Fast electrode reactions, potentiostatic

Cahan, Nagy and Genshaw examine design criteria for an electrochemical measuring system to be used for potentiostatic transient investigation of fast electrode reactions. They emphasise the importance of co-design of the experimental cell and electronics. 

A cell system has been designed for the potentiostatic transient investigation of fast electrode reactions.9 The main novel feature of the cell is the elimination of the classical Luggin capillary, as shown in Figure 6.4. The design provides a low-ohmic-resistance reference electrode with low stray capaitances. 

The mean surface concentrations enforced by depend on many factors (a) the way in which is varied (b) whether or not there is periodic renewal of the diffusion layer (c) the applicable current-potential characteristic and (d) homogeneous or heterogeneous chemical complications associated with the overall electrode reaction. For example, one could vary sequential potentiostatic manner with periodic renewal of the diffusion layer, as in sampled-current voltammetry. This is the technique that is actually used in ac polarography, which features a DME and effectively constant during the lifetime of each drop. Alternatively one could use a stationary electrode and a fairly fast sweep without renewal of the diffusion layer. Both techniques have been developed and are considered below. The effects of different kinds of charge-transfer kinetics will also be examined here, but the effects of homogeneous complications are deferred to Chapter... 

The potentiostatic coulometry, involves holding the electric potential (the voltage) of the working electrode constant during the reaction. This avoids parasitic reactions. The current decreases as fast as the analyte disappears from the solution. A galvanostat (or amperostat) is used to compute the quantity of current used. 

With a potentiostat the potential at the working electrode is linearly increased from 1.0 to 1.6 V and then decreased back to 0 V. In the first interval 1 is oxidized to the radical cation l+ with a peak potential of p.a = 1-38 V. 1 is stable in this solvent and is reduced in the reverse scan back to 1 at p,c = 1-32 V. The ratio of the current for reduction and oxidation ip c-ip.a = 1 indicates the stability of the radical cation. All of 1, that is formed by oxidation of 1 is reduced back to 1. This behavior is termed chemically reversible. Upon addition of 2,6-lutidine, the radical cation 1 reacts with the nucleophile to afford 2 , which is further oxidized to a dication, which yields the dication with 2,6-lutidine. This can be seen in the decrease of /p,c fp,a and an increase of due to the transition from an le to a 2e oxidation. From the variation of the ratio ip.c-ip,n with the scan rate, the reaction rate of the radical cation with the nucleophile can be determined [9]. This can also be aehieved by digital simulation of the cyclovoltammogram, whereby the current-potential dependence is calculated from the diffusion coefficients, the rate constants for electron transfer and chemical reactions of substrate and intermediates at the electrode/electrolyte interface [10]. With fast cyclovoltammetry [11] scan rates of up to 10 Vs- can be achieved and the kinetics of very short-lived intermediates thus resolved. 

Electron diameter d is am important experimental variable. For a typical planar disk WE and reference probe, solution resistance is proportional to 1M whereas the current is proportional to the area, or d. As a consequence, IR drop is proportional to d. The capacitive rise time decreases as the electrode area is made smaller. (This property is of particular consequence in potential step experiments.) With the appearance of microelectrodes and fast potentiostatic circuits, the time scale of CV has been extended into the submicrosecond range. As a consequence, faster following chemical reactions can be examined. Note that for fast scan experiments to give useful results, electron transfer must be rather facile. (See, e.g.. Figure 2-24). If the heterogeneous rate constant is too... 

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