Potential single-step experiments

A- Single Step Experiments. Potential step experiments were performed in order to determine the reaction mechanism and the reaction rate. As described above, the platinum surface was initially covered by a monolayer of CO at a controlled potential, Ef = 0.40 V (referred to as the initial potential) and then CO was removed from the bulk of the solution. Next, the electrode potential was suddenly changed to a more positive value, Ef, (referred to as the final potential) where the adsorbed CO was oxidized and the rate of oxidation was followed by recording the resulting current transients. 

As we have noted, potential step methods are particularly attractive for the determination of chemical rate constants in electrochemical mechanisms because the potential can be stepped to a potential at which the forward electron transfer is fast and irreversible, so that the current response depends only on the rates and mechanism of coupled chemical reactions. A complete quantitative evaluation of the mechanism was achieved by combining the potential step results with a series of simulations. The chemical reaction rate constants were determined by single-step experiments (the oxidation of NO2). Early in the step, the single-step response is determined by the equilibrium concentration of NO2. At later times, the response reflects the rate of conversion of N2O4 to NOj. Simulated potential step response curves could be compared to experimental data to extract the Ke, and k, and k, (see Figure 3-1). 

Figure 2. Three-dimensional plots of the CO oxidation transients determined in the single potential step experiments on the Pt(100) electrode in 0.10 M HC10., solution. The potential step was applied from E = +0.40 V to Ep displayed on the third axis of the figure.

Figure 6. A plot of the logarithm of the square root of the initial slope of the current time transients determined in the double step experiments against the final potential for the three single crystal surfaces investigated.

Since the effects of the preceding chemical reaction substantially affect the forward response, the single potential step experiment can be adequately used. 

Let us consider, for example, a single potential step experiment that takes place in the equivalent circuit illustrated in Figure 3 (see Chapter 1, Section 5). 

After first verifying that bead-bound monomers had minimal affinity for target RNA 12, the entire 11,325-compound library was screened with fluorescently-labeled HIV-1 FSS RNA as a pool. Fluorescent beads were then removed, cleaved, and cleaved material analyzed. As the stoichiometry of the library screen was set up such that bead-bound monomers were in excess (a choice made so as to favor dimmer formation on bead rather than in solution), mass spectrometry revealed only the identity of component monomers rather than the full structure of selected compounds. Nevertheless, this provided a substantial simplification of the library in a single step three monomers were identified as being selected in replicate experiments, potentially representing six unique compounds, or 0.05% of the library (ignoring terminus differentiation due to resin attachment taking this into consideration raises the number of compounds to a still-low 9). 

Interestingly, no and Do can also be obtained from a single CA experiment if an ultramicroelectrode is used [6]. Following the potential step, planar diffusion will initially dominate, as the diffusion layer is smaller than the radius of the electrode. The current thus follows the usual Cottrel equation (Eq. 39). Later in the CA experiment, however, the diffusion layer grows larger than the radius of the ultramicroelectrode, and spherical diffusion will now dominate. This results in a time-independent current, given by Eq. 48 if the electrode is disc shaped, or Eq. 49 for microsphere geometry. 

Let us consider the ac response at a renewable stationary mercury drop electrode immersed in a solution containing initially only species O in the nernstian process O ne The dc potential starts at a value considerably more positive than and is scanned slowly in a negative direction. During the lifetime of a single drop, is effectively constant hence the dc part of the experiment is conventional polarography and is treated as a series of individual step experiments (see Sections 7.1 and 7.2). 

In chronoamperometric (CA) experiments (see Fig. 3) the working electrode potential is changed instantaneously from the initial potential to the first step potential, and it is held at this value for the first step time. This is a single potential step experiment. In a double potential step experiment, the potential is changed to the second step potential after the first step time, and it is then held at this value for the second step time. The current is monitored as a function of time. 

The time dependence of reductive desorption was explored by single potential step experiments (from E to E ), Selected transients (dotted lines) for freshly prepared BP3 monolayers on Au(lll)-(1 x 1)/0.1 M NaOH are displayed in Fig. 2IB. The traces are characterized by an initial exponential decay t <5 ms, q<5 iC cm ), followed by a well-developed maximum, /max at fmax) which increases and shifts towards shorter times with more nega-... 

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