Voltage polarogram, current

Microelectrodes are less sensitive than larger electrodes to fall off in current at very high oxygen tensions, but they tend to be somewhat unstable. It is not possible to relate the current-voltage polarogram with overall stability (6). 

A current/voltage polarogram between 0-1.1 volts is constructed. The best electrodes show a reasonably flat plateau between 0.6 and 0.75 volts, but lack of a flat plateau does not necessarily mean that the probe is faulty. 

Polarographic maxima. Current-voltage curves obtained with the dropping mercury cathode frequently exhibit pronounced maxima, which are reproducible and which can be usually eliminated by the addition of certain appropriate maximum suppressors . These maxima vary in shape from sharp peaks to rounded humps, which gradually decrease to the normal diffusion-current curve as the applied voltage is increased. A typical example is shown in Fig. 16.3. Curve A is that for copper ions in 0.1 M potassium hydrogencitrate solution, and curve B is the same polarogram in the presence of 0.005 per cent acid fuchsine solution. 

Commercial polarographs are also available in which the voltage scan is carried out automatically while a chart recorder plots the current-voltage curve. A counter-current control is incorporated which applies a small opposing current to the cell which can be adjusted to compensate for the residual current this leads to polarograms which are better defined. Most of these instruments also incorporate circuits which permit the performance of alternative, more sensitive types of polarography as discussed in Section 16.9... 

If the current-voltage curves of the reagent and of the substance being titrated are not known, the polarograms must first be determined in the supporting... 

Polarography Electrochemical reaction Exchange of charge Diffusion current Current, voltage i=m Polarogram ... 

Figure 4.20 A polarographic titration curve. A polarogram has oscillations about a mean current-voltage curve owing to the regularly changing area of the mercury drop.

Polarogram The current/voltage plot obtained from polaro-graphic measurements. 

Fig. 21. Current-voltage curves for DC and AC polarography 1) = DC polarogram 2) = AC polarogram...

The effect of Triton X-100 on the current-voltage curve of the iron (III)-diethylenetriaminepentaacetic acid complex has been explained as follows [226] at low levels of surfactant the mercury is only partly covered by surfactant and the reversible reduction proceeds at the uncovered portions of the undisturbed surface and at the covered areas a penetration-controlled reaction occurs. As the Triton concentration is increased so the penetration-controlled reaction contribution increases. When the coverage is complete only the second wave is observed on the polarogram. The shift in the half-wave potential with increasing surfactant concentration is probably due to increased adsorbed layer thickness and is shown along with the polarograms and electrocapillary capillary curves in the absence and presence of Triton X-100 in Fig. 11.35. 

Figure 3. Current-voltage plots (polarograms) of platinum oxygen sensing microelectrodes in an air saturated 0.9% NaCl solution. A Saturated Calomel Electrode was used as reference. Note that the plateau region is extended on the coated electrodes although the magnitude of the current has dropped.

Half-wave potentials. The salient features of a typical current-applied voltage curve (polarogram) are shown in Fig. 16.4. 

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