Dropping mercury electrode , double-layer

Electrochemical impedance measurements of the physical adsorption of ssDNA and dsDNA yields useful information about the kinetics and mobihty of the adsorption process. Physical adsorption of DNA is a simple and inexpensive method of immobilization. The ability to detect differences between ssDNA and dsDNA by impedance could be applicable to DNA biosensor technology. EIS measurements were made of the electrical double layer of a hanging drop mercury electrode for both ssDNA and dsDNA [34]. The impedance profiles were modeled by the Debye equivalent circuit for the adsorption and desorption of both ssDNA and dsDNA. Desorption of denatured ssDNA demonstrated greater dielectric loss than desorption of dsDNA. The greater flexibility of the ssDNA compared to dsDNA was proposed to account for this difference. 

We stipulate the electrode to be smooth (though not necessarily flat) and of constant area A. By smooth we mean that any undulations in the electrode surface should not exceed the thickness of the double layer. For an electrode that is less smooth than this, the concept of electrode area is somewhat vague and the effective electrode area may change with time. By prescribing a constant electrode area, we exclude one of the most practical electrodes the dropping mercury electrode treated in Chap. 5. 

In practice, differential pulse polarography is usually performed with the dropping mercury electrode. This means that appropriate expressions are needed for j F(tm ) and y F(f0), whereas for sufficiently small tp values, eqn. (50) for AyF(fp) remains valid. Some technical refinements, especially to reduce the effect of double-layer charging, have been described in the literature [51]. 

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,... 

Fig. 17.2 Tafel plots for the (normalized, dimensionless) current, yjy, that accompanies hydrogen evolution in a solution containing 3.4 mM HCl + 1.0 M KCl, corrected for diffuse-double-layer effects, mass transport controlled kinetics and ohmic potential drop, measured at three temperatures (5, 45, 75°C all results fall on the same line of this reduced plot) at a dropping mercury electrode. The slope obtained from this plot is 0.52, independent of temperature. (Based on data from E. Kirowa-Eisner, M. Schwarz, M. Rosenblum, and E. Gileadi, J. Electroanal. Chem. 381, 29 (1995) and reproduced by the authors.)...

This technique was the first used to measure double-layer parameters (principally of the dropping mercury electrode) and later to measure electrode impedance in the presence of a faradaic reaction to determine the kinetics of electrode processes. The use of ac bridges provides meas-... 

H. L. F. V. HELMHOLTZ (1821-1894) introduces the dropping mercury electrode, pubhshes a model of the electrical double layer (1879) Wied Ann 7 337... 

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... 

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. 

Reference electrodes of mercury have been used by several investigators in an attempt to measure single electrode potentials. Stastny and Strafelda (5 ) concluded that the zero charge potential of such an electrode in contact with an infinitely dilute aqueous solution is -0.1901V referred to the standard hydrogen electrode. Hall ( ) states that the potential drop across the double layer under these conditions is independent of solution composition when specific adsorption is absent. Daghetti and Trasatti (7, ) have used mercury reference electrodes to study the absolute potential of the fluoride ion-selective electrode and have compared their estimates of ion activities in NaF solutions with those provided by other methods. Their method is based on the assumption that the potential drop across the mercury I solution interface is independent of the electrolyte concentration once the diffuse layer effects are accounted for by the Gouy-Chapman theory. 

Fig. 1.6 A scheme of a double layer charging current (Iq) and the Faradaic current (7p) during the second half of the last half-cycle of square-wave signal applied to the dropping mercury electrode. E-E 2= -0.016 V, = 0.1 V, C = 40pF/cm, R= 10 2, = 0.01 cm, D = 9 x...

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

---