Rotating current step

The aforementioned experiments at rotating electrodes concerned merely steady-state conditions so-called transients123 at these electrodes, e.g., with potential or current steps, as well as with hydrodynamic modulation, i.e., variation of co with time, are, as a consequence of their non-steady-state conditions, less important in analysis and therefore will not be treated here. 

Consider a series of steady state current-potential measurements with, say, a rotating disc electrode, supplemented with determination of and from the sudden jump and the following linear rise of potential with time, observed after application of a very short current-step pulse. If we consider this from the point of view of the information content, we realize that in these experiments we have, in effect, measured each quantity when its information content was unity, or very close to it. This procedure yields the best results, but it is limited to relatively slow reactions. Thus, we could say that the concept of... 

Fig. 8. Typical results for the rotation speed step experiment. The values of t are found from the current transient using eqn. (8). These are plotted against t according to eqn. (7). A value of D can then be found from the gradient.

In the concentration step the metal is deposited at a potential sufficiently negative to maintain its limiting current while stirring the solution or rotating the working electrode. 

Experimental results obtained at a rotating-disk electrode by Selman and Tobias (S10) indicate that this order-of-magnitude difference in the time of approach to the limiting current, between linear current increases, on the one hand, and the concentration-step method, on the other, is a general feature of forced-convection mass transfer. In these experiments the limiting current of ferricyanide reduction was generated by current ramps, as well as by potential scans. The apparent limiting current was taken to be the current value at the inflection point in the current-potential curve. 

In Talon Charbonnel (1998, TC98), Charbonnel Talon (1999, CT99) and Palacios et al. (2003) we went one step further We included in the models the most complete description currently available for rotation-induced mixing, and we computed self-consistently the transport of the chemicals and that of AM due to wind-driven MC. We used the same input physics than that used with success by the Geneva group to explain several observational patterns of more massive stars (e.g. Maeder Meynet 2000 and Talon Charbonnel 2003 and references therein). 

As the field of electrochemical kinetics may be relatively unfamiliar to some readers, it is important to realize that the rate of an electrochemical process is the current. In transient techniques such as cyclic and pulse voltammetry, the current typically consists of a nonfaradaic component derived from capacitive charging of the ionic medium near the electrode and a faradaic component that corresponds to electron transfer between the electrode and the reactant. In a steady-state technique such as rotating-disk voltammetry the current is purely faradaic. The faradaic current is often limited by the rate of diffusion of the reactant to the electrode, but it is also possible that electron transfer between the electrode and the molecules at the surface is the slow step. In this latter case one can define the rate constant as ... 

The electrode processes on the voltammetric and the preparative electrolysis time scales may be quite different. The oxidation of enaminone 1 with the hydroxy group in the ortho position under the controlled potential electrolysis gave bichromone 2 in 68% yield (Scheme 4.) with the consumption of 2.4 F/mol [21], The RDE voltammogram of the solution of 1 in CH3CN-O.I mol/1 Et4C104 showed one wave whose current function, ii/co C, was constant with rotation rates in the range from 1(X) to 2700 rpm and showed one-electron behavior by comparison to the values of the current function with that obtained for ferrocene. The LSV analysis was undertaken in order to explain the mechanism of the reaction which involves several steps (e-c-dimerization-p-deamina-tion). The variation of Ep/2 with log v was 30.1 1.8 mV and variation of Ep/2 with logC was zero. Thus, our kinetic data obtained from LSV compare favorably with the theoretical value, 29.6 mV at 298 K, for a first order rate low [15]. This observation ruled out the dimerization of radical cation, for... 

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