Potential scan

Stripping voltammetry involves the pre-concentration of the analyte species at the electrode surface prior to the voltannnetric scan. The pre-concentration step is carried out under fixed potential control for a predetennined time, where the species of interest is accumulated at the surface of the working electrode at a rate dependent on the applied potential. The detemiination step leads to a current peak, the height and area of which is proportional to the concentration of the accumulated species and hence to the concentration in the bulk solution. The stripping step can involve a variety of potential wavefomis, from linear-potential scan to differential pulse or square-wave scan. Different types of stripping voltaimnetries exist, all of which coimnonly use mercury electrodes (dropping mercury electrodes (DMEs) or mercury film electrodes) [7, 17]. 

Sensitivity In many voltammetric experiments, sensitivity can be improved by adjusting the experimental conditions. For example, in stripping voltammetry, sensitivity is improved by increasing the deposition time, by increasing the rate of the linear potential scan, or by using a differential-pulse technique. One reason for the popularity of potential pulse techniques is an increase in current relative to that obtained with a linear potential scan. 

Selectivity Selectivity in voltammetry is determined by the difference between half-wave potentials or peak potentials, with minimum differences of+0.2-0.3 V required for a linear potential scan, and +0.04-0.05 V for differential pulse voltammetry. Selectivity can be improved by adjusting solution conditions. As we have seen, the presence of a complexing ligand can substantially shift the potential at which an analyte is oxidized or reduced. Other solution parameters, such as pH, also can be used to improve selectivity. 

Time, Cost, and Equipment Commercial instrumentation for voltammetry ranges from less than 1000 for simple instruments to as much as 20,000 for more sophisticated instruments. In general, less expensive instrumentation is limited to linear potential scans, and the more expensive instruments allow for more complex potential-excitation signals using potential pulses. Except for stripping voltammetry, which uses long deposition times, voltammetric analyses are relatively rapid. 

In hydrodynamic voltammetry the solution is stirred either by using a magnetic stir bar or by rotating the electrode. Because the solution is stirred, a dropping mercury electrode cannot be used and is replaced with a solid electrode. Both linear potential scans or potential pulses can be applied. 

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

The purity of a sample of K3Fe(CN)6 was determined using linear-potential scan hydrodynamic voltammetry at a glassy carbon electrode using the method of external standards. The following data were obtained for a set of calibration standards. 

Rapid-Scan Corrosion Behavior Diagram (CBD) Basically, all the same equipment used in the conductance of an ASTM G5 slow-scan polarization study is used for rapid-scan CBDs (that is, a standard test cell, potentiostat, voltmeters, log converters, X-Y recorders, and electronic potential scanning devices). The differences... 

Fig. 19.43 Effect of potential scan rate on the value of for Type 304 stainless steel in O-I mol dm NaCI (after Leckie )...

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 ... 

The selection of the pulse amplitude and potential scan rate usually requires a trade-off among sensitivity, resolution, and speed. For example, larger pulse amplitudes result in larger and broader peaks. Pulse amplitudes of 25-50 mV, coupled with a 5 mV s 1 scan rate, are commonly employed. Irreversible redox systems result in lower and broader current peaks (i.e., inferior sensitivity and resolution) compared with those predicted for reversible systems (6). In addition to improvements in sensitivity and resolution, the technique can provide information about the chemical form in which the analyte appears (oxidation states, complexa-tion, etc.). 

Cathodic stripping voltammetry (CSV) is the mirror image of ASV It involves anodic deposition of the analyte, followed by stripping in a negative-going potential scan ... 

Potential of zero charge, 20, 23, 25, 66 Potential scanning detector, 92 Potential step, 7, 42, 60 Potential window, 107, 108 Potentiometry, 2, 140 Potentiometric stripping analysis, 79 Potentiostat, 104, 105 Preconcentrating surfaces, 121 Preconcentration step, 121 Pretreatment, 110, 116 Pulsed amperometric detection, 92 Pulse voltammetry, 67... 

Figure 4. Intensity as a function of potential vs. SCE for two of the Raman bands (1346 cm and 699 cm ) of Fe-TsPc adsorbed on a silver electrode at different pH values. These measurements were obtained at a potential scan rate of 10 mV s. See caption Fig. 2.

FIGURE 123 Schematic shape of polarization curves during an anodic and a cathodic potential scan. 

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. 

A linear potential scan (LPS) is applied to the electrode with the aid of the potentiodynamic equipment, that is, a potential that has a constant rate of variation v =... 

The current is recorded as a function of time. Since the potential also varies with time, the results are usually reported as the potential dependence of current, or plots of i vs. E (Fig.12.7), hence the name voltammetry. Curve 1 in Fig. 12.7 shows schematically the polarization curve recorded for an electrochemical reaction under steady-state conditions, and curve 2 shows the corresponding kinetic current 4 (the current in the absence of concentration changes). Unless the potential scan rate v is very low, there is no time for attainment of the steady state, and the reactant surface concentration will be higher than it would be in the steady state. For this reason the... 

In a round-trip potential scan the values of corresponding to the anodic and cathodic direction are different. For reversible reactions the difference is minor, according to Eq. (12.9) (i.e., only 0.056/n V regardless of the component concentrations and of the potential scan rate v). It is typical for irreversible reactions that the difference between these potentials is much larger (Fig. 12.9) the gap between the maxima increases with decreasing value of the reaction rate constant and increasing scan rate v. 

The limits of transition region BC are not very distinct and depend on the experimental conditions. At high potential scan rates (short duration of the experiment), passivation will start later (i.e., potential will be somewhat more positive, and for a short time the currents may be higher than i ). 

The first version of a polarographic technique was put forward in 1922 by the Czech scientist Jaroslav Heyrovsky. Classsical polarography is the measurement of quasisteady-state polarization curves with linear potential scans applied to the DME sufficiently slowly (v between 1 and 20mV/s), so that within the lifetime, of an individual drop, the potential would not change by more than 3 to 5 mV. With special instruments (polarographs), one can record the resulting 7 vs. E curves (polaro-grams) automatically. 

Voltammograms with characteristic current maxima are obtained (see Fig. 12.9) when linear potential scans (LPS) which are not particularly slow are apphed to an electrode. The potentials at which a maximum occurs depend on the nature of the reactant, while the associated current depends on its concentration. When several reactants are present in the solution, several maxima will appear in a curve. 

---