads the difference across the electrodes to be 1.05 Volts. The following standard reduction potentials at 25°C apply ... 

The (standard) reduction potentials at pH 7 of some important biogeochemi cal redox couples are given in Table 5 together with the reduction potentials ofl some half-reactions involving xenobiotic organic species. From the data in Tubtifl 5 we can, for example, conclude that, from a thermodynamic point of view, thtfl... 

TABLE 5. (Standard) Reduction Potentials at 25°C and pH 7 of Some Redox Couples Important in Natural Redox Processes0 and of Some Half-Reactions Involving Organic Compounds6. Half-Reactions Ordered in Decreasing (VV) Values"... 

Table 7.1 Selected standard reduction potentials (at 298 K) further data are listed in Appendix 11. The concentration of each substance in aqueous solution is Imoldm and the pressure of a gaseous component is 1 bar (10 Pa). Note that where the halfcell contains [OH] , the value of E° refers to [OH ] = Imoldm , and the notation °[oh-] = i should be used (see Box 7.1).

A typical example is the recently purified cssa from the green algae, Kirchneriella obesa (308). It has one heme per 12,000 molecular weight, absorption bands at 553, 522, and 417 nm, an acid isoelectric point, and a rather high standard reduction potential at pH 7 of - -400 mV. Oxidized C663 from K. obesa is easily reduced by ascorbate or dithionite, and the reduced form does not bind CO and is not autoxidized, but can be oxidized by ferricyanide, all being traits familiar from respiratory c. The c-type heme is held to the polypeptide chain by two thioether links. 

Fig. 41. Experimentally studied electron transport chain from D. desulfuricans H29). Abbreviations PyDH, p3 ruvate dehydrogenase FD, ferredoxin. Standard reduction potentials at pH 7 are given below, where known.

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