Dropping mercury electrode capacitive current

Figure 3.4 Faradaic (a), capacitive (b), and total (c) current at the dropping-mercury electrode.

At the dropping mercury electrode the fact that DPV is better than NPV is due to the residual capacitive current contribution, which is subtracted out in the differential technique. It is relatively easy to demonstrate that the diminution factor, f, is given by... 

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... 

If a constant potential is imposed on a solid electrode of fixed size this capacitive current dies off rapidly once all the charges are in place. However the dropping mercury electrode is growing in surface area all the time, fresh charge is required just to keep the charge density constant. Thus a capacitive current continues to flow. 

Charging Currents. A small current ( 10 A) is usually observed in the absence of an electroactive substance and is due to the charging of the electrical double layer at the mercury/solution interface as each new mercury drop forms. In this respect, the dropping mercury electrode behaves as a varying capacitance. The magnitude of this capacitative current in the presence of oxidizable and/or reducible substances may indicate the onset of adsorption or desorption processes which can occur as the applied potential is increased. 

This limitation led to the development of normal pulse polarography (NPP) introduced by G.C. Barker, which minimized background responses (principally the capacitance cm-rent of the growing mercury drop) and maximized the analytical response. NPP requires the application of a series of potential pulses at the working electrode for each drop. The pulse increases in potential with every drop. The current is sampled just before the end of the drops lifetime, minimizing the effect of the capacitance current on the faradaic current. As the potential pulse for each drop starts at a potential lower than the redox potential, no depletion of the analyte would have occurred. 

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