Mercury anion adsorption

In the following discussion some important features of the experimental results for anion adsorption are presented. Emphasis is placed on data obtained at mercury electrodes in aqueous solutions. This simply reflects the fact that a large fraction of the existing data were obtained for these systems. This is followed by a presentation of the theory applied to ion adsorption at polarizable interfaces. 

Frumkin has also reported electrocapillary measurements for NH4CNS and NHJ solutions in pyridine which show that CNS and I are less strongly adsorbed from pyridine than from water. Pyridine itself is evidently strongly adsorbed on mercury due to interaction of the aromatic nucleus with the metal. However, the relative importance of ionic solvation and solvent co-adsorption to the anion adsorption process is unknown. 

Introduction Anions have a strong tendency to adsorb specifically at metal surfaces, for example, to estabKsh a direct bond with the electrode by partial loss of their hydration shell. As a consequence of the contact with the electrode, the ionic character of the anions is markedly reduced, resulting in a higher surface concentration than in case of nonspecific adsorption. This effect was first observed in double-layer studies on mercury [229, 230] and later confirmed and studied in detail on single-crystal solid electrodes [231-234]. Specifically adsorbed anions can form various types of ordered structures, either more open (cf. sulfate on Au(hld) [235, 236]) or close-packed as reported for halides on different solid electrodes [21]. Cyclic current-potential curves often reveal sharp current peaks, indicative of phase transitions within the anionic adlayers and hence of the existence of ordered phases [21, 237]. Thermodynamic data of specific anion adsorption was obtained in surface tension studies (on mercury only [229,238-240]), capacitance measurements [231-233], cyclic voltammetry, and chronocoulometry [234]. As an... 

In the case of specific anion adsorption, many metals exhibit another effect which has not been observed in studies involving a mercury cathode, namely, the adsorption of anions produces not only accelerating, but also... 

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. 

Certain negative ions such as Cl , Br, CNS , N03 and SO2 show an adsorption affinity to the mercury surface so in case (a), where the overall potential of the dme is zero, the anions transfer the electrons from the Hg surface towards the inside of the drop, so that the resulting positive charges along the surface will form an electric double layer with the anions adsorbed from the solution. Because according to Coulomb s law similar charges repel one another, a repulsive force results that counteracts the Hg surface tension, so that the apparent crHg value is lowered. 

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. 

Fig. 4.8 compares data on the adsorption of lauric acid (C12) and caprylic acid (Cs) at a hydrophobic surface (mercury) as a function of the total bulk concentration for different pH-values. As is to be expected the molecular species becomes adsorbed at much lower concentrations than the carboxylate anions. The latter cannot penetrate into the adsorption layer without being accompanied by positively charged counterions (Na+). As was shown in Fig. 4.4, the adsorption data of pH = 4 can be plotted in the form of a Frumkin (FFG) equation. Fig. 4.9 compares the adsorption of fatty acids on a hydrophobic model surface (Hg) with that of the adsorption on Y-AI2O3. 

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 potential of zero charge, ac, can be obtained from the condition at which Om = - (dy/d.E)=O.This is the potential at which the interfacial tension is maximum in an electrocapillary curve (yvs.E) and is called the electrocapillary maximum. Figure 5-17 illustrates the electrocapillary curves observed for a liquid mercury electrode in aqueous solutions of varioxis anions. It is found that the greater the adsorption affinity of the anions (Cl" < Br" < I") on mercury, the more negative is the potential of zero charge (the potential of electrocapillary maximum). 

Fig. 5-17. Interfacial tension y of a mercury electrode observed in aqueous solutions of various anions as a function of electrode potential pu = potential of zero diarge in sodium fluoride solution in which no contact adsorption occurs. [From Grahame, 1947.]...

There are numerous analytically oriented studies developed upon adsorption coupled electrode reactions (2.144) and (2.146), which are summarized in the Sect. 3.1. For the purpose of verification of the theory, electrode mechanisms including reductions of a series of metallic ions in the presence of anion-induced adsorption [110], as well as electrode mechanisms at a mercury electrode of methylene blue [92], azobenzene [79], midazolam [115], berberine [111], jatrorubine [121], Cn(lI)-sulfoxine and ferron complexes [122], Cd(II)- and Cu(II)-8-hydoxy-qninoline... 

This phenomenon is known as surface denaturation. Adsorption area and, consequently, the extent of denaturation are decreased with an increasing specific adsorption of the anion. The dependence of the molecular weight versus adsorption area shows that the proteins of large molecular weight (above 15 kD) behave differently from the smaller proteins [94], as they possibly do not denaturate and spread on mercury to the same extent as... 

Formation of 2D phase accompanying electrochemical reduction of 4,4 -pyridine on mercury in the presence of iodide ions occurred via adsorption-nucleation and reorientation-nucleation mechanisms [139]. The first reduction step of Bpy on Hg in the presence of iodide as counterion in acidic medium at 15 involved the BpyH2 " /BpyH couple and led to the formation of a 2D phase. The increased contribution of the reorientation term in the formation of the condensed phase was consistent with the increased adsorption strength of the anion to the electrode surface. 

Adsorption of a condensed 1-hydroxy-adamantane layer at the Hg elec-trode/(Na2S04 or NaF) solution interface has been studied as a function of temperature by Stenina et al. [174]. Later, Stenina etal. [175] have determined adsorption parameters and their temperature dependence for a two-dimensional condensation of adamantanol-1 at a mercury electrode in Na2S04 solutions. They have also studied coadsorption of halide (F , Cl , Br ) anions and 1-adamantanol molecules on Hg electrode [176]. More recently, Stenina etal. [177] have described a new type of an adsorption layer comprising organic molecules of a cage structure condensed at the electrode/solution interface. This phenomenon was discovered for adsorption of cubane derivatives at mercury electrode. 

Values of the parameter 0 may be experimentally evaluated for the mercury-water surface from electrocapillary studies. The displacement of the coordinates of the electrocapillary maxima in Figure 7.23 reflects differences in the intrinsic adsorbability of various ions. Electrocapillary studies reveal that the strength of specific adsorption at the mercury-water interface for some monovalent anions follows the order... 

Class IV adsorbates are transition metal cations whose adsorption is induced by complexation with NCS" and N3 anions, both of which adsorb strongly on mercury. 

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