Hydrodynamic voltammograms

In contrast to cyclic voltammetry, this technique is a steady state method in which the rate of mass transport is relatively high and occurs by both convection and diffusion. 

Electrochemically reversible reactions produce steep curves whereas for irreversible reactions the slope may be much shallower, and it may be necessary to go to high potentials to reach the limiting current. 

2 Using Flow Injection Analysis HDVs can be constructed faster using a FIA approach on the HPLC system, just by replacing the analytical column with a length of narrow bore stainless steel tubing, that is, about 2 m of 0.1 mm i.d. tubing. This creates 

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The following data were obtained from the linear scan hydrodynamic voltammogram of a reversible reduction reaction... 

A linear-potential scan hydrodynamic voltammogram for a mixture of Le + and Le + is shown in the figure, where and... 

Heterogeneous rate constants, 12, 113 Hofmeister sequence, 153 Hybridization, 183, 185 Hydrodynamic boundary layer, 10 Hydrodynamic modulation, 113 Hydrodynamic voltammetry, 90 Hydrodynamic voltammogram, 88 Hydrogen evolution, 117 Hydrogen overvoltage, 110, 117 Hydrogen peroxide, 123, 176... 

LCEC is a special case of hydrodynamic chronoamperometry (measuring current as a function of time at a fixed electrode potential in a flowing or stirred solution). In order to fully understand the operation of electrochemical detectors, it is necessary to also appreciate hydrodynamic voltammetry. Hydrodynamic voltammetry, from which amperometry is derived, is a steady-state technique in which the electrode potential is scanned while the solution is stirred and the current is plotted as a function of the potential. Idealized hydrodynamic voltammograms (HDVs) for the case of electrolyte solution (mobile phase) alone and with an oxidizable species added are shown in Fig. 9. The HDV of a compound begins at a potential where the compound is not electroactive and therefore no faradaic current occurs, goes through a region... 

Before the measurement of HOR activity, a pretreatment of the alloy electrode was carried out by potential sweeps (10 V s ) of 10 cycles between 0.05 and 1.20 V in N2-purged 0.1 M HCIO4. The cyclic voltammograms (CVs) at all the alloys resembled that of pure Pt. As described below, these alloy electrodes were electrochemically stabilized by the pretreatment. Hydrodynamic voltammograms for the HOR were then recorded in the potential range from 0 to 0.20 V with a sweep rate of 10 mV s in 0.1 M HCIO4 saturated with pure H2 or 100 ppm CO/H2 at room temperature. The kinetically controlled current 4 for the HOR at 0.02 V was determined from Levich-Koutecky plots [Bard and Faulkner, 1994]. 

FIGURE 13.3 Hydrodynamic voltammograms of Prussian blue-modified electrodes in a wall-jet cell with continuous flow of 0.8ml/min ( ) background in air saturated solution (0.1 M KC1 + 0.01 M phosphate, pH 6.0), ( ) 0.1 mM H2O2. 

A useful variation of the amperometric titration involves measuring the current resulting from a small fixed potential applied across two working electrodes. One electrode functions as an anode and the other as a cathode. Once again, the expected current behavior during a titration can be explained by means of hydrodynamic voltammograms. Position a in Figure 3.43A shows the small po-... 

Fig. 11.10. Hydrodynamic voltammograms for (A) DCPs (6.2 pM each) and (B) TCPs (5.1 pM each) at anodically oxidized diamond electrodes.

According to Eq. (24.2), the injection of RSH causes an increase in the concentration of the PBred in the proximity of the electrode, resulting in an increase of the anodic peak current. By contrast, the cathodic peak is proportional to the PBox concentration that is diminished at the electrode surface by the reaction with thiol (RSH). On the basis of CV experiments and after performing a hydrodynamic voltammogram, a potential of 200 mV to be applied at the PB-modified sensor for thiocholine detection was chosen. 

Fig. 34.7. Hydrodynamic voltammograms for AsA ( ),pAP ( ) and H202 ( ) using a polymer-microchip with an end-channel Pt wire (A) or Au wire (B) detector.

Record the hydrodynamic voltammograms for a mixture of H2O2 and AsA in order to determine the optimal detection potential. Injection and separation is performed as described in Section 48.4. Injection is carried out by applying a voltage of + 2000 V for 10 s. + 2000 V is applied for separation using 50 mM Tris-based buffer pH 9.0. The detection potential is varied between 0 and +0.9 V. Thus, hydrodynamic curves of H202 and AsA have to reach a plateau where the optimal potential is chosen. 

Figure 4. Normalized hydrodynamic voltammograms obtained from liquid chromatograms (conditions as per Figure 3A)...

The hydrodynamic voltammograms of picric acid and p-nitro-phenol depicted in Figure 12 illustrate the influence of mechanism on the detection process. If the detector potential is set at the potential on the plateau of the voltammogram of p-nitro-phenol, both compounds are detected. If the potential is set at the plateau of the first wave of picric acid, it alone is detected. At more negative potentials the unit response for the polynitroaromatic compounds will be greater than the response for mononitroaromatic compounds since more electrons are involved in the electrochemical reduction of polynitroaromatics. 

Figure 12. Hydrodynamic voltammograms of picric acid (Q)) and p-nitrophenol (O) (0 is the normalized peak current response to repeated injection of 20 ng of each compound mobile phase pH 3.7 citrate/phosphate at 0.5 mL/min)...

The oxidizable compounds A, B, and C display the following hydrodynamic voltammograms ... 

Fig. 6 Effect of uncompensated resistance on (a) a cyclic voltammetric response and (b) a steady-state hydrodynamic voltammogram.

Figure 2. Hydrodynamic voltammogram for 2 mM thiocholine at A) Unmodified SPE and B) MWCNT modified SPE. Measurement conditions 50 mM phosphate buffer containing 0.1 M KC1, pH 7.4 (Reproduced with permission from Joshi et al. Electroanalysis 2005, 17, 54-58).

Figure 4.23 shows hydrodynamic voltammograms of the four VPs. The oxidation of 4-VG and 4-EG starts at potentials below 300 mV,... 

Figure 4.23 Hydrodynamic voltammograms of 4-ethylphenol (4-EP), 4-ethyl guaiacol (4-EG), 4-vinylphenol (4-VP), and 4-vinylguaiacol (4-VG)...

Electrochemical detection using amperometric or coulo-metric measurement is preferred for specific measurement of small quantities of 5-HIAA a modification of the method developed by Chou and Jaynes is is available on this book s accompanying Evolve site. Like serotonin, the oxidation potential for 5-HIAA is below 0.6 V and must be optimized for each apphcation. Very few interfering compounds are electrochemically active at such low voltage potentials. But if detection of 5-HIAA with other indoles and catecholamines is desired, then hydrodynamic voltammograms for each analyte should be studied to select the minimum potential that achieves maximum specificity. Some HPLC systems use fluorometric detection, with or without derivatization, for a less demanding measurement of 5-HIAA. A method combining fluorometric and electrochemical detection has also been described. ... 

The variation in peak height for the phenol signal as a function of the potential applied to the electrochemical cell is shown by the hydrodynamic voltammogram in Fig. 4. Maximum sensitivity for phenol detection is achieved at a potential of +850 mV versus Ag/AgCl, or greater. Our experiments were conducted at a potential of +870 mV. 

Fig. 4. Hydrodynamic voltammogram of 7.0 x 10 M phenol. See conditions for Fig. 3. (From Wehmeyer et al., 1983, with permission.)...

Fig. 15. Hydrodynamic voltammogram of 20 y.M NADH in 0.1 M phosphate buffer, pH 7.4. Potential versus Ag/AgCl.

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