Constant current voltammetry

Controlled-current voltammetry generally involves either a sinusoidal waveform (see Chap. 4) or a constant current [Fig. 18(a)]. Constant-current voltammetry, or chronopotentiometry [64, 65], generates a potential—time signal, as in Fig. 18(b), characterized by a transition time r. 

Now returning to the coulometric analysis proper we can. say that any determination that can be carried out by voltammetry is also possible by coulometry whether it should be done by means of the controlled-potential or the titration (constant-current) method much depends on the electrochemical properties of the analyte itself and on additional circumstances both methods, because they are based on bulk electrolysis, require continuous stirring. 

Thin films of a composite nickel-iron (9 1 Ni/Fe ratio) and iron-free oxyhydroxides were deposited from metal nitrate solutions onto Ni foils by electroprecipitation at constant current density. A comparison of the cyclic voltammetry of such films in 1M KOH at room temperature (see Fig. 6) shows that the incorporation of iron in the lattice shifts the potentials associated formally with the Ni00H/Ni(0H)2 redox processes towards negative potentials, and decreases considerably the onset potential for oxygen evolution. The oxidation peak, as shown in the voltammo-gram, is much larger than the reduction counterpart, providing evidence that within the time scale of the cyclic voltammetry, a fraction of the nickel sites remains in the oxidized state at potentials more negative than the reduction peak. 

The simulation of other electrochemical experiments will require different electrode boundary conditions. The simulation of potential-step Nernstian behavior will require that the ratio of reactant and product concentrations at the electrode surface be a fixed function of electrode potential. In the simulation of voltammetry, this ratio is no longer fixed it is a function of time. Chrono-potentiometry may be simulated by fixing the slope of the concentration profile in the vicinity of the electrode surface according to the magnitude of the constant current passed. These other techniques are discussed later a model for diffusion-limited semi-infinite linear diffusion is developed immediately. 

B. Chronopotentiometry (Formerly called Voltammetry at Constant Current). These terms were applied by Delahay et al (Refs 4 5) to measurements in which the course of polarization of an electrode (immersed in an unstirred soln) under forced constant current was followed potentiometrically as a function of time. The potential-time curve recorded in the presence of a depolarizer is characterized by a transition time, during which the rate of change of potential is relatively small. This... 

Delahay G.Mamantov, AnalChem 27, 478-83 (1955) (Voltammetry at constant current theoretical principles. The term "chronopotentiometry is recommended as less cumbersome... 

Fig. 6.26 Values of /°x recorded by voltammetry at 1-minute intervals during constant current coulometry...

The electrolysis of the studied systems was carried out in the same cell as voltammetry measurements under the mode of either constant current or voltage. In the constant current mode, the applied current density was in the range of 0.01 0.2 A/ sm2 with reference to the surface area of the cathode before starting the electrolysis. Semi-immersed glassy carbon plate electrodes (cathode area - 5 sm2, anode area - 10 sm2) were used while electrolysis experiments. A powder product was either settled down onto the crucible bottom or assembled on the cathode in the view of electrolytic pear . The deposit was separated from salts by successive leaching with hot water. Thereafter, the precipitate was washed with distilled water by decantation method several times and dried to a constant mass at 100 - 150 °C. The electrolysis products were analyzed by chemical and X-ray phase analyses, methods of electron diffraction and electronic microscopy (transmission and scanning). 

A steady state is independent of the details of the experiment used in attaining it. Thus, under conditions where a steady state is attained, e.g., under convective conditions in an - electrochemical cell, the application of a constant current leads to a constant potential and similarly the application of a constant potential leads to the same constant current. Voltammetric steady states are most commonly reached using linear potential sweeps (or ramps) in a single or cyclic direction at a UME or RDE. A sigmoidally shaped current (l)-potential (E) voltammogram (i.e., a steady-state voltammogram) is recorded in the method known as steady-state voltammetry as shown in the Figure. Characteristics of the... 

Delahay, P., and G.Mamantov Voltammetry at Constant Current—Review... 

In the development of electrochemical genosensors, carbon screen-printed electrodes have been coupled with modern electro analytical techniques such as square wave voltammetry or chronopotentiometry at constant current, and... 

According to eq. 2 a constant current appears in the cyclic voltammogram (CV) when Q is plotted versus U. In real systems such as porous carbon electrodes, both load resistances due to the spatial distributed capacitance in the pores (circuit model in fig.l) and surface functional groups cause a deviation from the rectangular CV-shape. While the first induces a finite time constant in the charging process, the latter are identified by current peaks in the CV [14,6]. The voltage range used for cyclic voltammetry was -0.2 to 0.8 Volt vs.. g/, gCl at a scanrate of 5 mV/s, respectively. 

The mean surface concentrations enforced by depend on many factors (a) the way in which is varied (b) whether or not there is periodic renewal of the diffusion layer (c) the applicable current-potential characteristic and (d) homogeneous or heterogeneous chemical complications associated with the overall electrode reaction. For example, one could vary sequential potentiostatic manner with periodic renewal of the diffusion layer, as in sampled-current voltammetry. This is the technique that is actually used in ac polarography, which features a DME and effectively constant during the lifetime of each drop. Alternatively one could use a stationary electrode and a fairly fast sweep without renewal of the diffusion layer. Both techniques have been developed and are considered below. The effects of different kinds of charge-transfer kinetics will also be examined here, but the effects of homogeneous complications are deferred to Chapter... 

For constant current experiments on Ei-CiEr systems with > 0, the results are analogous to those found in chronoamperometry and linear scan voltammetry (10, 64). For small A in zone DP, as i oo, (8.2.14) applies with n = 1. In zone KP with large A as / 0, (8.2.14) again holds with n = 2. The overall dependence of on A involves a rather complicated expression the general trend is similar to that in Figure 12.3.33. On current reversal, only B is oxidized near the forward wave, and the ratio of transition times r /rf as a function of A is shown in Figure 12.3.37. 

Ostatna, V., Uslu, B., Dogan, B., Ozkan, S., Palecek, E. (2006). Native and denatured bovine serum albumin d.c. polarography, stripping voltammetry and constant current chronopotentiometry. 7 ZecfraawaZ C/zem 593, 172-178. 

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