Half-wave potential accurate determination

The half wave potential (Ejy2 versus standard calomel electrode) was determined as -1.45 volts in lithium chloride in methanol-. The method was not considered sufficiently accurate to be used as a quantitative assay. 

Spreadsheet Summary Polarography is considered in the voltammetry exercise in Chapter 11 o( Applications of Microsoft Excel in Analytical Chemistry. A polarographic calibration curve is constructed first then an accurate determination of half-wave potential is made. Finally, the formation constant and formula of a complex are deteimined from polarographic data. 

For catalytic ORR measured by a rotating electrode technique, other two measures for catalytic activity are also commonly used the onset potential (Eonset) and the half-wave potential (E1/2), as shown in Figure 3.3. Onset potential is defined as the potential at which the catalyzed ORR current starts to appear. The more positive the onset potential, the more active the electrocatalyst would be. There are no simple theoretical expressions about E onset and 1/2. Fortunately, they can be easily obtained from the current—potential curves recorded by rotating electrode techniques. However, there exists some arbitrary in determining this onset potential point from the current—potential curve shown in Figure 3.3. The more accurate way may be the half-wave potential. The more positive the half-wave potential, the more active the electrocatalyst would be. Note that this half-wave potential is... 

In earlier studies, sigmoidal waves of simple IT were treated using simple steady-state theory (e.g.. Equation 15.12) and assuming that their shape is independent of geometry of the pipette inside. More recent simulations and experiments showed that this simplification is not realistic, and the reversible half-wave potential of simple IT from the external solution to the pipette under steady state depends on pipette angle, 0p. It was suggested that kinetic and thermodynamic parameters of simple ITs determined without taking into account the effects of ion diffusion in the inner shaft of a nanopipette may not be accurate. 

In 1966 the relative electron affinities of charge transfer complex acceptors were calculated from spectral data and half-wave reduction potentials. Unfortunately, at the time, no accurate electron affinities of typical n charge transfer complex acceptors existed so one could obtain absolute electron affinities from either half-wave reduction potentials or charge transfer complexes. Thus, the magnetron Ea of 1.40 eV for the electron affinity of benzoquinone was selected. This is now known to be about 0.5 eV too low, making all the values low. This emphasizes the difference between the determination of relative electron affinities that depend on the absolute electron affinity of a reference compound and absolute ones from experimental measurements and fundamental constants. 

---