Chronoamperogram

Example 6-1 Chronoamperogram a was obtained for the biosensing of glucose in whole blood. Subsequent standard additions of 1 x 10-3M glucose yielded the chronoamperograms b to d. Find the concentration of glucose in die sample. 

Nucleationlike processes appear on the experimental anodic chronoamperograms only after polymeric compaction by cathodic polarization, during a constant time t, if the cathodic potential surpasses a... 

These two equations quantify the evolution of the relaxation current and the relaxation charge as a function of the polarization time when the conducting polymer is submitted to a potential step from Ec to E. They are the relaxation chronoamperogram and the relaxation chronocoulogram,... 

Therefore these equations, even though the relaxation charge represents only a small fraction of the overall charge consumed during the complete oxidation, fulfill all the requirements for simulating the point in time at which the chronoamperograms attain the maximum current as a function of the different variables ... 

For chronoamperograms obtained by potential steps from the same cathodic potential to different anodic potentials ... 

Equations (37) and (38), along with Eqs. (29) and (30), define the electrochemical oxidation process of a conducting polymer film controlled by conformational relaxation and diffusion processes in the polymeric structure. It must be remarked that if the initial potential is more anodic than Es, then the term depending on the cathodic overpotential vanishes and the oxidation process becomes only diffusion controlled. So the most usual oxidation processes studied in conducting polymers, which are controlled by diffusion of counter-ions in the polymer, can be considered as a particular case of a more general model of oxidation under conformational relaxation control. The addition of relaxation and diffusion components provides a complete description of the shapes of chronocoulograms and chronoamperograms in any experimental condition ... 

The cathodic overpotential tjc controls the compactness of the polymeric structure included in the constant a of the equation through AH. Any variation in rjc promotes a change in the current required to oxidize the system at any time because a is contained by the two terms of Eq. (43). Figure 42 shows both theoretical and experimental chronoamperograms. 

The influence of the solvent on the oxidation of film under conformational relaxation control is illustrated in Fig. 47, which shows chronoamperograms obtained by steps from -2000 to 300 mV vs. SCE at room temperature (25°C) over 50 s in 0.1 M LiC104 solutions of different solvents acetonitrile, acetone, propylene carbonate, (PC), dimethyl sulfoxide (DMSO), and sulfolane. Films were reduced over 120 s in the corresponding background solution. Despite the large differences observed in the relative shape of the curves obtained in different solvents, shifts in the times for the current maxima (/max) are not important. This fact points to a low influence of the solvent on the rate at which confor-... 

As in chronoamperograms, the fraction of the overall oxidation charge involved in relaxation processes is quite small in the absence of any external stress. The share of the overall current at every potential between electrochemical processes occurring under relaxation control and those driven by swelling-diffusion control can be observed in Fig. 66. I(r) has... 

From Eq. (68), following a procedure similar to that described for chronoamperograms and voltammograms, theoretical coulovoltagrams were obtained as a function of the variables studied. The results189 can be observed in Fig. 67. Some new effects can be deduced from these experimental curves, which will allow us to provide a complete description of the electrochemistry of conducting polymers. 

Fig. 18b. 7. (a) Chronoamperogram showing the response due to a triple pulse 500-0-500 with a 3 mm diameter glassy carbon working electrode in 2.0 mM Potassium Ferricyanide in 0.1 M KC1. No current was recorded for the initial potential, 500 mV, where no faradaic reduction took place, (b) The same solution, except with a 10 pm diameter Pt working electrode. Current was recorded for the initial potential at 500 mV for 0-4000 ms where no faradaic reduction took place. Note the magnitude of current scale. 

The linear rate equation, eqn. (18), was assumed to hold throughout Sect. 2 because it is the most simple case from a mathematical point of view. Evidently, it is valid in the case of the linear mechanism (Sect. 4.2.1) as it is also in some special cases of a non-linear mechanism (see Table 6 and ref. 6). The kinetic information is contained in the quantity l, to be determined either from the chronoamperogram [eqn. (38), Sect. 2.2.3] or from the chronocoulogram [eqn. (36), Sects. 2.2.2 and 2.2.4], A numerical analysis procedure is generally preferable. The meaning of l is defined in eqn. (34), from which ks is obtained after substituting appropriate values for Dq2 and for (Dq/Dr)1/2 exp (< ) = exp (Z) [so, the potential in this exponential should be referred to the actual standard potential, see Sect. 4.2.3(a)]. 

Figure 3.3 Chronoamperometry. (A) Potential excitation signal for double potential step. (B) Current-time response signal (chronoamperogram).

Current as a function of time is the system response as well as the monitored response in chronoamperometry. A typical double-potential-step chronoamperogram is shown by the solid line in Figure 3.3B. (The dashed line shows the background response to the excitation signal for a solution containing supporting electrolyte only. This current decays rapidly when the electrode has been charged to the applied potential.) The potential step initiates an instantaneous current as a result of the reduction of O to R. The current then drops as the electrolysis proceeds. 

Understanding the shape of the chronoamperogram requires consideration of concentration-distance profiles for a potential-step excitation in conjunction with Faraday s law. Faraday s law is so fundamental to dynamic electrochemical experiments that it cannot be emphasized too much. It is important to keep in mind that the charge Q passed across the interface is related to the amount of material that has been converted, and the current i is related to the instantaneous rate at which this conversion occurs. Current is physically defined as the rate of charge flow therefore,... 

As shown in the preceding reaction sequence, a rate-determining chemical step is interposed between the two electrode reactions. (See Chap. 2 for an explanation and an example of this mechanism.) The two dashed lines in Figure 3.4A show hypothetical chronoamperograms for the le reduction of O to R and for the direct 2e reduction of O to P with no kinetic complications. The solid line shows a typical chronoamperogram for an ECE mechanism. The current is intermediate between the le and 2e reductions, since the reduction of X to P is controlled by the rate of the chemical reaction of R to generate X. The exact position of the solid line is determined by the value of the rate constant k. 

Fig. 27.10. Chronoamperograms for transducer with inserted Catalyst 3. Solution contents (1) Background 0.25M NaOH (2) 0.25M NaOH+10 4M urea (3) 0.25M NaOH+2 x 10 4M urea.

Electrochemical experiments were carried out using a hand-held po-tentiostat equipped with dedicated software for the elaboration of current data (Fig. 29.5). The current was sampled 2 min after the reaction started. Figure 29.5 shows the chronoamperogram where current versus time is plotted. The current shape was reproducible from time to time and hence the precision assured sampling the current, 2 min after the start of the measurement. After 2 min the current was continuously increasing due to the residual activity of AChE in solution. The biosensor... 

Record nickel oxidation chronoamperogram in the supporting electrolyte at the potential 0.54V. 

Fig. 39.2. Chronoamperograms and catalyst oxidation current-urea concentration plot. Background 0.25 NaOH. E — 0.54 V.

Fig. 39.3. Chronoamperograms observed with the use of transducer containing catalyst. Solution composition (1) background - 0.25 M NaOH (2) 0.25 M NaOH+10-4 M urea (3) 0.25 M NaOH+10-4 M urea+sample.

Note that the reversible l(E, t) response is expressed as a product of a potential-dependent function ((c 0 - c Rt l)/( + ye 1)) and a time-dependent function (FA sjDo/(nt)). This behavior is characteristic of reversible electrode processes. In the next sections the current-time curves at fixed potential (Chronoamperograms) and current-potential curves at a fixed time (Voltammograms) will be analyzed. 

Note that the appearance of the anodic peak implies that, beyond the peak potential, smaller anodic currents are obtained at more anodic potentials. This situation is shown in Fig. 4.14b where it can be seen that at a given time there is a crossing of the chronoamperograms so that the current corresponding to the... 

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