Double Pulse Chronoamperometry

They are applicable to electrodes of any shape and size and are extensively employed in electroanalysis due to their high sensitivity, good definition of signals, and minimization of double layer and background currents. In these techniques, both the theoretical treatments and the interpretation of the experimental results are easier than those corresponding to the multipulse techniques treated in the following chapters. Four double potential pulse techniques are analyzed in this chapter Double Pulse Chronoamperometry (DPC), Reverse Pulse Voltammetry (RPV), Differential Double Pulse Voltammetry (DDPV), and a variant of this called Additive Differential Double Pulse Voltammetry (ADDPV). A brief introduction to two triple pulse techniques (Reverse Differential Pulse Voltammetry, RDPV, and Double Differential Triple Pulse Voltammetry, DDTPV) is also given in Sect. 4.6. 

Scheme 4.1 Double Pulse Chronoamperometry. (a) Potential-time program (b) DPC response...

The use of double potential pulse chronoamperometry is of great interest in electrochemistry for an accurate determination of both diffusion coefficients DQ and Dr, and this interest is enhanced when this technique is applied to small size spherical electrodes like the SMDE or gold microhemispheres or microspheres. There is a great number of redox couples for which highly unequal diffusion coefficients appear such as room temperature ionic liquids [23], ferrocene/... 

In single- and double-step chronoamperometries, one or two constant potential pulses are applied and the signal corresponds to the variation of the... 

In chronoamperometry linearity of the current the sum of the convolution terms is obtained only with the correct choice of thus providing a route to the latter. Alternatively the return of I2 to zero in a suitable double pulse experiment provides one procedure. 

The electrode reaction can be inverted by a second potential step of different sign and same amplitude at = t (double potential step or pulse chronoamperometry). The first transient follows Eq. 2 and the second one, however, is given by [7, 8] ... 

Chronoamperometry is often used for measuring the diffusion coefficient of electroactive species or the surface area of the working electrode. Analytical applications of chronoamperometry (e.g., in-vivo bioanalysis) rely on pulsing of the potential of the working electrode repetitively at fixed tune intervals. Chronoamperometry can also be applied to the study of mechanisms of electrode processes. Particularly attractive for this task are reversal double-step chronoamperometric experiments (where the second step is used to probe the fate of a species generated in the first step). 

Fig. 15. Waveforms used for in vivo electrochemical analysis. A = chronoamperometry, B = double chronoamperometry (response = SI — S2), C = linear sweep, D = differential pulse (response = S2 — SI), S = sample window...

To improve the selectivity of chronoamperometric in vivo analysis, a differential measurement ta hnique has been employed Instead of a single potential pulse, the potential is alternately pulsed to two different potentials giving rise to the name double chronoamperometry. This waveform is shown in Fig. 15 B. Because the current contributions of individual electroactive components add linearly to produce the observed current output, the difference in current response at the two potentials is the current due to only those compounds which are oxidized at the higher potential and not oxidized at the lower potential. This system provides two responses, the current due to easily oxidized compounds and the current due to harder to oxidize compounds. This gives greater selectivity than the direct chronoamperometric method. 

Here, the electrode reaction is followed by a first-order irreversible chemical reaction in solution that consumes the primary product B and forms the final product C. The rate of this chemical reaction can be measured conveniently with cyclic voltammetry, double-potential-step chronoamperometry, reverse pulse voltammetry, etc. However, this is only true if the half-life of B is greater than or equal to the shortest attainable time scale of the experiment. 

Double-potential step chronoamperometry This method was proposed in 1965 by Schwarz and Shain [18] for the investigation of follow-up reactions especially for the mechanism. During the first potential pulse the product B is produced at a stationary electrode under diffusion-controlled conditions for a timed interval tp. During this interval substance B diffuses into the solution and simultaneously undergoes a chemical reaction. Then, the potential is suddenly switched to a value where B is converted back into A. The backward current indicates the amount of B which has not reacted and can be related to the rate constant kf. The forward current-time dependence is given by the Cottrell equation... 

Chronoamperometry (or potential step) can be regarded as an extreme fast potential sweep of CV which is especially beneficial for the practical IL-based sensor development due to its unique double-layer stmcture as discussed in the earlier section. However, as with all pulsed voltammetric techniques, chronoamperometry generates high charging currents, which decay exponentially with time as in any Randles circuit. As shown in our early work [90], the time-dependent oxygen reduction currents, i(t), in ILs, can be described as the sum of the Faradaic current for EC reaction, if, and the double-layer charging current, ic, on the electrode as shown in Eq. (2.4) ... 

---