Bjørg Aalstad

Normalized potential sweep voltammetry: Part II. Application to heterogeneous charge transfer kinetics

Abstract Plotting theoretical voltammetric electrode potential data for Nernstian charge transfer on the X -coordinate, that for quasi-reversible charge transfer on the Y -coordinate and normalized current ( I/I p ) on the Z -coordinate revealed that projections on to the X−Y plane resulted in: ( E − E p/2 ) quasi = m ( E − E p/2 ) Nern where the slopes m are directly related to the normalized heterogeneous rate constant Λ (Λ= k s ( DnF/RT ) −1/2 ν −1/2 ). Working curves, m −1 vs. 1/Λ, dependent upon the value of the transfer coefficient, α, were calcualted for the determination of k s . For the experimental evaluation of k s , values of m −1 are obtained as a function of the voltage sweep rate, ν, which allows both k s and α to be obtained from the working curves. The method is applicable to processes with k s −1 and is most suitable for processes having rate constants ranging from 10 −3 to 10 −1 cm s −1 . After obtaining the rate constant and transfer coefficient from the working curves, the appropriate theoretical current-voltage curve can be calculated and used in the three-dimensional analysis of the experimental data. An experimental verification of the method is presented.

Normalized potential sweep voltammetry: Part I. Application to electrode mechanism analysis

Abstract Linearization of linear sweep voltammetric data is achieved by a redefinition of the current-potential scale. Experimental electrode potentials expressed relative to the half-peak potential, ( E−E p/2 ) exp , and the corresponding theoretical values for a particular electrode mechanism, ( E−E p/2 ) theor , are plotted vs. the current, normalized to the peak current ( I/I p ). The three-dimensional plot gives a straight line of slope equal to 1.0 when projected onto the ( E−E p/2 ) exp −( E−E p/2 ) theor plane. Deviations from unit slope occur either when the wrong mechanism was chosen for the theoretical data or when the data do not fit a theoretical mechanism. Theoretical data for Nernstian charge transfer and several mechanisms under purely kinetic conditions were calculated for values of I/I p ranging from 0.1 to −0.5 where the negative sign refers to values beyond the peak. Thus, the analysis involves data over the entire voltammetric wave. The analysis is illustrated with experimental data from Nernstian as well as kinetic systems.