Polarography current-sampled

Figure 3.30 (A) Current-time curves illustrating the sampling sequence for current-sampled polarography if, diffusion-controlled faradaic current id), double-layer charging current it = if + idl. (B) Current-sampled polarogram.

Current-Sampled Polarography. As follows from the Ilkovic equation [Eq. (3.10)], the current at the electrode that results from the electrolysis of an electroactive species (Faradaic current) increases proportionally to tU6 during the life of the mercury drop. However, the electrical double layer is a capacitor that must be charged as potential is applied via a charging current (icc)... 

Current-sampled polarography offers only a marginal improvement on the sensitivity of the classic method because there is a more favorable faradaic-to-charging-current ratio at the end of the drop time when the drop is almost stationary. Thus, its only real benefit over the classic method is the clearer polarogram obtained. 

Current-sampled DC and often are used in voltammetry superimposed AC polarography... 

In practice, derivative polarography must be carried out on a smooth curve such as is obtained in current-sampled, current-averaged or rapid DC... 

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,... 

Fig. 3.36. Normal pulse polarography, (a) sampling scheme, (b) current sampled. 

However, in contrast to the condition of a constant potential and current sampling in pulse polarography, which yields36 the Cottrell equation ... 

Detection sensitivity better than that in NPV can be achieved by differential current sampling with small pulses. Figure 18b.lib shows this scheme known as differential pulse voltammetry (DPV). Here, the potential is changed from an initial potential in small steps (2-5 mV) and a voltage pulse of a short duration (50 ms) is superimposed at the end of a long step (500-5000 ms). The current is sampled before the beginning of the pulse and near the end of the pulse as shown in Fig. 18b. 12b. In differential pulse polarography (DPP), this is near the... 

Tast polarography (sampled curent polarography) -> DC polarography with -> current sampling at the end of each drop life. This way the ratio of faradaic current to -> charging current is enhanced and the - limit of detection can be decreased. 

The fast polarography consists of measuring the current only for a very short period, late in the lifetime of the drop. This current sampling avoids the saw-toothed spikes on the current curve. Yet it leads to a reduction in sensitivity since the diffusion current decreases as the analyte flux moves from the bulk of the solution to the surface of the droplet (Figure 20.3). 

Figure 7.3.2 Staircase waveform and sampling scheme for tast polarography and staircase voltammetry. The experiment is a series of cycles in which a potential is established and held constant for a period, a current sample is taken at time r after the start of the period, then the potential is changed by an amount AF. In tast polarography, the mercury drop is dislodged at the end of each cycle, as indicated by the vertical arrows. In staircase voltammetry, this step is omitted. The time between the current sample, drop dislodgment, and the change in potential is exaggerated here. Usually it is negligible and the cycle period is essentially the same as r.

Alternatively, air is passed through 1-pm PTFE membrane and 1% sodium bisulfite solution. The solution is treated with chromotropic and sulfuric acid mixture. The color development due to formaldehyde is measured by a visible spectrophotometer at 580-mn absorbance (NIOSH 1984, Method 3500). In polarography analysis, a Girard-T reagent is used. Formaldehyde forms a derivative that is analyzed by sampled direct current DC polarography (NIOSH 1984, Method 3501). Auel et al. (1987) reported a similar electrochemical analysis of industrial air using an iridium electrode backed by a gas-permeable fluorocarbon-based membrane. 

The NPV technique was first developed as NP polarography (NPP) at dropping mercury electrodes. In these cases, the pulses are synchronized with the drops, the pulse being applied near the end of drop life when the increase in the surface area of the drop is least. Current sampling in NPP also serves to remove the oscillations resulting from drop formation. 

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