Voltammogram hydrodynamic flow

The potential for the direct-current mode was selected from a so-called hydrodynamic flow voltammogram (Figure 5), The potential was chosen on the plateau, before the reduction wave of sulphite ions (Figure 6), Between pH 7 and pH 9 MMC produces only one reduction wave[23], From Figure 5 the pulse base and the pulse height for the differential pulse mode were selected. The optimum that could be achieved was -390 mV and -100 mV, To overcome changes in the reference solution of the reference electrode[21], the potential was checked every day and the inner solution was refreshed every day, if necessary. Most of the mercury detectors described, make use of very short drop times, resulting in small electrode-surfaces, fast renewal of the surface and a low noise level due to low capacity current. 

Fig. 5. Hydrodynamic flow-voltammogram of MMC. Injections of 0.50 iJg MMC, flow rate 1 ml/min, sensitivity 0.5 jjA f.Sa, d.c. —mode.

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... 

Wangfuengkanagul and Chailapakul [9] described the electroanalysis of ( -penicillamine at a boron-doped diamond thin film (BDD) electrode using cyclic voltammetry. The BDD electrode exhibited a well-resolved and irreversible oxidation voltammogram, and provided a linear dynamic range from 0.5 to 10 mM with a detection limit of 25 pM in voltammetric measurement. In addition, penicillamine has been studied by hydrodynamic voltammetry and flow injection analysis with amperometric detection using the BDD electrode. 

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. 

Cyclic voltammograms can be presented in an alternative format to that shown in Fig. 5 by using a time rather than potential axis, as shown in Fig. 8. The equivalent parameters in steady-state voltammetric techniques are related to a hydrodynamic parameter (e.g. flow-rate, rotation speed, ultrasonic power) or a geometric parameter (e.g. electrode radius in microdisc voltammetry). 

Fig.5. Hydrodynamic voltammogram for the oxidation of cystine at the mvRu electrode. Conditions are those in Fig.4 except the applied potential of the flow injection experiment was varied. Cystine concentration, 3.4 /iM.

Fig.2. Hydrodynamic voltammogram of glucose. Constructed from the peak height of the flow injection response. The electrode was reconditioned via potential program (-200 to +700 to 450 mV, 15 minutes at each step) between each change of potential. Flow rate 1 ml/min, injection volume 65 /il, analyte ImM glucose in 1 M NaOH, carrier stream 1 M NaOH.

Figure 2.5 Hydrodynamic voltammograms generated by repeat injections of physostigmine ( ) and its hydrolysis product, the phenol eseroline (o). Chromatographic conditions Column 250 x 4.6 mm(i.d.) Spherisorb S5W Eluent methanol-aq. ammonium nitrate (1.0 mol L , pH 8.5) (90 + 10) Flow rate 0.75 mL min Detection glassy carbon electrode (GCE). The half-wave potentials (from the fitted curves) were - -0.69 V and - -0.21 V vs Ag/AgCl.

Figure 5.1 Hydrodynamic voltammograms for captopril (100 nmol injected) at various electrodes Eluent 1(X) mmol potassium dihydrogen orthophosphate, pH 2 Flow-rate I inL min .

A hydrodynamic voltammogram is a current-potential curve which shows the dependence of the chromatographic peak height on the detection potential. The technique used to obtain the necessary information is voltammetric flow injection analysis. in which an aliquot of the analyte is injected into the flowing eluent prior to the detector and the peak current recorded. This is repeated many times, the detector potential being changed after each injection, until the peak current - potential plot reaches a plateau or a maximum, as shown... 

Figure 2 Hydrodynamic voltammogram of 8.1 pM cystine at amvRuOx-coated glassy carbon electrode with 0.2 M K SO at pH 2.0 as the carrier solution. Flow rate, 1.0 mL mininjection volume, 7.5 pL electrode area, 0.071 cm. Each plotted entry is the average and standard deviation of the peak current from five trials at the given potential. The current is not corrected for the onset of the catalyzed oxidation of water.

To record curves I = f(E), also called voltammograms, preferably the potential is varied arbitrarily either step by step or continuously, and the actual current value is measured as the dependent variable. The opposite procedure is possible also but less common. The shape of the curves depends on the speed of potential variation and on whether the solution is stirred or quiescent. Two basic shapes are foimd (Fig. 2.26). The right curve in Fig. 2.26 is sigmoidal in shape. Such curves appear if a continuous convection is stimulated, either by stirring the solution or by movement of the electrode vs. solution. An important group are the hydrodynamic electrodes. They have in common a laminar, steady flow of solution at the electrode surface. This flow is generated by mechanical devices. 

In potential-sweep techniques, the current flowing at the WE/solution interface is monitored as a function of the potential applied to it. We consider three such volta-mmetric techniques linear sweep voltammetry (LSV), cyclic voltammetry (CV), and hydrodynamic voltammetry (Fig. 20.4). The voltammogram obtained in each case may be regarded as the electrochemical equivalent of a spectrum obtained in a spectrophotometric technique. Indeed, the term electrochemical spectroscopy has been applied to CV [56], and it is worth noting that the independent variable in both cases is related to energy—wavelength in the case of spectroscopy and potential in the case of CV. The potential is swept linearly at I V/s so that the potential at any time is (/) = E vt. 

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