Current-reversal chronopotentiometry

A more usual procedure for overcoming the disturbances from contaminants is current reversal chronopotentiometry here the current is reversed at the initial transition time tf of the forward reaction and the next transition time xb of the backward reaction is measured as a rule the reversal wave will not be influenced by the contaminant because it will react either before the forward or after the backward reaction of the analyte (see Fig. 3.60a) the entire procedure can be even repeated as cyclic chronopotentiometry (see Fig. 3.60b), which may provide a further check on the reliability. The reversal technique can be applied to initial reduction followed by re-oxidation and also to initial oxidation followed by re-reduction79. 

By multi-phase transient experiments , we mean those such as cyclic voltammetry (see Chap. 3) or current reversal chronopotentiometry [33] in which some sudden charge is made in the imposed conditions at some definite time (t = r, for example) after the experiment commences. To address such multiphase experiments, one usually treats each phase separately, the solution of the first phase providing details of the concentration profiles at t — t, which then serve as the initial conditions for the second phase. Likewise, the final conditions of the second phase provide the initial conditions for the third phase, if any, and so on. 

If the polarity of the applied current in an ordinary chronopotentiometry experiment is reversed during the recording of the chronopotentiogram, the product R of the initial electrochemical reaction may now undergo the reverse reaction to give a current-reversal chronopotentiogram, as shown in Figure 4.5 [1-5]. A reverse transition time xr will result when the concentration of R becomes zero at the electrode surface (see Fig. 4.2C). Such reverse potential-time curves can be treated quantitatively for reversible and irreversible couples. 

Electrode mechanisms with following chemical reactions (EC mechanism see Chap. 2) are particularly amenable to study by current-reversal chronopotentiometry. A good example is the oxidation of p-aminophenol (PAP), which undergoes the following reaction sequence [9] ... 

Figure 4.5 Current-reversal chronopotentiometry. (A) Current excitation signal. (B) Potential response.

Current-reversal chronopotentiometry is also useful for detecting adsorption of the product generated during the forward electrolysis time. Complete adsorption of the product on the electrode surface causes tr to equal tf since no product is lost by diffusion from the electrode. This approach has been used to determine the amount of adsorbed material formed during the reduction of riboflavin and the oxidation of iodide [10]. 

Figure 4.7 Working curve for the calculation of the rate constant kf for a following chemical reaction from current-reversal chronopotentiometry. [From Ref. 9, adapted with permission. Copyright 1960 American Chemical Society.]...

Figure 6.20 illustrates a circuit that has been widely used. SW1 affords choice of anodic or cathodic current, SW2 initiates the experiment, and then SW1 may be used for current reversal. OA-3 is available for differentiating E with respect to t. A more advanced circuit (Fig. 6.21) incorporates an additional feedback loop and a comparator to perform cyclic chronopotentiometry with automatic switching. Operation of this circuit is perfectly analogous to the cyclic voltammetry circuit discussed in Section II.E. 

Figure 6.20 Current-reversal chronopotentiometry with derivative output.

Through the use of current-reversal chronopotentiometry, reaction sequences of the type... 

Chronopotentiometry — is a controlled-current technique (- dynamic technique) in which the - potential variation with time is measured following a current step (also cyclic, or current reversals, or linearly increasing currents are used). For a - nernstian electrode process,... 

Chronopotentiometry closely resembles chronoamperometry with the exception that the role of current and potential are reversed. In chronopotentiometry the current is controlled and is the variable and the electrode potential is the observable. A single step of the current or a double step can be employed. The double step method called current reversal chronopotentiometry is more information-rich as in previous comparisons. Both the applied currenttime wave form and the potential-time response for a reversible electrode process are illustrated in Fig. 4. The current step from 0 to a predetermined value depending upon the experimental conditions is maintained until time... 

Fig. I8K Current—reversal chronopotentiometry, for the oxidation of 1 niM PAP to PQl in 0.1 M H SO, at a platinum electrode, followed by hydrolysis to PBQ. i = 0.10 niAlcm. x is the transition time on the reverse pulse, following a forward pulse of duration t. From Gileadi, Kirowa-Eisner and...

When applied to the analysis of kinetic data, chronopotentiometry is most often used in the current-reversal mode, in which the direction of the current is changed after some time tf (Fig. 29). When only the direction, but not the magnitude, of the current is changed, the reverse transition time is given by Eq. (73) [232]. 

Figure 29. Current-reversal chronopotentiometry current-time program and potential-time curve.

Figure 30. Current-reversal chronopotentiometry working curve for the catalytic eC mechanism. The parameter tg on the figure corresponds to tp in the text. (From Ref. 234.)...

For current reversal chronopotentiometry involving the forward reduction of a species O under conditions of semi-infinite linear diffusion, the reverse transition time can be made equal to forward... 

Notwithstanding, the experimental simplicity of chronopotentiometry may still make it a first-choice electroanalytical technique for higher-temperature molten systems. Furthermore, in the current-reversal mode, it is one of only a few diagnostic techniques for assessing the classical reversibility of an electrode process. A recent review has surveyed many of its applications in this context, and so, bearing in mind the authors own interests, it seems appropriate to use examples of chronopotentiometric studies to illustrate some of the present aspects of current interest. 

A Bads (I ) B Bads- The sequence of these processes is indicated by arrows in the second column GS galvanostatic technique, CR current reversal method and x[j. transition times before resp. after the current reversal surface excess of reactant A at t = 0 reversal time tr < Tjr is considered. Further (a) reaction (I) starts after the total depletion of Aads> (t>) the rates of parallel reactions (I) and (II) depend on the ratio F /cJ (c) valid for F /cJ- O (d) for Fa/cJ- oo. Information about adsorption effects in chronopotentiometry is summarized in [224], principles of the method are discussed in chapter 3, section 3. 

Chronopotentiometry with current reversal was successfully exploited in the study of coupling reactions of 1,4-diaminobenzene with 1-naph-tol-2,4-disulfonic acid [104]. The acid-base catalytic dehydration was... 

The analysis by chronopotentiometry of a solution of titanium salt shows the existence of a potential plateau in the potential range -2.2, -2.35 V (Fig. 5). The length of the plateau (transition time, r) depends on the current intensity and on the concentration of titanium ions. In agreement with the Sand s law, it is shown that r is proportional to the reverse of the square of the current intensity. The reactions involving metallic titanium were studied by the current reversal technique which is useful for analysing the deposition and dissolution process (Fig. 6). The two bumps at the beginning and at the end of the chronopotentiogram are due to the reaction Ti " + — Ti +. These additional plateaux occur in the same potential... 

Chronopotentiometry is an important molten salt technique because it can be used with electrodes of relatively large areas, such as simple flag electrodes without an insulating seal By using current-reversal chronopotentiometry, preliminary diagnostic work to determine whether the electrode reaction product is soluble or insoluble, and whether the electrode reaction is reversible or irreversible has proven to be convenient, especially for coitplex reactions such as the reduction of chromate (30). The important... 

Chronopotentiometry is a controlled-current technique in which the potential variation with time (0 is measiued following a current step. Other ciurent perturbations such as linear, cyclic, or current reversals are also used [1,3,4,12]. For a reversible electrode reaction following a ciurent step, chronopotentiograms shown in Fig. 7 can be obtained. 

Feldbeig SW, Auerbach C (1964) Model for current reversal chronopotentiometry with second-order kinetic complications. Anal Chem 36 505-509... 

Ruac I, Blitz D (1991) Consistency proof of the sequential algorithm for the digital simulation of systems involving first-order homogeneous kinetics. Acta Chem Scand 45 1087-1089 Feldberg SW, Auerbach C (1964) Model for current reversal chronopotentiometry with second-order kinetic complications. Anal Chem 36 505-509... 

Chronopotentiometry at a dme appeared to be impossible until Kies828 recently developed polarography with controlled current density, i.e., with a current density sweep. He explained the method as follows. The high current density during the first stage of the drop life results in the initiation of a secondary electrolysis process at a more negative electrode potential followed by a reverse reaction with rapid (reversible) systems because of the increase in the electrode potential. 

In cyclic chronopotentiometry, the current is continually reversed at potentials corresponding to the forward and reverse transition times as shown in Figure... 

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