Limiting current voltammetric

In the previous section we saw how voltammetry can be used to determine the concentration of an analyte. Voltammetry also can be used to obtain additional information, including verifying electrochemical reversibility, determining the number of electrons transferred in a redox reaction, and determining equilibrium constants for coupled chemical reactions. Our discussion of these applications is limited to the use of voltammetric techniques that give limiting currents, although other voltammetric techniques also can be used to obtain the same information. 

Let us see now what happens in a similar linear scan voltammetric experiment, but utilizing a stirred solution. Under these conditions, the bulk concentration (C0(b, t)) is maintained at a distance S by the stilling. It is not influenced by the surface electron transfer reaction (as long as the ratio of electrode area to solution volume is small). The slope of the concentration-distance profile [(CQ(b, t) — Co(0, /))/r)] is thus determined solely by the change in the surface concentration (Co(0, /)). Hence, the decrease in Co(0, t) duiing the potential scan (around E°) results in a sharp rise in the current. When a potential more negative than E by 118 mV is reached, Co(0, t) approaches zero, and a limiting current (if) is achieved ... 

In a typical voltammetric experiment, a constant voltage or a slow potential sweep is applied across the ITIES formed in a micrometer-size orifice. If this voltage is sufficiently large to drive some IT (or ET) reaction, a steady-state current response can be observed (Fig. 1) [12]. The diffusion-limited current to a micro-ITIES surrounded by a thick insulating sheath is equivalent to that at an inlaid microdisk electrode, i.e.,... 

The key factor in voltammetry (and polarography) is that the applied potential is varied over the course of the measurement. The voltammogram, which is a current-applied potential curve, / = /( ), corresponds to a voltage scan over a range that induces oxidation or reduction of the analytes. This plot allows identification and measurement of the concentration of each species. Several metals can be determined. The limiting currents in the redox processes can be used for quantitative analysis this is the basis of voltammetric analysis [489]. The methods are based on the direct proportionality between the current and the concentration of the electroactive species, and exploit the ease and precision of measuring electric currents. Voltammetry is suitable for concentrations at or above ppm level. The sensitivity is often much higher than can be obtained with classical titrations. The sensitivity of voltammetric... 

Very limited investigations have been conducted with this alloy system. The results obtained to date suggest that A1 co-deposits with Fe from solutions of Fe(II) in both AlCl3-EtMeImCl at 25 °C [46] and AlCl3-NaCl at 160 °C [100]. In both studies, the A1 co-deposition process was found to commence at nearly the same potential as the Fe(II) reduction reaction therefore, it was difficult to observe a well-defined voltammetric limiting current for the latter process, regardless of the technique that was employed. This behavior is nearly identical to that seen in the Ni-Al system [47], However, a comparison of the voltammetric results recorded for some of the other... 

Garces, J. L., Mas, F., Cecilia, J., Puy, J., Galceran, J. and Salvador, J. (2000). Voltammetry of heterogeneous labile metal-macromolecular systems for any ligand-to-metal ratio. Part I. Approximate voltammetric expressions for the limiting current to obtain complexation information, J. Electroanal. Chem., 484, 107-119. 

The nanostructured Au and AuPt catalysts were found to exhibit electrocatalytic activity for ORR reaction. The cyclic voltammetric (CV) curves at Au/C catalyst reveal an oxidation-reduction wave of gold oxide at +200 mV in the alkaline (0.5 M KOH) electrolyte but little redox current in the acidic (0.5 M H2SO4) electrolyte. Under saturated with O2, the appearance of the cathodic wave is observed at -190 mV in the alkaline electrolyte and at +50 mV in the acidic electrolyte. This finding indicates that the Au catalyst is active toward O2 reduction in both electrolytes. From the Levich plots of the limiting current vs. rotating speed data, one can derive the electron transfer number (w). We obtained n = 3.1 for ORR in 0.5 M KOH electrolyte, and 2.9 for ORR in 0.5 M H2SO4 electrolyte. The intermittent n-value between 2 and 4 indicates that the electrocatalytic ORR at the Au/Ccatalyst likely involved mixed 2e and 4e reduction processes. 

From what is described in this section, it can be concluded that the kinetics of the oxidation reaction of sulphite and dithionite at a platinum electrode in alkaline solution are strongly affected by the nature of the platinum surface. This is important when a platinum electrode is used for a quantitative investigation of the kinetics of the oxidation of sodium dithionite and/or sulphite or as electrode material in the development of a sensor for the measurement and/or control of dithionite and/or sulphite concentrations. However, for sodium dithionite, it has no serious consequences because the limiting-current at 0.45 V vs. SCE does not change as a function of scan number. However, this oxidation is still irreversible (no return peak observed) which means that, in the onset of the voltammetric wave, the current is controlled by charge-transfer kinetics. Therefore, it is possible to investigate and obtain the mechanism of the oxidation of sodium dithionite, which is explained in the next section. 

This assumption is based on three relevant indications. First, this wave results in a limiting-current. This means that steady-state transport phenomena control the rate of this reaction, which is not compatible with a possible oxidation of metallic copper to Cu(I) or Cu(II). If the latter were to be valid, a peak-shaped response should have been obtained because of the limited available amount of metallic copper (initially deposited by reduction of Cu(II) or Cu(I) in the reduction wave). In addition, the second voltammetric oxidation wave in the backward scan direction is actually compatible with such a dissolution reaction. 

The first voltammetric methods met are stationary voltammetries performed on a dropping mercury electrode (polarography) or on a solid rotating disk electrode. The limiting current measured is directly proportional to the concentration of the electroactive species in the solution. Experimental potential scan rate is lower than lOrnVs-1. 

Under these conditions, the voltammetric response of a catalytic mechanism given by Eq. (3.228) remains valid since it is independent of time. This expression can be normalized by the diffusion-controlled limiting current at microspheres ... 

The theoretical study of other electrode processes as a reduction followed by a dimerization of the reduced form or a second-order catalytic mechanism (when the concentration of species Z in scheme (3.IXa, 3.IXb) is not too high) requires the direct use of numerical procedures to obtain their voltammetric responses, although approximate solutions for a second-order catalytic mechanism have been given [83-85]. An approximate analytical expression for the normalized limiting current of this last mechanism with an irreversible chemical reaction is obtained in reference [86] for spherical microelectrodes, and is given by... 

Figure 5.49 Time course for the limiting current at gold modified with the oligonucleotide 2/4 in methylviologen (MV+2) solution (0.1 mM in 0.1 NaCl) at an applied potential of 0 mV (vs. Ag/AgCl). The inset shows the time domain in which the electrode is alternatively exposed to and shielded from 346 nm illumination the arrows indicate when the shutter is open or closed. From R. S. Reese and M. A. Fox, Spectral and cyclic voltammetric characterization of self-assembled monolayers on gold of pyrene end-labeled oligonucleotide duplexes, Can.. Chem., 77,1077-1084 (1999), with permission from The National Research Council of Canada...

Changes in pH may affect the voltammetric curve in three ways, viz., by changing the half-wave potential, the limiting current and/or the shape of the wave. If there is no variation of these parameters with pH, the substrate itself is the electroactive species. 

In voltammetric experiments a normal type of calibration is the recording of voltammetric curves for a known system, constructing plots such as variation of limiting current with the transport parameter, or of current with concentration. In potentiometric experiments the equivalent would be the variation of potential with concentration. These curves are especially important in electroanalytical experiments working curves permit the immediate conversion of a measured current or potential into a concentration. 

Data obtained using these reference couples may be compared with analytical solutions for the peak current, limiting current or voltammetric waveshape (see Sections 3 and 4). Non-compliance of experiment and theory is indicative of malfunctioning instrumentation, poor experimental design (i.e. an unacceptably large uncompensated solution resistance) or a faulty electrode. 

Hydrodynamic voltammetric techniques have the major advantage of being steady-state techniques (see Section 1). Consequently, it is easy to measure limiting currents and half-wave potentials (see below for their definition) as a function of the convective parameter (i.e. flow rate, electrode angular velocity) in the absence of significant problems arising from capacitative charging currents. 

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