Dropping mercury electrode , double-layer capacitance

FIGURE 1-13 Double-layer capacitance of a mercury drop electrode in NaF solutions of different concentrations. (Reproduced with permission from reference 5.)... 

There are two reports that determined the double-layer capacitance of ionic liquids [31, 40]. By an electrocapillary curve measurement using dropping mercury electrode (DME), the integral double-layer capacitances of ionic liquids were shown to be smaller than those of aqueous solutions and larger than those of non-aqueous solutions, as summarized in Table 17.2 [31]. This behavior can be explained by the thinner double-layer being due to the higher ionic concentration than that of nonaqueous solutions. However, the correlation between the doublelayer capacitance and anion size [41] observed in PC solutions [8] is not clear. It was further shown that the double-layer capacitance of the ionic liquid was not dependent on the choice of electrode from among DME, GC, and activated carbon fiber [31]. 

D. C. Grahame, who is revered by electrochemists even today for his meticulous measurements and detailed and careful analysis of the data, believed that starting from double-layer-capacitance data yields more accurate results. He based his argument on the fact that capacitance can be measured on the dropping mercury electrode, with its periodically renewed surface, whereas eleclrocapillary measurements are taken on a stationary interphase, which is more prone to contamination. Also,... 

The ac bridge is based on a classical Wheatstone (or Wien for ac measurements) bridge in which one part is replaced by an electrochemical cell and the other compensating part by a variable, R or C. The dc potential is supplied by a potentiometer in the center and ac by the external source. The double layer capacitance measurements were initially carried out on a dropping mercury electrode (DME), and the bridge compensation had to be carried out always at the same surface area of the DME, that is, after exactly the same time from the beginning of... 

Historically, impedances were measured on dropping mercury or amalgam electrodes using an ac bridge [9, 10, 24, 30, 39] and registered as functions of the electrode potential. Information on the electrode process was included in the faradaic impedance. It may be obtained by subtracting the solution resistance and double layer capacitance from the total impedance, Zt ... 

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. 

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... 

Important thermoelectrochemical insight has been obtained by investigating the temperature dependencies of a, C and p. The temperature coefficients of a at the electrocapillary maximum, of C at the capacitive minimum and of are important sources for information about the double layer stmcture. The latter depends on temperature dependent changes of physical solution properties (see Sect. 2.5). Classical investigations have been done at ideal polarizable electrodes , i.e. at electrodes where no charge transfer across the interface electrode/solution is occurring during polarisation. Very often, the mercury drop electrode has been used as an example of an ideal polarisable electrode. 

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