Standard potential difference

Because of lithium s low density and high standard potential difference (good oxidation reduction characteristics), cells using lithium at the anode have a very high energy density relative to lead, nickel and even zinc. Its high cost limits use to the more sophisticated and expensive electronic equipment. 

Assuming unit ion activity the potential difference is equal to the first term this term is called standard potential difference or more commonly standard electrode potential. Eq. (1.5) thus simplifies to... 

Girault and Schiffrin [6] and Samec et al. [39] used the pendant drop video-image method to measure the surface tension of the ideally polarized water-1,2-dichloroethane interface in the presence of KCl [6] or LiCl [39] in water and tetrabutylammonium tetraphenylborate in 1,2-dichloroethane. Electrocapillary curves of a shape resembling that for the water-nitrobenzene interface were obtained, but a detailed analysis of the surface tension data was not undertaken. An independent measurement of the zero-charge potential difference by the streaming-jet electrode technique [40] in the same system provided the value identical with the potential of the electrocapillary maximum. On the basis of the standard potential difference of —0.225 V for the tetrabutylammonium ion transfer, the zero-charge potential difference was estimated as equal to 8 10 mV [41]. 

Fig. 8 (a) Variations of the forward rate constant, k, with the standard potential difference, — rx/rx -> a function of the rate constant of the follow-up reaction, k (values on each curve in s" ) for typical values of kj (10 m ... 

The values of the standard Gibbs energy between nitrobenzene and water and the values of the standard potential differences between these two solvents for various ions are given in table 2.1. Equations (2.2.8) and (2.2.10) can be generalized readily for an electrolyte of any type of charge. 

A 0f is the standard potential difference between phases a and p for this ion, kf is the standard rate constant for transfer of ion / and a is the charge-transfer coefficient. Concentrations c (a) and c (/3) correspond to the immediate vicinity of the phase boundary and are functions of the potential differences in the diffuse double layers according to the Boltzmann relationship... 

The two half reactions of any redox reaction together make up an electrochemical cell. This cell has a standard potential difference, E , which is the voltage of the reaction at 25 °C when all substances involved are at unit activity. E refers to the potential difference when the substances are not in the standard state. E for a particular reaction can be found by subtracting one half cell reaction from the other and also subtracting the corresponding voltages. For example for reduction of Fe to Fe by H2, E° = 0.77 - 0 = 0.77 V. A further example is the oxidation of Fe " by solid Mn02 in acid solution. The half cell reactions are. 

In addition, electrode reactions are frequently characterized by an irreversible, i.e., slow, electron transfer. Therefore, overpotentials have to be applied in preparative-scale electrolyses to a smaller or larger extent. This means not only a higher energy consumption but also a loss in selectivity as other functions within the molecule can already be attacked. In the case of indirect electrolyses, no overpotentials are encountered as long as reversible redox systems are used as mediators. It is very exciting that not only overpotentials can be eliminated but frequently redox catalysts can be applied with potentials which are 600 mV or in some cases even up to 1 Volt lower than the electrode potentials of the substrates. These so-called redox reactions opposite to the standard potential gradient can take place in two different ways. In the first place, a thermodynamically unfavorable electron-transfer equilibrium (Eq. (3)) may be followed by a fast and irreversible step (Eq. (4)) which will shift the electron-transfer equilibrium to the product side. In this case the reaction rate (Eq. (5)) is not only controlled by the equilibrium constant K, i.e., by the standard potential difference be-... 

The standard potential difference of the Ag/AgCl reference electrode E° is determined in cell (I) filled with HC1 at a fixed molality. For the molality of 0.01 mol kg-1, the values for the mean activity coefficient of the HC1 are given in [7] at various temperatures. 

The measurements to get the cell potential EIa of cell I filled with HC1 and Ex of cell I filled with buffer are performed simultaneously. The difference AE= Ei — Ia is therefore independent of the standard potential difference. 

The electric work W required for an electrochemical process in a practical electrolytic cell will be larger than AG °. For example, the indicated value of AG° in Equation (3) corresponds to a thermodynamic standard potential difference of ° = 1.23 V, while the voltage typically applied to the cell of Figure 3.1.1 is approximately U- 1.8 V. 

Separation of Metals by Electrolysis.—The complete separation of one metal from another is important in quantitative electro-analysis the circumstances in which such separation is possible can be readily understood from the preceding discussion of simultaneous deposition of two metals. The conditions must be adjusted so that the discharge potentials of the various cations in the solution are appreciably different. If the standard potentials differ sufficiently and there are no considerable deposition overvoltages, complete separation within the limits of analytical accuracy is possible this is, of course, contingent upon the metals not forming compounds or solid solutions under the conditions of deposition. Since the concentration of the ions of a deposited metal decreases during electrolysis, the deposition potential becomes steadily more cathodic, and may eventually approach that for the deposition of another metal. For example, if the ionic concentration is reduced to 0.1 per cent of its original value, the potential becomes 3 X 0.0295 volt more cathodic for a bivalent metal and 3 X 0.059 volt for a univalent metal, at ordinary... 

Fig. 15. Logarithm of the true forward rate constant lc vs. the inner layer potential difference relative to the standard potential difference (corrected Tafel plots) for Cs ion transfer between nitrobenzene solution of 0.05 M Pn4N[(CFj)3Ph]4B and an aqueous solution of (O) 0.05, ( ) 0.1, (V) 0.2, and (T) 0.5 M LiCl at 298 K. Vertical bars indicate the standard deviation in 0.1 M LiCl the broken line corresponds to a = 0.5. (After [163]).

Table 1. Standard potential differences diffusion coefficients or (in parentheses),...

According to the stochastic theory (Sec. 3.1.3), the hydrodynamic friction should influence mainly the pre-exponential factor of the rate constant. However, ions studied differ considerably in the standard potential difference which con-... 

Figure 2. (A) Dependence on A of the logarithm of the rate constants of reoxidation of the following radical anions (with increasing A ) biphenyl, 1-methylnaphthalene, naphthalene, 2-methylphenanthrene, phenanthrene, / -etrphenyl, and benzonitrile in the presence of chlorobenzene. From Ref. [7]. (B) Rate constants of the homogeneous electron exchange between chlorobenzene and redox catalysis as functions of the half-wave potential difference and of the standard potential difference (at room temperature in the considered solvent =). With increasing AE biphenyl, naphthalene, dibenzothiophene, phenanthrene, / -toluonitrile, m-toluonitrile, /7-terphenyl, and benzonitrile. From Refs 1 and 2.

ATP is synthesized from ADP and phosphate during electron transport in the respiratory chain. This type of phosphorylation is distinguished from substrate-level phosphorylation, which occurs as an integral part of specific reactions in glycolysis and the TCA cycle. The free energy available for the synthesis of ATP during electron transfer from NADH to oxygen can be calculated from the difference in the value of the standard potential of the electron donor system and that of the electron acceptor system. The standard potential of the NADH/NAD+ redox component is —0.32 V and that of H2O/5O2 is -1-0.82 V therefore, the standard potential difference between them is... 

If one defines the standard free energy required to transfer species i, with charge Zj, between the two phases, , in terms of a standard potential difference, (the... 

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