Dropping polarographic maxima

In a few instances, the polarographic wave is accompanied by a large peak (where the current rises to a maximum before returning to the expected diffusion current plateau). Such an undesired peak, known as the polarographic maximum, is attributed to a hydrodynamic flow of the solution around the expanding mercury drop, can be suppressed by adding a small amount of a surface-active material (such as Triton X-100). 

Polarographic maximum — A peak-shaped current signal that is caused by enhanced streaming of the solution, or mercury and solution, in measurements with a - dropping mercury electrode (DME) and sometimes also on - stationary mercury drop electrodes (SMDE). Two different types are to be distinguished (a) Polaro-... 

Sharp potential drops are observed on the chronopotenti( rams for the anodic processes in both media (Fig. 3, Curves 3 and 5), and these are retained at various current densities, including fairly high current densities where t is small and the drop on the curve cannot be due to a polarographic maximum. As shown by Zolotovitskii, Tedoradze and firshler [28], the presence of such drops on the E, t curves indicates the existence of an effect from large coverages. Consequently accumulation of dihydroriboflavin on the electrode surface retards the electrochemical oxidation of this compound both under conditions where a prewave is obtained on the polarographic curve and under conditions where there is no prewave. 

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. 

Polarographic waves often show a peak followed by a sharp fall to the limiting current plateau, the cause of which is related to streaming of the solution past the mercury drop. Known as a current maximum, it can be eliminated by adding a surfactant such as gelatin or methyl-red to the sample solution. 

A current maximum of the first kind has the form of a sharp, straight line which starts to form just before the main polarographic wave (curve a in Figure 6.32). Such a maximum can be considerably larger than the wave itself, although it will usually drop suddenly back to the normal wave. Maxima of the first kind are caused by convective effects, as electrolyte flows past the surface of the mercury drop, resulting from surface tension differences at various points on the surface of the drop. 

Another important phenomenon that occurs at the dropping mercury electrode is the polarographic maximum15, which occurs when, on reaching the limiting current plateau, the observed current exceeds /L (Fig. 8.10). The causes are mass transport within the electrode and surface adsorption. Three types of maximum have been identified ... 

The half-wave potentials are dependent on substrate concentration as well as on the drop time of the mercury electrode and the height of the mercury column, and frequently the current (i) versus potential ( ) curves exhibit large polarographic maxima due to adsorption processes. These effects can be minimized by addition of maximum suppressors like gelatine or Triton X-100. A consequence of this non-ideal polarographic behaviour is that results obtained under even slightly different conditions only can be compared in a semi-quantitative way. 

When only the inert electrolyte is present in the polarographic cell a residual current will still flow. This current, which is non-faradaic, is attributable to the formation of an electrical double layer in the solution adjacent to the electrode surface (Fig. 3). At all applied potentials, a current flows to develop this double layer, and the process may be considered analogous to the charging of a parallel plate capacitor. Therefore, the charging current is a capacitance current and varies during the drop lifetime, i.e., with the size of the mercury drop. When the drop surface area is increasing rapidly from the start of the drop lifetime, the capacitance current is a maximum, falling to a minimum near the end of the drop lifetime when the drop size is at a... 

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