Mercury electrode, adsorption

Electrochemical reduction of iridium solutions in the presence azodye (acid chrome dark blue [ACDB]) on slowly dropping mercury electrode is accompanied by occurrence of additional peaks on background acetic-ammonium buffer solutions except for waves of reduction azodye. Potentials of these peaks are displaced to cathode region of the potential compared to the respective peaks of reduction of the azodye. The nature of reduction current in iridium solutions in the presence ACDB is diffusive with considerable adsorptive limitations. The method of voltamiuetric determination of iridium with ACDB has been developed (C 1-2 x 10 mol/L). 

Some emphasis has been placed inthis Section on the nature of theel trified interface since it is apparent that adsorption at the interface between the metal and solution is a precursor to the electrochemical reactions that constitute corrosion in aqueous solution. The majority of studies of adsorption have been carried out using a mercury electrode (determination of surface tension us. potential, impedance us. potential, etc.) and this has lead to a grater understanding of the nature of the electrihed interface and of the forces that are responsible for adsorption of anions and cations from solution. Unfortunately, it is more difficult to study adsorption on clean solid metal surfaces (e.g. platinum), and the situation is even more complicated when the surface of the metal is filmed with solid oxide. Nevertheless, information obtained with the mercury electrode can be used to provide a qualitative interpretation of adsorption phenomenon in the corrosion of metals, and in order to emphasise the importance of adsorption phenomena some examples are outlined below. 

The authors [35] emphasize that their result regarding the first HgS monolayer, which involves reversible underpotential adsorption, suggests that nucleation cannot be considered as a universal mechanism for the formation of anodic films. Analogous conclusions have been inferred for cathodic HgSe films electrodeposited on mercury electrode by the reduction of selenous acid [37] the first monolayer appeared to be reversibly adsorbed, while formation of the following two layers was preceded by nucleation. 

Copper(II) ions in the presence of chloride ions are reduced at the dropping mercury electrode (dme) in two steps, Cu(II) -> Cu(I) and Cu(I) -> Cu(0) producing a double wave at -1-0.04 and 0.22 V versus sce half-wave potentials. In the presence of peroxydisulphate , when the chloride concentration is large enough, two waves are also observed the first limiting current corresponds to the reduction of the Cu(II) to Cu(I) plus reduction of a fraction of peroxydisulphate and the total diffusion current at a more negative potential is equal to the sum of the diffusion currents of reduction of Cu(II) to Cu(0) and of the peroxydisulphate. There is evidence that peroxydisulphate is not reduced at the potential of the first wave because of the adsorption of the copper(I) chloride complex at... 

Adsorption of surface-active substances is attended by changes in EDL structure and in the value of the / -potential. Hence, the effects described in Section 14.2 will arise in addition. When surface-active cations [NR] are added to an acidic solution, the / -potential of the mercury electrode will move in the positive direction and cathodic hydrogen evolution at the mercury, according to Eq. (14.16), will slow down (Fig. 14.6, curve 2). When I ions are added, the reaction rate, to the contrary, will increase (curve 3), owing to the negative shift of / -potential. These effects disappear at potentiafs where the ions above become desorbed (at vafues of pofarization of less than 0.6 V in the case of [NR]4 and at values of polarization of over 0.9 V in the case of I ). 

Opinions differ on the nature of the metal-adsorbed anion bond for specific adsorption. In all probability, a covalent bond similar to that formed in salts of the given ion with the cation of the electrode metal is not formed. The behaviour of sulphide ions on an ideal polarized mercury electrode provides evidence for this conclusion. Sulphide ions are adsorbed far more strongly than halide ions. The electrocapillary quantities (interfacial tension, differential capacity) change discontinuously at the potential at which HgS is formed. Thus, the bond of specifically adsorbed sulphide to mercury is different in nature from that in the HgS salt. Some authors have suggested that specific adsorption is a result of partial charge transfer between the adsorbed ions and the electrode. 

In the presence of iodide ions, the overpotential at a mercury electrode decreases, although the adsorption of iodide is minimal in the potential region corresponding to hydrogen evolution. The adsorption of iodide... 

The inhibition of electrode processes as a result of the adsorption of electroinactive surfactants has been studied in detail at catalytically inactive mercury electrodes. In contrast to solid metal electrodes where knowledge of the structure of the electrical double layer is small, it is often possible to determine whether the effect of adsorption on the electrode process at mercury electrodes is solely due to electrostatics (a change in potential 02)... 

A number of reports have appeared concerned with the adsorption of purines at a dropping mercury electrode 77"80> but these are confined to studies at potentials far removed from those where electrochemical oxidation occurs. More recently some qualitative studies on the adsorption of certain purines at the PGE have appeared with a view to understanding the adsorption of these compounds at positively charged electrodes. Since many biological reactions occur at charged membrane or ribosomal surfaces it is of considerable interest to investigate these phenomena. 

This mechanism has been probed by ab initio calculation.157 Structural modifications of the cyclam framework can drastically affect the catalytic efficiency, as a consequence of the modification of the metal ability to interact with C02 or of the modification of the adsorption ability of the complex.157-163 For instance it has been found that the increase of TV-methyl substitution of the cyclam increases the adsorption of the Ni11 complex on the mercury electrode, but decreases the stability of the Ni1 reduced form.164... 

F. Matsumoto, K. Tokuda, and T. Ohsaka, Electrogeneration of superoxide ion at mercury electrodes with a hydrophobic adsorption film in aqueous media. Electroanalysis. 8, 648-653 (1996). 

The adsorption of organic molecules offers a rich phenomenology. A large number of studies have been performed on mercury electrodes, where the surface tension can be measured directly, and the surface charge and the capacity obtained by differentiation. We will not attempt to survey the literature, but consider a simple example the adsorption of aliphatic compounds. 

In Situ Surface Fourier Transform Infrared Study of Adsorption of Isoquinoline at a Mercury Electrode... 

Buffle, J., Mota, A. M. and Simoes Gonsalves, M. L. S. (1987). Adsorption of fulvic-like organic ligands and their Cd and Pb complexes at a mercury electrode, J. Electroanal. Chem., 223, 235-262. 

Ulrich, H-J., W. Stumm, and B.Cosovic (1988), "Adsorption of Aliphatic Fatty Acids on Aquatic Interfaces. Comparison between 2 Model Surfaces The Mercury Electrode and 6-AI2O3 Colloids", Environ. Sd. Techn. 22, 37-41. 

Cytochrome c3 has a positive charge (pi = 10.5) and displays problems of adsorption at a mercury electrode. However, these problems are avoided using a glassy carbon electrode, Figure 13,26 in that, as said above, such a material behaves as a negatively charged surface. 

A similar conclusion arises from the capacitance data for the mercury electrode at far negative potentials (q 0), where anions are desorbed. In this potential range, the double-layer capacitance in various electrolytes is generally equal to ca. 0.17 F Assuming that the molecular diameter of water is 0.31 nm, the electric permittivity can be calculated as j = Cd/e0 = 5.95. The data on thiourea adsorption on different metals and in different solvents have been used to find the apparent electric permittivity of the inner layer. According to the concept proposed by Parsons, thiourea can be treated as a probe dipole. It has been cdculated for the Hg electrode that at (7 / = O.fij is equal to 11.4, 5.8, 5.1, and 10.6 in water, methanol, ethanol, and acetone, respectively. 

The electrode roughness factor can be determined by using the capacitance measurements and one of the models of the double layer. In the absence of specific adsorption of ions, the inner layer capacitance is independent of the electrolyte concentration, in contrast to the capacitance of the diffuse layer Q, which is concentration dependent. The real surface area can be obtained by measuring the total capacitance C and plotting C against Cj, calculated at pzc from the Gouy-Chapman theory for different electrolyte concentrations. Such plots, called Parsons-Zobel plots, were found to be linear at several charges of the mercury electrode. ... 

Most earlier papers dealt with the mercury electrode because of its unique and convenient features, such as surface cleanness, smoothness, isotropic surface properties, and wide range of ideal polarizability. These properties are gener y uncharacteristic of solid metal electrodes, so the results of the sohd met electrolyte interface studies are not as explicit as they are for mercury and are often more controversial. This has been shown by Bockris and Jeng, who studied adsorption of 19 different organic compounds on polycrystaUine platinum electrodes in 0.0 IM HCl solution using a radiotracer method, eUipsometry, and Fourier Transform Infrared Spectroscopy. The authors have determined and discussed adsorption isotherms and the kinetics of adsorption of the studied compounds. Their results were later critically reviewed by Wieckowski. ... 

Lipkowski et al determined the Gibbs energy of diethylether adsorption on single-crystals of gold in aqueous NaF solution and compared the data with the AG° value for a mercury electrode. The results are presented in Table 2. 

The observed disaepancies in experimental results is most likely caused by the ions of the supporting electrolyte. For example, fluoride ions do not adsorb on the mercury electrode but adsorb on the silver electrode. The adsorption on the latter metal strongly depends on the face orientation.The sequence of AG° values forn-hexanol adsorption from Na2S04 and KCIO4 solutions is AC° [Ag(lll)] > AG° [Ag(lOO)] (see Table 3). However, the sequence of AG°s of n-pentanol adsorption from KF solution is just the opposite [Ag(l 10)] > AG° [Ag(l ll)]. The... 

A change in ionic adsorption in the presence of organic molecules was also observed by Parsons and Zobel. They found that in the presence of acetanilide in the inner layer, the surface concentration of specifically adsorbed phosphate ions decreases. In another work it was suggested that specific adsorption of nitrate ions is markedly reduced in the presence of thiourea in the solution. Thiourea alters the properties of the mercury electrode, affecting even the adsorption of iodides. ... 

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