Mercury-solution interface

A typical example of an ideal polarizable interface is the mercury-solution interface [1,2]. From an experimental point of view it is characterized by its electrocapillary curve describing the variation of the interfacial tension 7 with the potential drop across the interface, 0. Using the thermodynamic relation due to Lippmann, we get the charge of the wall a (-a is the charge on the solution side) from the derivative of the electrocapillary curve ... 

Fig. 20.4 Lippmann electrometer for studying the variation of the excess charge on mercury with variation in potential difference at the mercury solution interface...

The evolution of nitrogen aids in removing dissolved air. A salt bridge (4 mm tube) attached to the saturated calomel electrode is filled with 3 per cent agar gel saturated with potassium chloride and its tip is placed within 1 mm of the mercury cathode when the mercury is not being stirred this ensures that the tip trails in the mercury surface when the latter is stirred. It is essential that the mercury-solution interface (not merely the solution) be vigorously stirred, and for this purpose the propeller blades of the glass stirrer are partially immersed in the mercury. 

The diffusion current Id depends upon several factors, such as temperature, the viscosity of the medium, the composition of the base electrolyte, the molecular or ionic state of the electro-active species, the dimensions of the capillary, and the pressure on the dropping mercury. The temperature coefficient is about 1.5-2 per cent °C 1 precise measurements of the diffusion current require temperature control to about 0.2 °C, which is generally achieved by immersing the cell in a water thermostat (preferably at 25 °C). A metal ion complex usually yields a different diffusion current from the simple (hydrated) metal ion. The drop time t depends largely upon the pressure on the dropping mercury and to a smaller extent upon the interfacial tension at the mercury-solution interface the latter is dependent upon the potential of the electrode. Fortunately t appears only as the sixth root in the Ilkovib equation, so that variation in this quantity will have a relatively small effect upon the diffusion current. The product m2/3 t1/6 is important because it permits results with different capillaries under otherwise identical conditions to be compared the ratio of the diffusion currents is simply the ratio of the m2/3 r1/6 values. 

Electrocapillary curves, i.e., the interfacial tension vs. the applied potential curves are well known for the mercury-solution interface and have been utilized also for interpreting the... 

Since this capacitance is supposed to be in series with that of the solution and since capacitances of mercury-solution interfaces are much larger than 2 F/cm2, this number is too low. The Thomas-Fermi theory as well as the neglect of interactions between metal electrons and the electrolyte are at fault. To reduce the metal s contribution to the inverse capacitance, a model must include6 penetration of the electron tail of the metal into the solvent region, where the dielectric constant is higher, as the models discussed below do. 

Subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS), has been used extensively to examine interactions of species at the electrode/electrolyte interface. In the present work, the method has been extended to probe interactions at the mercury solution interface. The diminished potential dependent frequency shifts of species adsorbed at mercury electrodes are compared with shifts observed for similar species adsorbed at d-band metals. 

Based on the discussion above, it seems evident that a detailed understanding of kinetic processes occurring at semiconductor electrodes requires the determination of the interfacial energetics. Electrostatic models are available that allow calculation of the spatial distributions of potential and charged species from interfacial capacitance vs. applied potential data (23.24). Like metal electrodes, these models can only be applied at ideal polarizable semiconductor-solution interfaces (25)- In accordance with the behavior of the mercury-solution interface, a set of criteria for ideal interfaces is f. The electrode surface is clean or can be readily renewed within the timescale of... 

Mercaptohexadecanol, adsorption, 979 Mercury in electrode kinetics, 1093, 1195 Mercury solution interface, ideal polarizable interface, 848 Metal capacity, 888 determination. 890 -water interactions, 896, 897... 

Fig. 6.65. (a) Representation of the Gouy-Chapman theory according to Eq. (6.130). (b) Plots of the differential capacity of the mercury/solution interface vs. electrode potential for various concentrations of A/,A/dimethylacetamide in 0.15 M Na2S04-water.(Reprinted with permission from W. R. Fawcett, G. Y. Champagne, S. Komo, and A. J. Motheo, J. Phys. Chem. 92 6368, Fig. 6, copyright 1988, American Chemical Society.)... 

Fig. 6.82. Surface potential of water at the mercury/solution interface as a function of charge density on the metal. (Reprinted from J. O M. Bockris and M. A. Habib, Electrochim. Acta 22 41, copyright 1977, Fig. 2, with permission from Elsevier Science.)...

Starting from the Lippmann equation, derive an expression for the variation of the radius of a mercury drop as a function of potential in a solution where there is no specific adsorption. Assume that the double layer at the mercury solution interface can be treated as a parallel-plate capacitor. (Contractor)... 

Consider a simple interfacial region at a mercury/solution interface. The electrolyte is 0.01 M NaF and the charge on the electrode is 10 iC negative to the pzc. The zeta potential is -10 mV on the same scale. What is the capacitance of the Helmholtz layer and that of the diffuse layer Galculate the capacitance of the interfaces. Take the thickness of the double layer as the distance between the center of the mercury atoms and that of hydrated K+in contact with the electrode through its water layer. (Bockris)... 

In this discussion the mercury-solution interface is assumed to be completely polarizable that is, there is no transfer of charge across the interface. Therefore each of the charged species (including electrons) occurs in only one of the phases. The requirement of complete polarizability means that the surface excess may be divided between that in the aqueous phase (W) and that in the mercury (Hg). In view of Equations (85) and (86), we write... 

Another interpretation of the electrocapillary curve is easily obtained from Equation (89). We wish to investigate the effect of changes in the concentration of the aqueous phase on the interfacial tension at constant applied potential. Several assumptions are made at this point to simplify the desired result. More comprehensive treatments of this subject may be consulted for additional details (e.g., Overbeek 1952). We assume that (a) the aqueous phase contains only 1 1 electrolyte, (b) the solution is sufficiently dilute to neglect activity coefficients, (c) the composition of the metallic phase (and therefore jt,Hg) is constant, (d) only the potential drop at the mercury-solution interface is affected by the composition of the solution, and (e) the Gibbs dividing surface can be located in such a way as to make the surface excess equal to zero for all uncharged components (T, = 0). With these assumptions, Equation (89) becomes... 

Mercury flows from the capillary in small drops. The size of these drops is determined by the interfacial tension at the mercury-solution interface. The internal pressure in the drop acting against Pt is, according to Kucera, given by the relation... 

This internal pressure Pb, often called back pressure, is proportional to the interfacial tension y at the mercury-solution interface and is inversely proportional to r, the radius of the drop. 

For reversible stripping reactions, the applied potential controls the concentration at the mercury-solution interface (according to the Nernst equation). Because of the rapid depletion of all the metal from thin mercury films, the stripping behavior at these electrodes follows a thin-layer behavior. The peak current for the linear scan operation at thin mercury film electrodes is thus given by... 

The liquid metal mercury-solution interface presents the advantage that it approaches closest to an ideal polarizable interface and, therefore, it adopts the potential difference applied between it and a non-polarizable interface. For this reason, the mercury-solution interface has been extensively selected to carry out measurements of the surface tension dependence on the applied potential. In the case of other metal-solution interfaces, the thermodynamic study is much more complex since the changes in the interfacial area are determined by the increase of the number of surface atoms (plastic deformation) or by the increase of the interatomic lattice spacing (elastic deformation) [2, 4]. 

For the thermodynamic study of the mercury-solution interface, the electrochemical potential will be used in Gibbs s isotherm instead of the chemical potential, due to the presence of charged species. In the metal side of the interface, the components are the electrons in excess and the mercury metal whereas in solution the two ions of the electrolyte and the solvent must be considered in the sum,... 

The experimental system is shown in Fig. 3.2. It consists of a capillary column containing mercury up to height h regulated so that, on altering the applied potential, the mercury/solution interface stays in the same position. Under these conditions surface tension counterbalances the force of gravity, according to... 

From their calculations of the surface excess entropy and volume of the electric double layer at a mercury-aqueous electrolyte interface, Hill and Payne (HP) [147] postulated an increase in the number of water molecules in the Stern inner region as the surface charge a of about 30 piC/m2, which is consistent with the results of TC on a silver surface obtained some 30 years later. HP used an indirect method to determine the excess entropy and volume by measuring the dependence on temperature and pressure of the double layer capacitance at the mercury-solution interface. 

Electrocapillarity — (a) as a branch of science, this term covers all phenomena related to the thermodynamics of charged - interfaces, esp. of metal-solution interfaces. The term is practically synonymous with -> capillarity, but emphasizes the electric aspects, (b) The term electrocapillarity is often used in a restricted sense to mean the study of the equilibrium properties of metal solution interfaces, such as the - interfacial tension of mercury solution interfaces, the height of a mercury column (in the case of the - Lippmann capillary electrometer), or the -> drop time (in the case of the - dropping mercury electrode). More generally, however, the equilibrium properties of many other interfaces fall... 

In general, however, the quality of polarizability of liquid-liquid interfaces is far behind the polarizability of electrode-solution interfaces, e.g., a mercury-solution interface, in view of both the magnitude of residual current density and the width of the potential window. 

Figure 3.47. Sketch of a Lippmann-type capillary electrometer. The mercury-solution interface resides in the slightly conical capillary A. in which a certain height h is chosen at which the measurements are carried out. The potential is externally applied R is the reference electrode (in Llppmann s experiments it was a calomel electrode, connected to the solution S via a salt bridge). The interfacial tension between mercury and solution is obtained from the height h, defined in the sketch...

In connection with the foregoing, double layers of course also play an important role in electroanalysis. Transfer of. say, electroactive ions through the polarized mercury-solution interface is preceded by passage through the double layer. Therefore current-potential plots depend in principle on double layer properties. Historically, it was his interest In charge-transfer and corrosion problems that induced Grahame to start his seminal double layer Investigations. 

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