Potentiometry with electron transfer

Potentiometry True Equilibrium and Monitoring Systems with Electron Transfer... 

Eddowes and Hill found [48,49] that essentially reversible cyclic voltammetry of horse mitochondrial cytochrome c could be achieved with a Au electrode onto which was adsorbed, from the same solution, the reagent 4,4 -bipyridyl. The result is shown in Fig. 4. The criteria described by Nicholson and Shain [50] for a one-electron process controlled by linear diffusion of species to a planar electrode surface are met very closely indeed. The value of E°, given by (Ep -f- Epa)/2, was 255 mV, in good agreement with values determined by potentiometry. It could be argued that free, reduced 4,4 -bipyridyl played no part in the mechanism, since its reduction potential is much lower than that of cytochrome c. It was proposed [7] that the organic adsorbate allowed electron transfer to occur directly by providing, at the electrode surface, functionalities with which the protein could interact specifically and reversibly and thereupon donate or accept electrons rapidly. It was thus termed a promoter as opposed to a mediator, in which the latter is considered to convey electrons in bulk solution. 

Potentiometry While most SECM studies are carried out with amperometric tips that drive a Faradaic (electron transfer) reaction, it is also possible to nse potentiometric tips that produce a potential change in response to concentration changes of species. These are usually typical ion-selective electrode tips, although other types, such as Sb tips for pH detection have been described. Probes of this type and their applications are discussed in Chapter 10. They are particnlarly nseful for studies of species that do not show electroactivity, like Na+, K+, and Ca +. Note, however, that ions of this type can alternatively be determined in an amperometric mode by the use of micropipette electrodes that respond to the transport of ions across an interface between two immiscible liquids. ... 

E0 = -0.042 V (Pierick et al., 1993). The reduced P-cluster of Av2 transfers at least one electron to FeMocoN to form the ESR silent "super-reduced" FeMoco, (FeMoco) 1. (Ohrme-Johnson et al., 1972 Miinck et al., 1975). A combination of cyclic voltampermetry, potentiometry and ESR spectroscopy has allowed the observation in isolated FeMoco of two redox transitions with essentially different potentials, E0 = - 0.3 eV and E0 = -1.0 eV (Newton et al., 1996). The first transition of FeMoco is from oxidize state to semireduced state of FeMoco and the second transition is related to the substrate-reducing state in FeMoco during the nitrogenase reaction turnover. Independently, for the redox pair FeMocoN ( FeMoco)2", the Eo was estimated as... 

---