Fully automated and computer assisted heuristic data analysis approaches have been applied to a series of AC voltammetric experiments undertaken on the [Fe(CN)6](3-/4-) process at a glassy carbon electrode in 3 M KCl aqueous electrolyte. The recovered parameters in all forms of data analysis encompass E(0) (reversible potential), k(0) (heterogeneous charge transfer rate constant at E(0)), α (charge transfer coefficient), Ru (uncompensated resistance), and Cdl (double layer capacitance). The automated method of analysis employed time domain optimization and Bayesian statistics. This and all other methods assumed the Butler-Volmer model applies for electron transfer kinetics, planar diffusion for mass transport, Ohm's Law for Ru, and a potential-independent Cdl model. Heuristic approaches utilize combinations of Fourier Transform filtering, sensitivity analysis, and simplex-based forms of optimization applied to resolved AC harmonics and rely on experimenter experience to assist in experiment-theory comparisons. Remarkable consistency of parameter evaluation was achieved, although the fully automated time domain method provided consistently higher α values than those based on frequency domain data analysis. The origin of this difference is that the implemented fully automated method requires a perfect model for the double layer capacitance. In contrast, the importance of imperfections in the double layer model is minimized when analysis is performed in the frequency domain. Substantial variation in k(0) values was found by analysis of the 10 data sets for this highly surface-sensitive pathologically variable [Fe(CN)6](3-/4-) process, but remarkably, all fit the quasi-reversible model satisfactorily.

Many electrode processes that approach the "reversible" (infinitely fast) limit under voltammetric conditions have been inappropriately analyzed by comparison of experimental data and theory derived from the "quasi-reversible" model. Simulations based on "reversible" and "quasi-reversible" models have been fitted to an extensive series of a.c. voltammetric experiments undertaken at macrodisk glassy carbon (GC) electrodes for oxidation of ferrocene (Fc(0/+)) in CH3CN (0.10 M (n-Bu)4NPF6) and reduction of [Ru(NH3)6](3+) and [Fe(CN)6](3-) in 1 M KCl aqueous electrolyte. The confidence with which parameters such as standard formal potential (E(0)), heterogeneous electron transfer rate constant at E(0) (k(0)), charge transfer coefficient (α), uncompensated resistance (Ru), and double layer capacitance (CDL) can be reported using the "quasi-reversible" model has been assessed using bootstrapping and parameter sweep (contour plot) techniques. Underparameterization, such as that which occurs when modeling CDL with a potential independent value, results in a less than optimal level of experiment-theory agreement. Overparameterization may improve the agreement but easily results in generation of physically meaningful but incorrect values of the recovered parameters, as is the case with the very fast Fc(0/+) and [Ru(NH3)6](3+/2+) processes. In summary, for fast electrode kinetics approaching the "reversible" limit, it is recommended that the "reversible" model be used for theory-experiment comparisons with only E(0), Ru, and CDL being quantified and a lower limit of k(0) being reported; e.g., k(0) ≥ 9 cm s(-1) for the Fc(0/+) process.

Commonly, significant discrepancies are reported in theoretical and experimental comparisons of dc voltammograms derived from a monolayer or close to monolayer coverage of redox-active surface-confined molecules. For example, broader-than-predicted voltammetric wave shapes are attributed to the thermodynamic or kinetic dispersion derived from distributions in reversible potentials (E(0)) and electrode kinetics (k(0)), respectively. The recent availability of experimentally estimated distributions of E(0) and k(0) values derived from the analysis of data for small numbers of surface-confined modified azurin metalloprotein molecules now allows more realistic modeling to be undertaken, assuming the same distributions apply under conditions of high surface coverage relevant to voltammetric experiments. In this work, modeling based on conventional and stochastic kinetic theory is considered, and the computationally far more efficient conventional model is shown to be equivalent to the stochastic one when large numbers of molecules are present. Perhaps unexpectedly, when experimentally determined distributions of E(0) and k(0) are input into the model, thermodynamic dispersion is found to be unimportant and only kinetic dispersion contributes significantly to the broadening of dc voltammograms. Simulations of ac voltammetric experiments lead to the conclusion that the ac method, particularly when the analysis of kinetically very sensitive higher-order harmonics is undertaken, are far more sensitive to kinetic dispersion than the dc method. ac methods are therefore concluded to provide a potentially superior strategy for addressing the inverse problem of determining the k(0) distribution that could give rise to the apparent anomalies in surface-confined voltammetry.

Estimation of thermodynamic and kinetic parameters in electrochemical studies is usually undertaken via comparison of the experimental results with theory based on a model that mimics the experiment. The present study examines the credibility of transient d.c. and a.c. voltammetric theory-experiment comparisons for recovery of the parameters needed to model the ubiquitous mechanism when an electron transfer (E) reaction is followed by a chemical (C) step in the EC process ([Formula: see text]). The data analysis has been undertaken using optimization methods facilitated in some cases by grid computing. These techniques have been applied to the simulated (5% noise added) and experimental (reduction of trans-stilbene) voltammograms to assess the capabilities of parameter recovery of E(0) (reversible potential for the E step), k(0) (heterogeneous electron transfer rate constant at E(0)), α (charge transfer coefficient for the E step), and k(f) and k(b) (forward and backward rate constants for the C step) under different kinetic regimes. The advantages provided by the use of a.c. instead of d.c. voltammetry and data optimization methods over heuristic approaches to "experiment"-theory comparisons are discussed, as are the limitations in the efficient recovery of a unique set of parameters for the EC mechanism. In the particular experimental case examined herein, results for the protonation of the electrochemically generated stilbene dianion demonstrate that, notwithstanding significant advances in experiment and theory of voltammetric analysis, reliable recovery of the parameters for the EC mechanism with a fast chemical process remains a stiff problem.