Surface chronopotentiometry

A complicated situation arises when ox or red is insoluble, for instance in the reductive chronopotentiometry of an ion of a non-amalgamating metal then the metal precipitates on the stationary Hg electrode surface without further diffusion, so that... 

Chronopotentiometry. Paunovic and Oechslin (8) measured the adsorption of peptone on lead-tin alloy electrodes using chronopotentiometric and double-layer measurements. This case is different from the adsorption of HCOOH because peptone is not an electroactive species in the conditions smdied but only blocks the surface used for the electrodeposition of lead-tin alloys from solutions containing Sn and Pb ions. Chronopotentiometric analysis is based on the following principles (7). In the absence of adsorption, the relationship between the transition time r (for reduction of Sn and Pb in this case), the bulk concentration c° of the substance reacting at the electrode, and the current I is given by the equation... 

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. 

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]. 

In practice two methods are used for stationary planar electrodes in quiescent solution chronoamperometry and chronopotentiometry. By use of an electroactive species whose concentration, diffusion coefficient, and n value are known, the electrode area can be calculated from the experimental data. In chronoamperometry, the potential is stepped from a value where no reaction takes place to a value that ensures that the concentration of reactant species will be maintained at essentially zero concentration at the electrode surface. Under conditions of linear diffusion to a planar electrode the current is given by the Cottrell equation [Chapter 3, Eq. (3.6)] ... 

Analyses of insertion electrodes include structural analysis by XRD, neutron diffraction, HRTEM with electron diffraction, chemical analysis by EDAX, XPS and dissolution followed by ICP, morphological analysis by electron microscopy, surface area measurements by gas adsorption, and electrochemical analysis by voltammetry chronopotentiometry (primary techniques) and fine electrochemical tools such as EIS, PITT, GITT, and... 

Transition time (for chronopotentiometry) — Electrolysis time required before the surface concentration of a redox species drops to zero in - chronopotentiometry. In constant-current chronopotentiometry (see also -> constant-current techniques), the transition time r is given by the - Sand equation ... 

Reversal results are usually very sensitive to perturbing chemical reactions. For example, in the Ej-Ci case for cyclic voltammetry, would be 1 in the absence of the perturbation (or in chronopotentiometry r lrf would be 1/3). In the presence of the following reaction, /pa//pc 1 (or Tf/Tf < 1/3) because R is removed from near the electrode surface by reaction, as well as by diffusion. A similar effect will be found for a catalytic (EC ) reaction, where not only is the reverse contribution decreased, but the forward parameter is increased. 

Integration of equation (1.48) with initial and boundary conditions appropriate to the particular experiment is the basis of the theory of instrumental methods such as chronopotentiometry, chronoamperometry and cyclic voltammetry. The first law applied at the electrode surface, x = 0, is used to relate the current to the chemical... 

R. Gabai, N. Sallacan, V. Chegel, T. Bourenko, E. Katz, I. Willner, Characterization of the swelling of acrylamidophenylboronic acid-acrylamide hydrogels upon interaction with glucose by faradaic impedance spectroscopy, chronopotentiometry, quartz crystal microbalance (QCM), and surface plas-mon resonance (SPR) experiments, Journal of Physical Chemistry B 2001, 105, 8196. 

Electrodes coated with Pd-(Ni-Zn)/C and Pd-(Ni-Zn-P)/C are apparently stable for the oxidation of ethanol as shown by the chronopotentiometry experiments illustrated in Fig. 21. hi fact, there is some potential oscillation in the first two-three honrs of ethanol oxidation at a constant current density of 3.0 mA cm, then no oscillation at all, as well as a potential increase by only a few tens of mV after 5.5 h. Both features suggest the absence of effective catalyst deterioration, for example by the formation of strongly adsotbed species on the catalysts surface. 

The extent to which the electrode surface is covered by chemisorbed hydrogen atoms has been classically demonstrated by cyclic voltammetry and chronopotentiometry, particularly with respect to elucidation of the mechanism of the hydrogen evolution reaction and in the electrochemistry of fuel cells. The involvement of such intermediates is also consistent with the known mechanisms of catalytic hydrogenation, both from the vapor phase and the liquid phase. These results also indicate coadsorption of R species. 

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. 

Follow-up chemical reactions (ECi r cind EC ev mechanisms) The situation in the case of the following chemical reactions looks quite different in comparison with the CrevE mechanism. The current quantities (as peak currents in LSV, limiting currents in DC polarograpy and transition times in chronopotentiometry) are, as a matter of fact, not influenced by the following chemical steps. Nevertheless, the concentration of the product B at the electrode surface is decreased by this reaction. Since the electron transfer is reversible the Nernst equation for the surface concentrations of A and B can be applied... 

Chronopotentiometry at dropping mercury electrodes At the end of the 1970s all problems with chemical kinetics in chronopotentiometry appeared to be resolved. Innovations were proposed in papers written by Galvez et al. [106-111]. Their ideas were based on the application of dropping mercury electrodes for these measurements. The authors considered the time-dependent current passing the growing electrode surface so that... 

We therefore covered the surface of the electrodes with a molybdenum layer prior to each determination. Chronopotentiometry was used in stabilised solutions. Figure 14 shows a characteristic chronopotentiogram. 

One of the most important methods of testing of the EASPs is their gatvano-static reduction and oxidation. The analogous method in electroanalytical chemistry is called chronopotentiometry (CPM). In CPM, the transient time corresponds to the utilization of the battery electrode. The transient time is the time when the surface concentration of the depolarizer drops to zero the utilization is the fraction of inserted (extracted) lithium atoms. Obviously, at constant current conditions the difference between these characteristics is solely a matter of units. However, since the guest s concentration varies from (sometimes) zero to a certain maximal value, the DC will suffer aU possible variations as weU, and aU the CPM equations cannot be used, except the single CPM relationship given below. 

In chronopotentiometry, a current is applied to the electrode and the potential changes to a value at which the flux of the electroaclive species is sufficient to supply the applied current. After a certain time, the flux of redox species to the surface cannot sustain this current and the potential changes rapidly to a new value at which another species (often solvent or electrolyte) is reduced (or oxidized). This time, termed the transition time t, follows the Sand equation ... 

Different electronic transduction methods to probe DNA recognition events on surfaces will be addressed in the present account. These include electrochemical transduction means such as amperometry, faradaic impedauce spectroscopy or chronopotentiometry, and microgravimetric, quartz-crystal-microbalance (QCM) measurements. 

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