Polarographic charging current

Electrical circuits for an automatic compensation of charging currents and a direct recording of the faradaic current are available in modem polarographs to reduce the influence of the charging currents. However, the accuracy of such compensation is limited, particularly at low reactant concentrations. 

A charging current (non-faradaic) due to the formation of an electric double layer on the surface of the growing drop most polarographs permit a... 

Residual or condenser current flowing in a polarographic cell is the sum of the Faradic current (if) and the charging current... 

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... 

The d.c. polarographic experiment is performed at a slow scan rate, virtually at constant potential. However, charging currents in polarography are appreciable, owing to the time derivative of Eq. (a) at constant applied voltage ... 

At higher concentrations of the electroactive species, adsorption of components of the redox couple, coupled with the fluidity of the liquid-liquid interface, may lead to mechanical streaming of the solution at the interface, generating a polarographic maximum. Charging currents and polaro-graphic maxima typically limit classical polarography to concentrations of the electroactive species between 10 3 and 10 5 M. 

Fig. 6.10-1 Top the simulated faradaic current if for a one-electron reduction at a halfwave potential of - 0.7 V Middle panel the simulated charging current ic. Bottom panel the total simulated polarographic current i = if + ic. Parameters used E1/2 = - 0.7, 7 t-1/6 = 0,T t 1,6 = — 1, FIRT= 40, C= 0.03, = -0.4.

The phrase pulse voltammetry encompasses a sizable suite of methods whose practice has changed substantially since the first edition appeared. The methods originated in a classical polarographic context and were based on the desire to suppress the charging current arising from continuous expansion of the mercury drop at the DME. Since 1980, practice has departed from the DME, because the SMDE has become the dominant electrode for practical polarographic work and because the use of these methods at stationary electrodes has become more common (35). 

---