Chronopotentiometrics

The determination of such compounds was measured by their effect on the oxidation signal of tire guairine peak of calf tlrymus DNA immobilised on tire electrode surface and investigated by chronopotentiometric or voltammetric analysis. Applicability to river and wastewater samples is demonstrated. 

FIGURE 6-14 DNA hybridization biosensors detection of DNA sequences from the E. coli pathogen. Chronopotentiometric response of the redox indicator upon increasing the target concentration in 1.0 pg/ml steps (a-c), in connection with a 2 min hybridization time. (Reproduced with permission from reference 46.)... 

Weaver MJ, Inman D (1975) The sulphur-sulphide electrode in molten salts-1 Chronopotentiometric behaviour in lithium chloride-potassium chloride eutectic. Electrochim Acta 20 929-936... 

For the individual types of transient measuring techniques, special names exist but their terminology lacks uniformity. The potentiostatic techniques where the time-dependent current variation is determined are often called chronoamperometric, and the galvanostatic techniques where the potential variation is determined are called chronopotentiometric. For the potentiodynamic method involving linear potential scans, the term voltammetry is used, but this term is often used for other transient methods as well. 

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... 

Fig. 5.11 Chronopotentiometric curves for a reversible electrode reaction (Eq. 5.4.16) in E-t (upper curve) and E-tV2 (lower curve) coordinates, r denotes the transition time...

The simplest of the methods employing controlled current density is electrolysis at constant current density, in which the E-t dependence is measured (the galvanostatic or chronopotentiometric method). The instrumentation for this method is much less involved than for controlled-potential methods. The basic experimental arrangement for galvanostatic measurements is shown in Fig. 5.15, where a recording voltmeter or oscilloscope replaces the potentiometer. The theory of the simplest applications of this method to electrode processes was described in Section 5.4.1 (see Eqs 5.4.16 and 5.4.17). 

From chronopotentiometric curves such as those shown, the authors obtained plots of ... 

Table 3.3 The values of finc and logtfcft calculated from the chronopotentiometric experiments of Haynes and Sawyer 1967)...

Chronopotentiometry has also been used to determine chloride ions in seawater [31]. The chloride in the solution containing an inert electrolyte was deposited on a silver electrode (1.1 cm2) by the passage of an anodic current. The cell comprised a silver disc as working electrode, a symmetrical platinum-disc counter-electrode and a Ag-AgCl reference electrode to monitor the potential of the working electrode. This potential was displayed on one channel of a two-channel recorder, and its derivative was displayed on the other channel. The chronopotentiometric constant was determined over the chloride concentration range 0.5 to 10 mM, and the concentration of the unknown solution was determined by altering the value of the impressed current until the observed transition time was about equal to that used for the standard solution. 

Fig. 6 Chronopotentiometry (a) typical excitation signal (b) potential response (c) concentration profiles of educt for a chronopotentiometric experiment (three profiles at various times, increasing time shown by arrow).

Occasionally, we may wish to measure a potential while it changes with time, for example, chronopotentiometric measurements are often a useful means of studying the processes occurring during corrosion. 

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... 

Einally, the chronopotentiometric equation for a given system and for a constant current in the presence of adsorption is obtained by substimting A in Eq. (10.14) with A2 from Eq. (10.16) thus. 

Electrodeposition of lead-tin alloy films is usually performed in the presence of peptone as an additive. Peptone is adsorbed on the metal surface during the electrodeposition process. The fractional surface coverage Q of the lead-tin electrode may be determined from the double-layer capacitance C measurements, and/or chronopotentiometric measurements. For a solution containing 9.0 g/L of tin and 13.0 g/L of lead, the following relationship between the concentration of peptone, the double-layer capacitance C, and the transition time At is observed (8). 

Various oxidation and reduction reactions of substituted metallocenes have already been discussed. A large number of substituted metallocenes have been oxidized chronopotentiometrically at a platinum foil in acetonitrile solution (39, 46). Electron-withdrawing substituents decrease the ease of oxidation, while electron-donating substituents increase the ease of oxidation with respect to the parent metallocenes. A plot of chronopotentiometric quarter-wave potentials, El, vs. Hammett para-sigma constants shows a definite linear relationship. The Ei s for ruthenocene and osmocene indicate these metallocenes are more difficultly oxidized than ferrocene, in agreement with earlier qualitative observations (18). 

Fig. 18. A chronopotentiometric experiment, (a) Current step (b) potential vs. time response.

Further guidelines for the interpretation of chronopotentiometric results can be found in refs. 21 and 22. Its diagnostic value, especially, should be emphasized, as well as its sensitivity to a wide range of chemical rate constants, because its time window can be adapted by choosing proper values for the controlled current density j. The ECE mechanism, left out of the discussion here, is treated in detail in ref. 22. 

The better accuracy of current vs. potential or current vs. time measurements at controlled potential makes these methods preferable to the chronopotentiometric method. However, at controlled potential, the mathematical evaluation of the requisite relationships is more complex due to the difficult inverse transformation of the terms s 1/2 jF(s) and (s + ki) in jF (s), or the solution of the corresponding convolution... 

CURRENT STEP, CHRONOPOTENTIOMETRIC AND PULSED-CURRENT TECHNIQUES... 

Figure 4.4 Chronopotentiometric it1/2 variations for a range of impressed currents as a diagnostic for electrode mechanisms.

Although chronopotentiometric experiments were included among the earliest thin-layer studies, there has been very little work in recent years even though... 

The reader familiar with controlled-potential methodology will have no trouble understanding a controlled-current apparatus. Figure 6.18 illustrates classical approaches to two- and three-electrode constant-current chronopotentiometric experiments (see Chap. 4). The simplicity of these circuits was for many years an attractive feature of chronopotentiometry. Improvements in potentiostats have been largely responsible for a decline in the popularity of chronopot in recent years. Nevertheless, constant-current experiments are even more important with respect to coulometric titrations and stripping potentiometry (Chap. 24). 

As indicated previously, in the simulation of chronopotentiometric behavior, one writes the computer program to calculate L, the number of iterations associated with t, the transition time. This is achieved by defining a constant current ic such that... 

Figure 20.8 Model used in the digital simulation of chronopotentiometric behavior. A constant amount (5) is removed from the contents of the first element following the diffusion step. The electrode surface concentration is determined by extrapolating the line of known slope through the point representing first element concentration to the electrode surface.

In kinetically uncomplicated simulations of chronopotentiometric behavior, the value of L that is calculated during the simulation is found to agree with L within a fraction of one iteration out of a thousand. The corresponding simulated potential-time behavior is generally accurate within 0.1 mV. 

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