Half reduction potentials

According to Ohnishi 121), the half-reduction potentials of the various iron-sulfur centers of the respiratory chain at pH 7.2 are as shown in Fig. 18. The value for center 1 is essentially in agreement with the results of Orme-Johnson et al. 46, 54) who found that reduced acetylpyridine adenine dinucleotide (Bo = —248 mV) can only reduce this center by 50%, while its oxidized form can effectively oxidize iron-sulfur center... 

Dutton, P. L., Petty, K. M., Bonner, H. S., and Morse, S. D., 1975, Cytochrome c and reaction center of Rhodopseudomonas sphaeroides Ga membranes. Extinction coefficients, content, half-reduction potentials, kinetics and electric field altertions. Biochim. Biophys. Acta, 387 5369556. 

Half- reduction potential -305mV -245mV -20mV +40mV... 

The Half-Reduction Potentials of the Components of the Mitochondrial Respiratory Chain (mV), pH = 7.2 ... 

P.L. Dutton, K.M. Petty, H.S. Bonner, and S.D. Morse, Cytochrome C2 and Reaction Center of Rhodopseudomonas spheroides Ga. Membranes. Extinction Coefficients, Content, Half-Reduction Potentials, Kinetics and Electric Field Alterations, Biochim. Biophys. Acta 387 536 (1975). 

Since the potential for a single half-reaction cannot be measured, a reference halfreaction is arbitrarily assigned a standard-state potential of zero. All other reduction potentials are reported relative to this reference. The standard half-reaction is... 

The electrochemical potential for the reaction is the difference between the reduction potentials for the reduction and oxidation half-reactions thus,... 

A voltammogram for the two-electron reduction of M has a half-wave potential of —0.226 V versus the SCE. In the presence of an excess of the ligand L, the following half-wave potentials are recorded... 

This experiment describes the determination of the stability (cumulative formation) constant for the formation of Pb(OH)3 by measuring the shift in the half-wave potential for the reduction of Pb + as a function of the concentration of OH . The influence of ionic strength is also considered, and results are extrapolated to zero ionic strength to determine the thermodynamic formation constant. 

The limiting current was 5.67 tA. Verify that the reduction reaction is reversible, and determine values for n and 1/2. The half-wave potentials for the normal pulse polarograms of Pb + in the presence of several different concentrations of OH are shown in the following table. 

Polarographic studies on haloisoxazoles in anhydrous DMF containing R4N" were performed and the magnitude of the half-wave potentials were recorded. Cleavage of the C—X bond was faster in phenylhaloisoxazoles than in halobenzenes. Substitution patterns affected the reduction (79ZOB1322). 

The polarographic half-wave reduction potential of 4-nitroisothiazole is -0.45 V (pH 7, vs. saturated calomel electrode). This potential is related to the electron affinity of the molecule and it provides a measure of the energy of the LUMO. Pulse radiolysis and ESR studies have been carried out on the radical anions arising from one-electron reduction of 4-nitroisothiazole and other nitro-heterocycles (76MI41704). 

Standard, reduction potentials are determined by measuring the voltages generated in reaction half-cells (Figure 21.2). A half-cell consists of a solution containing 1 M concentrations of both the oxidized and reduced forms of the substance whose reduction potential is being measured, and a simple electrode. 

If electron flow between the electrodes is toward the sample half-cell, reduction occurs spontaneously in the sample half-cell, and the reduction potential is said to be positive. If electron flow between the electrodes is away from the sample half-cell and toward the reference cell, the reduction potential is said to be negative because electron loss (oxidation) is occurring in the sample halfcell. Strictly speaking, the standard reduction potential, is the electromotive force generated at 25°C and pH 7.0 by a sample half-cell (containing 1 M concentrations of the oxidized and reduced species) with respect to a reference half-cell. (Note that the reduction potential of the hydrogen half-cell is pH-dependent. The standard reduction potential, 0.0 V, assumes 1 MH. The hydrogen half-cell measured at pH 7.0 has an of —0.421 V.)... 

Figure 21.2a shows a sample/reference half-cell pair for measurement of the standard reduction potential of the acetaldehyde/ethanol couple. Because electrons flow toward the reference half-cell and away from the sample half-cell, the standard reduction potential is negative, specifically —0.197 V. In contrast, the fumarate/succinate couple and the Fe /Fe couple both cause electrons to flow from the reference half-cell to the sample half-cell that is, reduction occurs spontaneously in each system, and the reduction potentials of both are thus positive. The standard reduction potential for the Fe /Fe half-cell is much larger than that for the fumarate/ succinate half-cell, with values of + 0.771 V and +0.031 V, respectively. For each half-cell, a half-cell reaction describes the reaction taking place. For the fumarate/succinate half-cell coupled to a H Hg reference half-cell, the reaction occurring is indeed a reduction of fumarate. 

Some typical half-cell reactions and their respective standard reduction potentials are listed in Table 21.1. Whenever reactions of this type are tabulated, they are uniformly written as reduction reactions, regardless of what occurs in the given half-cell. The sign of the standard reduction potential indicates which reaction really occurs when the given half-cell is combined with the reference hydrogen half-cell. Redox couples that have large positive reduction potentials... 

Standard Reduction Potentials for Several Biological Reduction Half-Reactions ... 

The half-reactions and reduction potentials in Table 21.1 can be used to analyze energy changes in redox reactions. The oxidation of NADH to NAD can be coupled with the reduction of a-ketoglutarate to isocitrate ... 

In a spontaneous reaction, electrons are donated by (flow away from) the half-reaction with the more negative reduction potential and are accepted by (flow toward) the half-reaction with the more positive reduction potential. Thus, in the present case, isocitrate donates electrons and NAD accepts electrons. The convention defines as... 

When an element can exist in several oxidation staie.s it is sometimes convenient to display the various reduction potentials diagramaltcally. the corresponding half-reactions under standard conditioas being implied. Thus, in acidic aqueous soiultons... 

In addition to simple dissolution, ionic dissociation and solvolysis, two further classes of reaction are of pre-eminent importance in aqueous solution chemistry, namely acid-base reactions (p. 48) and oxidation-reduction reactions. In water, the oxygen atom is in its lowest oxidation state (—2). Standard reduction potentials (p. 435) of oxygen in acid and alkaline solution are listed in Table 14.10- and shown diagramatically in the scheme opposite. It is important to remember that if or OH appear in the electrode half-reaction, then the electrode potential will change markedly with the pH. Thus for the first reaction in Table 14.10 O2 -I-4H+ -I- 4e 2H2O, although E° = 1.229 V,... 

The potential at the point on the polarographic wave where the current is equal to one-half the diffusion current is termed the half-wave potential and is designated by 1/2. It is quite clear from equation (9) that 1/2 is a characteristic constant for a reversible oxidation-reduction system and that its value is independent of the concentration of the oxidant [Ox] in the bulk of the solution. It follows from equations (8) and (9) that at 25 °C ... 

The shift of the half-wave potentials of metal ions by complexation is of value in polarographic analysis to eliminate the interfering effect of one metal upon another, and to promote sufficient separation of the waves of metals in mixtures to make possible their simultaneous determination. Thus, in the analysis of copper-base alloys for nickel, lead, etc., the reduction wave of copper(II) ions in most supporting electrolytes precedes that of the other metals and swamps those of the other metals present by using a cyanide supporting electrolyte, the copper is converted into the difficultly reducible cyanocuprate(I) ion and, in such a medium, nickel, lead, etc., can be determined. 

E° values have been measured for many reactions and tabulated as standard half-cell potentials. Table 9.3 summarizes half-cell potentials as standard reduction potentials for a select set of reactions.aa In the tabulations, E° for... 

---