Mercury electrosorption

Most frequently, this electrode mechanism has been analyzed in connection with a mercury electrode hence the following reaction schemes are pertinent to this electrode. Note, that the electrosorption mechanism can serve as a theoretical basis for these processes as well [135, 139, 140], The simplest case of an accumula-tion/stripping equilibrium is given by the following equation ... 

Sander and Henze [50] have performed ac investigations of the adsorption potential of metal complexes at Hg electrode. Later, Sander etal. [51] have studied electrosorption of chromium - diethylenetriaminepentaacetic acid (DTPA) on mercury in 0.1 M acetate buffer at pH 6.2 using a drop-time method. The changes in the interfacial activity of the Cr(III)-DTPA complex with the bulk concentration obeyed the Frumkin adsorption isotherm. 

The electrosorption valency of several neutral aliphatic molecules on mercury was examined by Koppitz at al.54 As usual, the experimental l values were referred to the potential of zero charge in the absence of specific adsorption, Ez, and to low surface coverages. With the remarkable exception of thiourea, all these molecules do not undergo pet (A = 0). Since they are also neutral (z = 0), their electrosorption valency is exclusively determined by the two dipole terms ... 

Table 1 summarizes a few values of the dipole moment fx and of the electrosorption valency l of halide and alkali metal ions adsorbed on mercury. The experimental values of l are relative to low coverages near the potential of zero charge and are taken from Schultze and Koppitz,50 while the corresponding // values were calculated from Eq. (75). The theoretical values in the last column are from a hard sphere electrolyte model. Further data can be found in the article by Schmickler49 Note that, in the electrochemical environment, the dipole moments are much smaller than in vacuo, where they can reach values of the order of 7 D for the alkali metal ions. No doubt, this difference is caused by the screening of the adsorbate dipole by the solvent molecules. 

Electrosorption Valency of Three Mercury-Supported Thiol Monolayers... 

After making an approximate estimate of the absolute potential difference y/ across the mercury/aqueous solution interphase, let us detennine the electrosorption valency of three mercury-supported thiol monolayers for which uM was measured by potential-step chronocoulometry. As already reported in Sec. IV.1, n-octadecanthiol... 

In the case of electrosorption, it is best to use a dimensionless scale of C/C(sat) when comparing the adsorjTtion of different solute molecules in the same solvent, or the same solute in different solvents. This scale permits us to compensate for the differences in the free energy of interaction between the solvent and the solute, and the effects seen arise from the different interactions of the solutes with the surface. A good example is the adsorption of phenol on mercury from two different solvents, shown in Fig. IJ. The solubility of phenol in water is much lower than in methanol. It takes therefore a much higher concentration in methanol to reach a given value of the fractional coverage 0 than in an aqueous solution. 

The electrosorption of pyridine on mercury is shown in Fig. 2J. One should note that the dependence of 0 on is roughly bell-shaped for most compounds, with the maximum of adsorption occurring at a potential that is slightly negative with respect to the potential of zero charge. 

Fig. 2J Potential dependence of the electrosorption of pyridine (0.3 mM in 0.1 M KCl) on mercury. Data from Damaskin, Electrochim. Acta, 9, 231, (1964).

A much simpler case is presented by the electrosorption of naphthalene on gold, shown in Fig. 6J. The higher stability of this compound combines with the lower catalytic activity of gold, to ensure that partial oxidation does not occur in the range of potential shown. Also, hydrogen adsorption does not occur on the cathodic branch of the curve. Thus, the potential dependence of 0 ii this case can be interpreted in terms of the properties of the double layer, just as in the case of mercury. 

Electrosorption has been studied on mercury more than on any other... 

Fig. IJ The electrosorption of phenol from (a) aqueous and (b) methano-lic solutions, as a function of the charge density on a mercury electrode. Supporting electrolyte-. 0.] M LiCl, the concentrations of phenol are marked on the curves. From Muller, Ph.D dissertation, Univ. of Pennsylvania, 1965.

Regarding the source of electrosorption information, the situation is the same as with double layers without adsorbed organics most of it is obtained on mercury. Few common organic molecules have escaped such measurements. Electrosorption on charged colloids as such has also been abundantly investigated, but systematic combination with surface charge measurements is relatively rare this will be the theme of the present section. Electroklnetic or stability measurements are sometimes enlightening but are insufficient to understand the composition and fine-structure of the adsorbate. 

By "congruence analysis" is meant a rather formal and general procedure to find out whether A G reacts on the surface potential or on the surface charge. In the field of electrosorption on mercury this procedure is fairly well established ) to obtain "the primary electric variable". 

The redox reaction (Eq. II.7.18) can be considered as an electrosorption reaction in which the ligand X- is adsorbed to the electrode surface by forming a more or less polarized covalent bond with mercury atoms [37, 38]. When the quantity of adsorbed ligand is less than a monolayer, the adsorbate on the surface behaves as a two-dimensional gas. The adsorption constant of the ligand depends on the electrode potential... 

The adsorption of ions and molecules on the surface of mercury electrodes is a thoroughly investigated phenomenon [51 ]. Surface-active substances are either electroactive [52] or electroinactive [53]. The former can be analyzed by adsorptive stripping voltammetry [54]. This is the common name for several electroanalytical methods based on the adsorptive accumulation of the reactant and the reduction, or oxidation, of the adsorbate by some voltammetric technique, regardless of the mechanisms of the adsorption and the electrode reaction [55, 56]. Frequently, the product of the electrode reaction remains adsorbed to the electrode surface. Hence, the term stripping should not be taken literally in all cases. Besides, some adsorbates may be formed by electrosorption reactions, so that their reduction includes covalently bound mercury atoms. The boundary between adsorption followed by reduction, on the one hand, and electrosorption, on the other, is not strictly defined. Moreover, it is not uncommon that, upon cathodic polarization, the current response is caused by a catalytic evolution of hydrogen, and not by the reduction of the adsorbate itself [57]. However, what is common to all methods is a hnear relationship between the surface concentration of the adsorbate and the concentration of analyte at the electrode surface ... 

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