Elisabet Ahlberg

Quantitative cyclic voltammetry: Part I. Acquisition of precise electrode potential data during measurements in resistive media

Abstract Data obtained during cyclic and linear sweep voltammetry using current practice in peak potential measurements are somewhat qualitative, due to uncertainties in peak potential measurements and the lack of precise knowledge of the magnitude of uncompensated solution resistance. The precision problem is effectively eliminated by using derivative techniques and the IRu complication can be overcome by correlation of peak potential and peak current data. Linear correlations are suggested by the following equation: ΔEp=ΔErealp+IpRPu Correlation of the peak potential difference as a function of the peak current at constant Ru or as a function of Ru at constant Ip results in linear equations in which ΔErealp is derived from the intercepts. Both linear relationships were observed experimentally. Precise peak potential differences could be obtained for use in heterogeneous charge transfer kinetic studies. The method was applied to the quasi-reversible reduction of benzonitrile in DMF containing Bu4NBF4. The heterogeneous charge transfer rate constant was found to be 0.35±0.03 at 23.5°C at a supporting electrolyte concentration of 0.1 M.

Quantitative cyclic voltammetry: Part II. Measurement of homogeneous chemical reaction kinetics by derivative cyclic voltammetry

Abstract Use of the derivative peak current ratio during cyclic voltammetry for the measurement of rate constants of reactions coupled to charge transfer was evaluated. The protonation of anthracene anion radical by phenol in N,N-dimethylformamide was selected as a model reaction following the ECE—nuance mechanism. Working curves calculated by digital simulation were linearized by using a logarithmic relationship which greatly facilitated data analysis. The peak current ratios could be determined to a high degree of precision. Rate constant measurements were made over a wide range of IR feedback adjustment, switching potential, sweep rate, anthracene concentration and phenol concentration. The variation in the measured second-order rate constants over all of the measurements was of the order of ±15% of the mean value 5.3×103 M−1s−1. Under a constant set of conditions the precision in the measurements was better than ±5% of the mean value.

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.