Mercury Electrode Surface

An important aspect is that of studying the effects of composition of the efectrolyte solution on the electrocapillary and capacitance curves. Identical curves are obtained for solutions of fluorides, sulfates, and certain other alkali metal salts having identical 

The effects of the anions (i.e., their specific adsorbabilities) increase in the order F Cr Br I . This trend is due to the fact that the solvation energy decreases with increasing crystal radius as one goes from F to I , and the transfer of the ions to the inner Helmholtz plane is facilitated accordingly. The opposite picture is seen for surface-active cations (e.g., [N(C4H5)4]+) the descending branch of the ECC is depressed, and the PZC shifts in the positive direction. 

Anion adsorption also inflnences the shape of the capacitance cnrves. In the region of the PZC and at positive snrface charge, the capacitance increases to valnes of 60 to 80pF/cm as EDL thickness drops to a valne of JCj. The capacitance ininimnm in dilute solutions is distorted, and its position no longer coincides with that of the PZC. 

FIGURE 10.8 Influence of the adsorption of organic substances (a) on the electrocapillary curve, (b) on the capacitance curve, and (c) on the plot of surface charge against potential (1) 0.1 M H2SO4 solution (2) the same, with 0.1 MC4H9OH. 

The values of EDL capacitance are strongly depressed in the region where the organic substances are adsorbed, which indicates that their molecules are wedged in between the metal surface and the solution side of the EDL. On the one hand, this leads to larger distances on the other hand, the value of s in the compact EDL part decreases. 

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A tunneling junction device was used to determine the water structure at the mercury electrode in an aqueous solution of 0.25MHg2 (N03)2 + 0.3M HNO3. It was found that the structure of water domains is the same as that of hexagonal ice. Hydrogen bonding is a dominant, structuredetermining factor in liquid water near the mercury electrode surface. ... 

One way to view UPD is as formation of a surface compound. In other words, deposition of the first atomic layer of an element on a second element involves a larger deposition driving force than subsequent layers, as it benefits from the AG of compound formation. For deposits formed at underpotential, once the substrate is covered the deposition stops because the reaction is surface limited. No more of the substrate element is available to react, unless it can quickly diffuse to the surface through or around the initially deposited monolayer (an example would be amalgam formation at a mercury electrode surface). Subsequent deposition is then only observed when the bulk deposition potential has been exceeded. 

Berberine is an alkaloid undergoing an irreversible four-electron and three-proton reduction to the electrochemically inactive compound canadine, which is also adsorbed on the mercury electrode surface. As predicted by the theory, the net peak current of berberine is a linear function of the frequency, whereas the peak current shifts linearly with log(/) with a slope of -45 mV. Based on the theoretically predicted value for the half-peak width, AE p/2 = (63.5 0.5) / c mV, the catho-... 

The theory for the reaction of an adsorbed redox couple (2.146) has been exemplified by experiments with methylene blue [92], and azobenzene [79], Both redox couples, methylene blue/leucomethylene, and azobenzene/hydrazobenzene adsorb strongly on the mercury electrode surface. The reduction of methlylene blue involves a very fast two-step redox reaction with a standard rate constants of 3000 s and 6000 s for the first and second step, respectively. Thus, for / < 50 Hz, the kinetic parameter for the first electron transfer is log(m) > 1.8, implying that the reaction appears reversible. Therefore, regardless of the adsorptive accumulation, the net response of methylene blue is a small peak, the peak current of which depends linearly on /J. Increasing the frequency above 50 Hz, the electrochemical... 

All ECi adsorption coupled mechanisms have been verified by experiments with azobenzene/hydrazobenzene redox couple at a hanging mercury drop electrode [86,128,130]. As mentioned in Sect. 2.5.3, azobenzene undergoes a two-electron and two-proton chemically reversible reduction to hydrazobenzene (reaction 2.202). In an acidic medium, hydrazobenzene rearranges to electrochemically inactive benzidine, through a chemically irreversible follow-up chemical reaction (reaction 2.203). The rate of benzidine rearrangement is controlled by the proton concentration in the electrolyte solution. Both azobenzene and hydrazobenzene, and probably benzidine, adsorb strongly on the mercury electrode surface. 

This system with lead amalgam is especially suitable to determine thermodynamic and electrochemical kinetic data related to Pb(II) owing to renewability of the mercury electrode surface. 

Analytical Applications In addition to the above-mentioned analytical aspects of the processes at Hg electrodes, in this section, we briefly review the papers focused on the subject of the affinity of various compounds to the mercury electrode surface, which allowed one to elaborate stripping techniques for the analysis of inorganic ions. Complexes of some metal ions with surface-active ligands were adsorptively accumulated at the mercury surface. After accumulation, the ions were determined, usually applying cathodic stripping voltammetry (CSV). Representative examples of such an analytical approach are summarized as follows. 

The area of the mercury electrode surface in such a cell is only about 0.45 mm2. Thus only a small amount of radical can be generated. To increase the surface area so that larger quantities can be generated, a foil or wire mesh electrode is used in place of the mercury. However, in this configuration, large potential gradients can promote convection, making the overall electrode behavior impossible to characterize. 

This is of course an oversimplification for all metal surfaces, except possibly that of liquid mercury. Electrode surfaces at solid metals and other solid materials are normally quite heterogeneous in the sense of exhibiting lots of cracks, crevices, comers, declivities, asperities, and edges, but this is for obvious reasons not possible to include at the present state of our knowledge (cf. Section 12). 

Pre-wave — In DC polarography, this is a part of the response of fast and reversible electrode reaction (- reversibility) complicated by the -+ adsorption of product on the - dropping mercury electrode surface Ox + ne (Red)ads Red, assuming that the reactant... 

Diffusion current is the limiting current observed in polarography when the current is limited only by the rate of diffusion to the dropping mercury electrode surface. 

In the rotating-disk method the thickness of the diffusion and reaction layers at the electrode surface is made less than at the dropping mercury electrode surface (Orsega et al., 1982). This reduces the extent of 07... 

When pcrtechnctate is electrochemically reduced in aqneous alkaline solution in the presence of gelatin using the techniques of controllcd-potential coulometry, chronoamperometry, and double potential step chronoamperometry, at the mercury electrode surface the technetate ion TcO " is reported to be produced ... 

These results also confirm previous voltammetric findings on the adsorption behaviour of mononucleotides and dinucleotide monophosphates at the mercury electrode surface Taking into account the respective potentials of zero charge... 

The first example is the measurement at a mercury (Hg) electrode. After the invention of polarography, an enormous body of work on the adsorption of organic molecules has been made at a mercury electrode surface. However, mercury is a liquid metal and its hanging drop changes its shape in response to the change of the surface tension, thus to the electrode potential. It is a difficult task to measure the reflection change at a mercury drop electrode surface, since exclusion of the perturbation due to the change of the shape of the electrode is critical. 

The use of a mercury drop bottom electrode placed on an underlying Nation fihn in ER measurement was demonstrated for the reaction of heptyl viologen incorporated in a Nafion film (Fig. 2.13) [48], A mercury drop was placed on a Nation 117 tihn of thickness 0.175 mm. The transparent nature of the Nafion as weU as the high conductivity for cations through its film made it actually possible to measure the ER spectrum of the redox reaction of heptyl viologen with a perpendicular incidence of the hght to the mercury electrode surface through the cell bottom window and the Nafion fihn. 

A special case of mixed reactions is an accumulation of insoluble mercuric and mercurous salts on the mercury electrode surface and their stripping off by SWV ... 

The insoluble salt Hg2X2 is precipitated as a submonomolecular layer on the mercury electrode surface. It is assumed that no lateral interactions between the deposited particles exist. At equilibrium, this redox reaction satisfies the Nemst... 

We have shown that the capacitance pit corresponding to the compact layer formed at the electrode surface is observed in the C solutions in 0.5 M NaCl at pH 5.5, but not at pH 7.0 [31,85,86]. The two-dimensional condensation of C molecules at the mercury electrode surface is thus obviously supported by protonation of a part of the C molecules. Ab initio quantum chemical calculations have proved that protonation significantly increases the stabilization energy of both stacked and hydrogen-bonded C dimers, which may support the two-dimensional condensation [89]. 

Direct observation of an ordered phase of NA bases on sohd electrodes by techniques, such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM), may help determine the orientation of the molecules in the compact film [83, 92-98], These techniques were also recently apphed to the surface of mercury [99-101], It was found that cationic detergent benzalkonium chloride (BAG), used for DNA spreading on mica in scanning force microscopy, forms a condensed film at the mercury electrode surface. The corresponding pit on C-E curves resembled the pits of bases and... 

It was found that the dissociation rate constant is dependent on the concentration of the complexing agent L if a hanging mercury drop electrode was used. For small values of c, kd = 90s At Nation coated mercury film electrode the value of k is lower, k = 6 s . The difference has been explained by the assumption of adsorption of proline anions on the bare mercury electrode surface. This effect induces the acceleration of the dissociation process. 

Polarographic determinations can be affected by low levels of surfactant, because of adsorption of surfactant at the dropping mercury electrode surface. 

It becomes important to highlight that, unlike for liquid eleetrodes, speeially mercury, for solid electrodes, the electroactive surfaee and its catalytic response depends directly on the effectively available area of such material. While for mercury electrode surface roughness has... 

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