Voltammetric theory

II. VOLTAMMETRIC THEORY OF ORGANIC BASES AND ACIDS AT 0/W INTERFACE... 

M. Senda, H. Katano, and M. Yamada, Amperometric ion-selective electrode. Voltammetric theory and analytical applications at high concentration and trace levels. J. Electroanal. Chem. 468, 34-41 (1999). 

On the other hand, adsorption can have serious negative effects on analytical response. Adsorbed reactants will increase the current over that predicted from theory based on diffusion-controlled mass transport. Thus the usually powerful methods based on Nicholson/Shain voltammetric theory are seriously perturbed by adsorption, particularly at high scan rates. In addition, the adsorption of nonelectroactive impurities or reaction products can eventually deactivate the electrode, thus requiring electrode renewal. 

In the theory described above, as well as previous theoretical treatments of ET rate constants, the effect of the molecular-level diffusion process is dealt with by including it in the overall (i.e., observed) rate constant. However, a somewhat different approach to this problem has been advanced by Senda [54], who proposed a model that includes the bimolecular-reaction effect in the voltammetric theory of ET at the O/W interface. 

The voltammetric behavior in the EC case is sensitive to the nature of the following reaction, which may be reversible or irreversible, a dimerization, a disproportionation, etc., and the voltammetric theory for each of these is different. Our treatment is restricted to the reversible charge transfer followed by an irreversible first-order following reaction ... 

Bond AM, O Halloran RJ, RuziC I, Smith DE (1976) Fundamental and second harmonic alternating current cyclic voltammetric theory and experimental results for simple electrode reactions involving solution-soluble redox couples. Anal Chem 48 872. 

Of course, this all assumes that the electron transfer is fast. The analysis for slow electron transfer and unequal diffusion coefficients is equally feasible, but rather more involved it is a recommended exercise in voltammetric theory which exceeds the scope of this book. 

In the presence of excess inert electrolyte, the voltammetric responses of Pt disk electrodes of 5-50 nm in radii do deviate from the predication of the conventional voltammetric theory as a result of the enhanced EDL effects at nanoscale electrochemical interfaces, but the deviations are quantitatively small (e.g., within 20% even at electrodes of a few nanometers), and in most cases might be hardly distinguished with the experimental uncertainties. In the absence of the excess of the supporting eleetrolyte, the voltammetric responses for disk electrodes larger than 200 nm in radii show reasonable agreements with the predications of the eonventional microelectrode voltammetric theory. However, for electrodes smaller than 200 nm, the voltammetric responses predicated by the present theory exhibit significant deviation from the microelectrode theory. The deviations are mainly resulted from the overlap between the diffuse double layer and the CDL at nanoscale electrochemical interfaces in weakly... 

Voltammetric simulations for ET reactions of various fe and X at electrodes of various sizes have shown that, for ET reactions with fe near 0.1 cm/s, the BV theory could predict voltammetric responses visibly deviating from that expected by the MHC model as the electrode radii are smaller than 50 mn, while this occurs as Tq approaches 10 nm for ET reactions with fe around 1.0 cm/s. According to the half-wave-potential difference in the polarization curves predicted by the BV and MHC models, the BV-based voltammetric analysis would give standard rate constants of ca. 0.6 and 0.5fe , respectively, for a reaction of 0.1 cm/s kP and 100 kJ/mol X at electrodes with radii of 20 and 10 nm. For the ET reaction with k° of 1.0 cm/s, apparent standard rate constants of 0.8fe and 0.6fe will be obtained by BV-based analysis at an electrode with radii of 10 and 5 nm. Considering that the EDL effect would result in enhanced apparent ET kinetics for cation reduction or anion oxidation (Table 2.1), the measured polarization curves at nanoelectrodes would be closer to that predicted by the BV formalism without including the EDL effect. For anion reduction or cation oxidation, the EDL effect and the MHC formalism both predict inhibited ET kinetics as compared with the conventional BV model combined with the diffusion-based MT theory. In this case, the measured polarization would significantly deviate from the prediction of conventional voltammetric theory. Therefore, BV-based voltammetric analysis would result in apparent rate constants that are significantly lower than the real fc . 

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