Transient spectroelectrochemical experiments

Transient spectroelectrochemical studies are possible with modem and fast spectroscopic probes, e.g. based on diode array spectrometers [55]. Compared to steady-state techniques, the design of transient spectroelectrochemical experiments is more challenging and the data analysis has to take into account the development of diffusion- and convection-driven concentration profiles [56]. However, these experiments open up the possibility to study time-dependent processes such as the development of a diffusion profile. For example, it is known, but usually ignored, that changing the redox state of a molecule changes its diffusivity. It has recently been shown [57] that, for the oxidation of AA A A -tetra-methylphenylenediamine (TMPD) in water, ethanol, and acetonitrile (Eq. II.6.1), the diffusion coefficients of the reduced form, TMPD, and the diffusion coefficient of the oxidised form. 

TMPD", can be determined separately and simultaneously in a single transient spectroelectrochemical experiment. 

The experiment was based on a channel flow cell system (see Fig. II.6.2) with UVMs detection downstream of the working electrode (Fig. n.6.2d). Switching the electrode potential from the potential region with no faradaic current into a region with diffusion-limited faradaic current allowed the transient change in the UVAfls absorption to be monitored. The data analysis for this transient UVAds response was based on a computer simulation model, which allowed T>(TMPD) and T)(TMPD ) to be varied independently. Interestingly, the difference in Z)(TMPD) and T)(TMPD ) was relatively high in water and ethanol (15% slower diffusion of the radical cation) and considerably lower (5%) in the less polar solvent acetonitrile. This example demonstrates the ability of transient spectroelectrochemical experiments, in conjunction with computer simulation-based data analysis, to unravel even complex processes. 

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