Chemically modified electrodes rate constant

In order to investigate the dependence of a fast reaction on the nature of the metal, Iwasita et al. [3] measured the kinetics of the [Ru(NH,3)6]2+/3+ couple on six different metals. Since this reaction is very fast, with rate constants of the order of 1 cm s-1, a turbulent pipe flow method (see Chapter 14) was used to achieve rapid mass transport. The results are summarized in Table 8.1 within the experimental accuracy both the rate constants and the transfer coefficients are independent of the nature of the metal. This remains true if the electrode surfaces axe modified by metal atoms deposited at underpotential [4]. It should be noted that the metals investigated have quite different chemical characteristics Pt, and Pd are transition metals Au, Ag, Cu are sd metals Hg and the adsorbates T1 and Pb are sp metals. The rate constant on mercury involved a greater error than the others... 

Equation (34) is equally applicable to finite electrochemical and chemical kinetics at the substrate. The only difference is that an electrochemical rate constant is a function of electrode potential, while for a one-step chemical process (R -4 O) kb is potential-independent. For more complicated surface reactions, the theory has to be modified. 

Figure 7.6 A shows SEIRAS spectra that foUow the adsorption of the protein cytochrome c oxidase onto the surface of a chemically modified Au electrode. Bands of the amide I and amide II modes of the protein backbone appear at 1658 cm" and 1550 cm", respectively. The peak intensity of the bands increases with time. The amide II band was shown to grow according to an exponential rate law with a time constant of 213 s [151]. The amide I band position is characteristic of an a-helical protein, consistent with the structure of cytochrome c oxidase. The protein was adsorbed onto the Au surface via affinity for a chemical layer that was constructed in a step-wise fashion upon exposure of the surface to a series of reagents. Initially, the metal surface was modified by self-assembly of DTSP [difhiobis-(suc-...

Transfer equilibria of halide ions between bulk water and association colloids have been followed electrochemically, e.g., by use of specific ion electrodes or conductimetrically [61,62], or by chemical trapping (Sec. Ill) [65]. Bromide ion is an effective nucleophile in S, 2 displacements at all l centers, and rate constants in aqueous and alcohol-modified micelles and in O/W microemulsions have been analyzed quantitatively in terms of local concentrations of substrate and Br in the interfacial region of the colloid microdroplets [99,105]. The local second-order rate constants are typically slightly lower in the colloidal pseudophases than in water but are similar for micelles and microemulsions prepared with CTABr, indicating that interfacial regions provide similar kinetic media for these Ss2 reactions. However, reactions with the same overall concentrations of Br , or other ionic reactant, are slower in microemulsions or alcohol-modified micelles than in normal micelles for two reasons (1) The fractional ionization, a, is lower in the normal micelles and (2) the increased volume of the reaction region, due to the presence of cosurfactant, dilutes Br in the pseudophase provided by the association colloid [66,69,105]. 

This technique involves the accumulation of the analyte at the electrode surface from diluted solutions, via favorable interaction with the electrode modifier, and its subsequent electrochemical detection. This enables to lower the detection limit and to increase the sensor sensitivity owing to effective concentration of the analyte. This is especially useful to enable quantitative determinations when they are not achievable by direct electrochemical measurement performed in the native medium. Compared to stripping voltammetric techniques, this one is based on chemical accumulation rather than on an electrolytic one, thus being basically independent on potentials. The typical experimental procedure involves successive steps (Fig. 16.19a) that must be optimized to get the best performance. The analyte is first accumulated at open-circuit under constant stirring (to enhance mass transport rates). The electrode is then removed from the preconcentration medium, rinsed with pure water, and immersed into the analysis cell containing an appropriate electrolyte, where the electrochemical quantification is carried out (analyte desorption is usually required, especially when the electrode modifier is an... 

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