Ultramicroelectrode current-potential curves

It is evident that the square wave charge-potential curves corresponding to surface-bound molecules behave in a similar way to the normalized current-potential ones observed for a soluble solution reversible redox process in SWV when an ultramicroelectrode is used (i.e., when steady-state conditions are attained), providing the analogous role played by 2sw (surface-bound species) and (soluble solution species), and also 2f (Eq- (7.93)) and the steady-state diffusion-limited current (7 css), see Sect. 2.7. This analogy can be made because the normalized converted charge in a surface reversible electrode process is proportional to the difference between the initial surface concentration (I ) and that... 

On the contrary, the radical cation of anthracene is unstable. Under normal volt-ammetric conditions, the radical cation, AH +, formed at the potential of the first oxidation step, undergoes a series of reactions (chemical -> electrochemical -> chemical -> ) to form polymerized species. This occurs because the dimer, tri-mer, etc., formed from AH +, are easier to oxidize than AH. As a result, the first oxidation wave of anthracene is irreversible and its voltammetric peak current corresponds to that of a process of several electrons (Fig. 8.20(a)). However, if fast-scan cyclic voltammetry (FSCV) at an ultramicroelectrode (UME) is used, the effect of the follow-up reactions is removed and a reversible one-electron CV curve can be obtained (Fig. 8.20(b)) [64], By this method, the half-life of the radical cat-... 

Fig. 4.1 Current density-time curves when both species are soluble in the electrolytic solution and only species O is initially present. Three electrode sizes are considered planar electrode (solid lines), spherical electrode with rs = 10 5 cm (dotted lines), and spherical ultramicroelectrode with rs = 10-5 cm (dashed lines), and three y values y = 0.5 (green curves), y = 1.0 (black curves), and y = 2.0 (red curves). The applied potential sequences are Ei -Ef -> -oo, E2 - E — +oo. n = T2 = 1 s, Cq = 1 mM, cR = 0, D0 = 10-5 cm2 s 1. Taken from [20] with permission...

The Qf E curve for a reversible two-electron transfer taking place in a monolayer is independent of time (i.e., it has a stationary character) and, therefore, is independent of the potential-time waveform applied to the electrode, as in the case of a reversible one-electron transfer reaction. It is also important to highlight that the normalized charge, has a identical expression to that for the normalized transient current 7 v N obtained for solution soluble species when the NPV technique is applied to an electrode with any geometry (see curves in Fig. 3.16, and Eq. (3.141)), and also to the normalized stationary current obtained for solution soluble species when any potential-time waveform is applied for ultramicroelectrodes with any geometry. 

Linear scan voltammetry (LSV) — It is an experimental method when the -> electrode potential is varied linearly with time (t) with a scan (sweep) rate v = dE/dt and the current (I) vs. E curve (which is equivalent with E vs. t curve) is recorded. Usually scan rates ranging from 1 mV s-1 to 1V s-1 are applied in the case of conventional electrodes with surface area between 0.1 and 2 cm2, however, at -> ultramicroelectrodes 1000 or even 106 V s-1 can also be used. The scan is started at a potential where no electrochemical reaction occurs. At the potential where the charge transfer begins, a current can be observed which increases with the potential, however, after a maximum value (current peak) it starts to decrease due to the depletion of the reacting species at the -> interface. 

---