Cyclic voltammetry midpoint potential

Figure 1 displays derivative cyclic voltammograms for the oxidation of Cp Mn(CO)2(NCMe) (1 left) and Cp Mn(CO)2(Pl%3) (2 right) at a voltage sweep rate v - 0.2 V/s. Cyclic voltammetry peak potentials correspond to the intersections between the DCV curves and the base line. The reversible potentials for the oxidation of 1 and 2, taken as the midpoints between the anodic and cathodic CV peaks, are located at -0.12 and +0.22 V vs the ferrocene/ferricinium couple (Fc), respectively. The unity ratio of the cathodic to anodic derivative peak currents demonstrates the chemical reversibility of the 1/1 and 2/2- couples. 

In this chapter, the voltammetric study of local anesthetics (procaine and related compounds) [14—16], antihistamines (doxylamine and related compounds) [17,22], and uncouplers (2,4-dinitrophenol and related compounds) [18] at nitrobenzene (NB]Uwater (W) and 1,2-dichloroethane (DCE)-water (W) interfaces is discussed. Potential step voltammetry (chronoamperometry) or normal pulse voltammetry (NPV) and potential sweep voltammetry or cyclic voltammetry (CV) have been employed. Theoretical equations of the half-wave potential vs. pH diagram are derived and applied to interpret the midpoint potential or half-wave potential vs. pH plots to evaluate physicochemical properties, including the partition coefficients and dissociation constants of the drugs. Voltammetric study of the kinetics of protonation of base (procaine) in aqueous solution is also discussed. Finally, application to structure-activity relationship and mode of action study will be discussed briefly. 

The various forms of adsorbed copper can alter the electrochemical behavior of modified carbon samples used as electrode materials (powdered working electrodes in cyclic voltammetry). Figures 45 and 46 show cyclic voltammograms (CVs) for powdered electrodes prepared from selected active carbon samples with and without preadsorbed copper recorded in solution, which do or do not contain Cu " ions. An aqueous solution of 0.5 M NaiS04 as background electrolyte was employed. The CV curves recorded in the solution containing copper ions exhibit a pair of cathodic and anodic peaks, the potentials of which are dependent on the carbon modification procedure and the electrolyte s pH. The estimated peak potentials and the midpoint potentials [formal potentials, Ef = ( p, — p,c)/2] are given in Table 13. 

One of the first reports on the quasireversible electrochemistry of redox proteins appeared in 1977 when Eddowes and Hill demonstrated (10) cyclic voltammetry of horse heart cytochrome c at a gold electrode in the presence of 4,4 -bipyridyl (Bipy) in solution. In the voltammo-grams (Fig. 1), the peak-to-peak separations were close to 60 mV and the faradaic currents varied linearly with (scan rate), indicating a quasireversible one-electron transfer process controlled by linear diffusion of redox species to the electrode surface. The midpoint potential... 

The electrochemical reduction of Cgo results in the formation of a series of fulleride ions, [Cgo] where n = 1-6. The midpoint potentials (obtained using cyclic voltammetry and measured with respect to the ferrocenium/ferrocene couple, Fc /Fc = 0 V, ferrocene see Section 23.13) for the reversible one-electron steps at 213 K are given in scheme 13.16. 

Fig. 13. Midpoint potential (E°) as a function of pH. Data points ( 3 mV) were obtained with cyclic voltammetry. Protein concentration was 0.10-0.15 roM in 50 mAf cacodylate/NaOH or HEPES/NaOH buffer. The solid circles and the left y axis refer to wild-t3T3e cyt C550, whereas the open circles and the right y axis are for the MIOOK cytochrome C550 variant.

An attempt to follow by direct electrochemistry the red-ox reactions involving the cofk tors of the RC embedded in lipid films on pyrolytic graphite electrodes has been recendy carried out, allovdng the evaluation of the peaks relative to quinones and the primary donor. Direct electrochemistry of cofactors was also realized for RC in a lipid film on graphite and ITO or sandvdched between polycation layers on gold, permitting the determination of their midpoint potentials by cyclic and square wave voltammetry. In this case evidence of the presence of peaks relative to the bacteriopheophytin was reported for the first time. ... 

Voltammetry of Adsorbed Proteins, Fig. 2 Non-tumover behavior during cyclic voltammetry at scan rates where the protein oxidation state is in equilihrimn with the electrode potential at all times in the experiment, (a) Cyclic voltammogram of a surface confined redox center undergoing redox transformation with a midpoint potential of 0.1 V and n =. The peak potentials of the reductive, Ep and oxidative, Ep , peaks are indicated. 

Voltammetry of Adsorbed Proteins, Fig. 3 Illustrative cyclic voltammograms from non-turnover processes measured at scan rates that are sufficiently high to preclude equilibration with the electrode potential, (a) Interfacial electron transfer is rate limiting when the peaks (black lines) are offset symmetrically from the midpoint potential of the redox... 

Clay platelets can serve as inorganic polyanions. Hb retained its native conformation in films made by casting aqueous dispersions of the protein and a DDAB-clay composite [63], Cyclic voltammetry of Hb-DDAB-clay films on pyrolytic graphite (PG) electrodes showed a pair of well-defined and nearly reversible peaks at a midpoint potential of about -0.06 V vs. NHE at pH 7. Only the inner layers of the film closest to the electrode were electrochemically active. These films had excellent stability over several months storage. Hb in these films is not located completely between the clay platelets. 

The formal or midpoint potential (E°) of a redox species is calculated by averaging the reduction and oxidative peak potentials. The rate at which an ET occurs between electrode and cyt P450-heme center can be determined by acquiring the cyclic voltammograms with increasing scan rates and by applying the Buder-Volmer and Marcus theories. One can also obtain information about non-electrochemical processes associated with an electrochemical event from cycUc voltammetry. One example is proton-coupled ET. 

Cyclic and pulsed voltammetric studies of Mb-surfac-tant films have been used to obtain electrochemical parameters such as electron transfer rate constants and formal potentials E° ), i.e. apparent standard potentials under given experimental conditions. Recent work has shown that a Gaussian distribution model for protein molecules with slightly different ° -values fits voltammetric data in thin surfactant films [26, 33]. This model was used with nonlinear regression to extract average ° and values from square wave voltammograms (SWV). Simple models for voltammetry of single species confined to the electrode surface did not fit the data, but formal potentials could be estimated from the midpoints of CV cathodic and anodic peaks. 

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