Polarography, dc

Normal-pulse voltammetry consists of a series of pulses of increasing amplitude applied to successive drops at a preselected time near the end of each drop lifetime (4). Such a normal-pulse train is shown in Figure 3-4. Between the pidses, the electrode is kept at a constant (base) potential at which no reaction of the analyte occurs. The amplitude of the pulse increases linearly with each drop. The current is measured about 40 ms after the pulse is applied, at which time the contribution of the charging current is nearly zero. In addition, because of the short pulse duration, the diffusion layer is thinner than that in DC polarography (i.e., there is larger flux of... 

An increase in co from 400 to 1600rpm tints results in a twofold increase of the signal. A deviation from linearity of a plot of z) vs. col/1 suggests some kinetic limitations. In addition, at veiy low rotation speeds (0-100 rpm), a slight upward bend is observed due to contribution by natural convection. The voltammetric wave has a sigmoidal shape for reversible systems it is identical to that common in DC polarography (described in Section 3-2), and independent of to. 

If sh 5 10 8 cm s 1 in dc polarography we arrive at a totally irreversible electrode process, where the backward reaction can be neglected we shall treat such a situation for a reduction process as the forward reaction. 

The difficulties in conventional polarography as mentioned in Section 3.3.1.1, especially the interference due to the charging current, have led to a series of most interesting developments by means of which these problems can be solved in various ways and to different extents. The newer methods concerned can be divided into controlled-potential techniques and controlled-current techniques. A more striking and practical division is the distinction between advanced DC polarography and AC polarography. These divisions and their further classification are illustrated in Table 3.1. In treating the different classes we have not applied a net separation between their principles, theory and practice, because these aspects are far too interrelated within each class. 

Rapid DC polarography These principles can be 5. Linear-sweep 1. Polarography with 2. Oscillographic ... 

For controlled-current DC polarography, especially its current density mode, see under Chronopotentiometry at a dme (p. 172). For charge-step polarography, i.e., a controlled charge of coulostatic technique, see ref. 9, pp. 424-429, and ref. 3, pp. 270-276. 

In normal DC polarography where E is assumed to be virtually constant for each Hg drop... 

It is clear, that the various modes of LSV at the dme require an integrated and coherent regulation of sweep time, current sampling and drop knocking, preferably by an electronic device and on with computer guidance. A disadvantage of LSV. at the dme, in contrast to normal DC polarography, is that for mixtures of components the latter yields a simple evaluation by curve extrapolation on the basis of additivity [see Fig. 3.34(a)], whereas the former suffers from an uncertain evaluation [see Fig. 3.34(b)],... 

Fig. 3.37 illustrates that tF starts much higher than in Tast DC polarography, because at the onset of the pulse we still have the analyte bulk concentration at the dme surface and rather near to the end of the drop life. Therefore, even if current sampling took place on an averaged basis over the whole pulse time,... 

The considerable gain in sensitivity with NPP compared with sampled DC polarography can be shown still more clearly on a calculative basis. Thus for a reversible process A 4- ne B and for a solution with ox only, we obtain from eqn. 3.50 by substituting P = exp(nF/RT) (E - El) the equation... 

The Model 384B (see Fig. 5.10) offers nine voltammetric techniques square-wave voltammetry, differential-pulse polarography (DPP), normal-pulse polar-ography (NPP), sampled DC polarography, square-wave stripping voltammetry, differential pulse stripping, DC stripping, linear sweep voltammetry (LSV) and cyclic staircase voltammetry. 

The Model 303A together with the Model 303A/99 accessory offers a DME operational mode for traditional DC polarography or special techniques that require a thin-bore capillary approach. The Model 305 stirrer, an instant off/on... 

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