Chronopotentiometric potential-time curves

CA 54, 9551 (I960) (Voltammetry at controlled current) 29)W.H.Reinmuth, Anal Chem 32, 1514-17 (I960) (Chronopotentiometric potential-time curves and their interpretation) 30)P.Delahay, "Advances in Electrochemistry and Electrochemical Engineering , Interscience, NY (1961)... 

The chronopotentiometric potential-time curves are often plotted and analysed in terms of semi-logarithmic functions ln(f / — =/(( ), or, in our... 

Reinmuth has examined chronopotentiometric potential-time curves and proposed diagnostic criteria for their interpretation. His treatment applies to the very limited cases with conditions of semi-infinite linear diffusion to a plane electrode, where only one electrode process is possible and where both oxidized and reduced forms of the electroactive species are soluble in solution. This approach is further restricted in application, in many cases, to electrode processes whose rates are mass-transport controlled. Nicholson and Shain have examined in some detail the theory of stationary electrode polarography for single-scan and cyclic methods applied to reversible and irreversible systems. However, since in kinetic studies it is preferable to avoid diffusion control which obscures the reaction kinetics, such methods are not well suited for the general study of the mechanism of electrochemical organic oxidation. The relatively few studies which have attempted to analyze the mechanisms of electrochemical organic oxidation reactions will be discussed in detail in a following section. 

Alternating current chronopotentio-metry) 17)T.Kambara I.Tachi, JPhysChem 61, 1405-07 (1957) (Chronopotentiometry-stepwise potential-time curves for. arbitrary number of reducible species) 18)R.H.Adams et al, AnalChem30, 471-75 (1958) (Chronopotentiometric studies with solid electrodes) 19)P. J.Elving A.F.Krivis, AnalChem 30, 1648-52 (1958) (Anodic chronopotentiometry with a graphite electrode analytical applications) 20)J.D.Voorhies N.H.Furman, AnalChem 30, 1656-59 (1958) (Quantitative anodic chronopotentiometry with Pt electrode) 21)D.E.Dieball, UnivMicrofilms (Ann Arbor, Mich), Publ No 24553, ll8pp Dissertation Abstr 18, 50-1 (1958) CA 52, 6051 (1958)... 

As is well known, the steady-state behavior of (spherical and disc) microelectrodes enables the generation of a unique current-potential relationship since the response is independent of the time or frequency variables [43]. This feature allows us to obtain identical I-E responses, independently of the electrochemical technique, when a voltammogram is generated by applying a linear sweep or a sequence of discrete potential steps, or a periodic potential. From the above, it can also be expected that the same behavior will be obtained under chronopotentiometric conditions when any current time function I(t) is applied, i.e., the steady-state I(t) —E curve (with E being the measured potential) will be identical to the voltammogram obtained under controlled potential-time conditions [44, 45]. 

Laviron55 has recently noted that linear potential sweep or cyclic voltammetry does not appear to be the best method to determine the diffusion coefficient D of species migrating through a layer of finite thickness since measurements are based on the shape of the curves, which in turn depend on the rate of electron exchange with the electrode and on the uncompensated ohmic drop in the film. It has been established that chronopotentiometric transition times or current-time curves obtained when the potential is stepped well beyond the reduction or oxidation potential are not influenced by these factors.55 An expression for the chronopotentiometric transition has been derived for thin layer cells.66 Laviron55 has shown that for a space distributed redox electrode of thickness L, the transition time (r) is given implicitly by an expression of the form... 

As CV, the potential-time chronopotentiometric curves also contain only one reduction wave (cf. Fig. 6.7). 

The case of the prescribed material flux at the phase boundary, described in Section 2.5.1, corresponds to the constant current density at the electrode. The concentration of the oxidized form is given directly by Eq. (2.5.11), where K = —j/nF. The concentration of the reduced form at the electrode surface can be calculated from Eq. (5.4.6). The expressions for the concentration are then substituted into Eq. (5.2.24) or (5.4.5), yielding the equation for the dependence of the electrode potential on time (a chronopotentiometric curve). For a reversible electrode process, it follows from the definition of the transition time r (Eq. 2.5.13) for identical diffusion coefficients of the oxidized and reduced forms that... 

To complement the coulometric measurements, chronopotentiometric experiments with these phases were performed (see Figure 2.8). It could be stressed that a saturated phase (evolution of the equilibrium potential with time for at least 1 h, curve a) appears stable, while lower levels of insertion (curves c and d. Figure 2.8) exhibit a progressive increase in the potential. This phenomenon seems provoked by a modification of the distribution of charges inside the layer. All these chronopotentiometric curves were found to depend strongly on the saturation level of charges injected in the platinum whatever the salt used. 

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