Quarter-wave potential

For the tetrabutylammonium salts of substituted acetate the quarter wave potentials have been determined by chronopotentiometry in acetonitrile. The ease of oxidation, as reflected in the Ej -values, decreases with increasing strength of the acid [88]. 

Various oxidation and reduction reactions of substituted metallocenes have already been discussed. A large number of substituted metallocenes have been oxidized chronopotentiometrically at a platinum foil in acetonitrile solution (39, 46). Electron-withdrawing substituents decrease the ease of oxidation, while electron-donating substituents increase the ease of oxidation with respect to the parent metallocenes. A plot of chronopotentiometric quarter-wave potentials, El, vs. Hammett para-sigma constants shows a definite linear relationship. The Ei s for ruthenocene and osmocene indicate these metallocenes are more difficultly oxidized than ferrocene, in agreement with earlier qualitative observations (18). 

Data are quarter wave potentials for I- oxidation reported in ref. 22 and measured at Pt, see Experimental. 

In the chronopotentiometric method, the quarter wave potentials are used and the corresponding function may be written as... 

The quarter-wave potential used in Eq. 50K is equal to the polarographic half-wave potential and is therefore characteristic of the electroactive species in solution. If the reaction rate is slow and the transient is measured in the linear Tafel region, Eq. 42K must be used... 

The first attempt to investigate substituent effects by this oxidation technique was that of Kuwana and co-workers 38). They observed that the quarter-wave potential determined by chronopotentiometry varied considerably with the nature of the substituent. It was quite evident from their data that ferrocenes which contained electron-donating groups were more easily oxidized than ferrocene and those which contained electron-withdrawing groups were more resistant to oxidation than ferrocene itself. These results were not surprising since it was known that amino- (/) and hydroxyferrocene (5) suffered oxidation within a very short period when exposed to the atmosphere. 

Chronopotentiometric Quarter-Wave Potentials for Reversibly Oxidized Ferrocene Derivatives in Acetonitrile at 25°C, Relative to the Quarter-Wave Potential for... 

Thus, the quarter-wave potential plays the same role in chronopotentiometry [228] as the half-wave potential Ej/o in polarography (see Sec, V). The mathematical formalism for chronopotentiometiy has been developed also for the application of ultramicroelectrodes [229]. 

This term denotes a potential whose nature depends on the technique used. Typical characteristic potentials are the half-wave potential in polarography, the quarter-transition-time potential in chronopotentiometry, and the peak or half-peak potential in stationary-electrode voltammetry. Regardless of its nature, the characteristic potential always depends on the identity of the electroactive substance, on the kinetics or thermodynamics of the electron-transfer process, and of course on the experimental conditions for any particular technique and under any completely defined set of experimental conditions the value of any characteristic potential is a reproducible property of the electroactive substance. 

This method has the usual advantages of null techniques, i.e. immunity to noise and potentially very high precision. The presence of a quarter-wave... 

DOS) appears conserved for a quantum particle in a ID nanoscale stepwise potential, on analogy with the conservation of optical DOS for the quarter-wave dielectric multilayers. 

In this equation is the potential at the one-quarter transition time (the same as the polarographic half-wave potential in the case of a mercury electrode) and t is any time from zero to the transition time. The trace represented by this expression is shown in Fig. 2.26. The corresponding expression for an irreversible process with one rate-determining step... 

Figure 9A A spline function consisting of two quarter-sine waves connected by a straight line is used to smooth the transition between high and low potentials during simulated square-wave voltammetry (black line). The open circle shows the time when current is measured for the end of peak potential. The gray line shows the ideal unsmoothed transition.

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