Standard potential of transfer

The standard potential of transfer for an individual ion, A cp , is not amenable to thermodynamic measurement. Its value can be determined by measuring the distribution ratio of its salt, for which the Gibbs free energy of transfer of the counterion is already known. From the experimentally accessible partition coefficient of the salt, the standard Gibbs free energy of transfer of the salt, AG aI7P, from phase a to phase p is calculated as... 

The quantity AGtr TPAsTpB is experimentally accessible from a partition ratio for the salt itself and was used to calculate individual Gibbs energies of transfer for many ions (17). Table I lists the values used in this work. A corresponding standard potential of transfer for an individual ion is calculated from the standard Gibbs free energy for the transfer of individual ion from phase a to the phase P as... 

Table I. Standard Potentials of Transfer between Water and Nitrobenzene for Ions Used in the Potentiometry Measurements...

This system behaves like a nonpolarizable interface. The salt concentration ratio will not be affected by potential applied from an extraneous source. The equilibrium potential depends only on the standard potentials of transfer of the ions in particular, it does not depend on the initial concentrations (ca and cp) nor is it a function of the phase volumes. Therefore, if only one salt is present in a LL system, the system is not amenable to potentiometric studies. It is thus essential that a supporting electrolyte be present to observe a potentiometric response of a third ion. The need to have a supporting electrolyte is similar to the need of immobilized ions in an ion exchanger membrane of an ion-selective electrode it also explains why it is essential that a supporting electrolyte or physiological concentration of salts must be present in measurements that employ fluorescent dyes. 

The minimal repartitioning when a potential difference is applied on this interface is reflected by only negligible current flow. This behavior is observed within the limits of the standard potentials of transfer of the ions present in the potential window region of the system (cf., Figure 3). For the previously mentioned TBATPB-LiCl system, the potential window is limited on the positive end by TPB (A cp0 = 372 mV) and on the negative end by TBA+ (A [Pg.72]

By analogy with the Nemst equation, one can define the first term on the right-hand side of equation (20.3.1-1) as the standard potential of transfer of the ion, i, between phases a and... 

The usual Tafel evaluation yielded a transfer coefficient a = 0.52 and a rate constant k of 4x 10 cm s at the standard potential of the MV /MV couple. This k value corresponds to a moderately fast electrochemical reaction. In this electrode-kinetic treatment the changes in the rate of electron transfer with pH were attributed only to the changes in the overpotential. A more exact treatment should also take into account the electrostatic effect on the rate of reaction which also changes with pH. 

Le Hung presented a general theoretical approach for calculating the Galvani potential Ajyj at the interface of two immiscible electrolyte solutions, e.g., aqueous (w) and organic solvent (s) [25]. Le Hung s approach allows the calculation of when the initial concentration (Cj), activity coefficients (j/,) and standard energies of transfer of ions (AjG ) are known in both solutions. 

An alternative electrochemical method has recently been used to obtain the standard potentials of a series of 31 PhO /PhO- redox couples (13). This method uses conventional cyclic voltammetry, and it is based on the CV s obtained on alkaline solutions of the phenols. The observed CV s are completely irreversible and simply show a wave corresponding to the one-electron oxidation of PhO-. The irreversibility is due to the rapid homogeneous decay of the PhO radicals produced, such that no reverse wave can be detected. It is well known that PhO radicals decay with second-order kinetics and rate constants close to the diffusion-controlled limit. If the mechanism of the electrochemical oxidation of PhO- consists of diffusion-limited transfer of the electron from PhO- to the electrode and the second-order decay of the PhO radicals, the following equation describes the scan-rate dependence of the peak potential ... 

The equilibrium constant for the disproportionation reaction, KD, may be expressed as a function of the standard potentials of the two-electrode electron transfer reactions according to... 

If the standard potential of the A/B couple, B, is known independently, we obtain the rate constant kc for decomposition of the transient intermediate B. If not, kc can be obtained when the following conditions are achieved. Upon increasing the mediator concentration, while keeping the excess factor, y = C /Cp, constant, the system tends to pass from kinetic control by the forward electron transfer step to control by the follow-up reaction (Figure 2.21). An ideal situation would be reached if the available concentration range would allow perusal of the entire intermediary variation between the two limiting situations. More commonly encountered situations are when it is possible to enter the intermediary zone coming from the forward electron transfer control zone or, conversely, to pass from the intermediary zone to the follow-up reaction control zone. In both cases the values of ke and Ke /kc can... 

Back electron transfer is at the diffusion limit because the homogeneous electron transfer reaction is uphill, owing to the fact that the standard potential of the redox catalyst is necessarily chosen as positive of the reduction potential of the substrate. 

The method consists of plotting the forward electron transfer rate constant against the standard potential of a series of redox catalysts as illustrated by Figure 2.29. Three regions appear on the resulting Bronsted plot, which correspond to the following reaction scheme (Scheme 2.14). The... 

FIGURE 3.15. Electrochemical reduction of 4-cyano-tert-butylperbenzoate in DMF showing the variation of the transfer coefficient, a, with the difference in standard potentials of the perbenzoate and the benzoate. Adapted from Figure 8 in reference 20, with permision from the American Chemical Society. 

Standard potential of the second electron transfer more cathodic than that of the first electron transfer (AE0 negative). One can consider the case where the formal electrode potential of the second couple is more cathodic, by at least 180 mV, with respect to the first couple (which has, for example, E01 = 0.00 V). If kf is low (compared to the intervention times of cyclic voltammetry i.e. if k[< n F- v/R T), the response will be due to the first electron transfer process, without complications caused by the following chemical reaction. As increases, the second process will have increasing effect up to the limiting case in which kt >n-F-v/R-T. In this limiting case the voltammogram will display two forward peaks, but only the second electron transfer will exhibit a return peak. 

Standard potential of the second electron transfer equal to that of the first electron transfer (AE° = 0). When the potential of the couple Ox/Red is equal to that of the couple Ox /Red it is quite probable that the entire system displays a single forward peak associated with a single reverse peak. 

Standard potential of the second electron transfer more anodic than that of the first electron transfer (AE01 positive). The case in which the product Ox, generated by the chemical reaction following the first electron transfer, is more easily reduced than the original species Ox constitutes another common ECE mechanism in inorganic electrochemistry. 

The outcome of the competition is represented in Fig. 5 in terms of the location of the half-wave potential of the RX reduction wave (i.e. the current-potential curve), relative to the standard potential of the RX/ RX- couple, E° (Andrieux et al., 1978). As concerns the competition, three main regions of interest appear in the diagram. On the left-hand side, the follow-up reaction is so slow (as compared to diffusion) that the overall process is kinetically controlled by the parameter A, i.e. by electron transfer and diffusion. Then, going upward, the kinetic control passes from electron transfer to diffusion. In the upper section d in the lower section... 

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