INFERENCE OF MECHANISM FROM KINETIC-ANALYSIS OF PULSE VOLTAMMETRIC DATA

Voltammetry provides direct access to kinetic information in that the measured quantity, current, is itself the rate. Kinetic analysis of voltammetric data generally focuses on the potential dependence of the current. For historical reasons, the most common method of analyzing data is to transform the data, often by very elaborate methods, to yield a potential-dependent rate constant, which is then plotted as semilogarithmic function of potential. This procedure requires extrinsic normalization factors which easily can introduce systematic error. In a few instances, statistically sound methods have been employed for analysis of data. One approach employs a nonlinear least squares procedure equivalenyt to the method of maximum likelihood. In addition to providing optimal values of kinetic parameters without recourse to other data, this method alos provides confidence regions at a known level of confidence. This method is implemented by the COOL algorithm, which has been described. An important ancillary factor is that the COOL algorithm runs in 'real-time' for many problems. This paper describes these alternative methods of analysis by using the particular example of slow charge transfer. The sensitivity of the analysis to changes in values of parameters is examined by computation of confidence regions. Then three specific kinetic problems are used to illustrate the types of questions which arise in the inference of mechanism. The first involves the search for a second order dependence of current on potential, this having been predicted by theoretical treatments. The second can arise in cases where two electrons are transferred. Under what conditions is it possible to determine the rate parameters for both transfers? What criteria ensure that the variance in the data is explained by only one charge transfer step (i.e., the other is too fast to see)? The third problem concerns heterogeneous charge transfers coupled by a homogenous chemical step. When the second charge transfer is more favored than the first, when does it take place through a homogeneous reaction route, and under what conditions can this be detected? The experimental examples include the reduction of Zn(II) and the reduction of p-nitrosophenol, both at mecury electrodes. The data are confounded to some degree by experimental artifacts; nonrandom distribution of residuals may arise from these artifacts or from choice of overly simple models.