Mercury electrodes aqueous solution

Mercury in aqueous solutions is undoubtedly the most investigated electrode interface and has been discussed in many reviews.1-1,84,99-109,120,121 There s jittle to add to what is already known. 

Consider the formation of hemispherical nuclei of mercury on a graphite electrode. The intefacial tension of mercury with aqueous solutions is about 426 mN m-1. From Eq. (10.16) calculate the critical cluster sizes for 7 = —10, —100, —200 mV. Take z — 1 and ignore the interaction energy of the base of the hemisphere with the substrate. 

As a result, the plots of o- / vs. E for different bulk concentrations of the solute intersect at the same point (Umax. max). which corresponds to the adsorption maximum. This implies that at the adsorption maximum does not vary with increasing F. Consequently, the surface dipole potential due to the adsorbing molecules exactly matches that of the desorbed solvent molecules, " provided that the solvent and solute molecules assume only one orientation at the surface. In practice, however, a single point of intersection of the vs. E plot for different bulk solute concentrations is observed rather rarely. For example, a gradual inaease in the surface concentration of 2-propanol on mercury in aqueous solution brings about a small positive shift of a. A much bigger shift has been observed for hexafluoro-2-propanol (HFP). " It has been ascribed to the reorientation of the HFP molecules in such a way that both -CF3 groups are directed toward the electrode, which in turn results in... 

The adsorption of a species from solution on the creation of fresh electrode surface proceeds in several steps. The inherent rate of adsorption, at least on mercury from aqueous solution, usually is rapid, so that the overall rate is... 

Here the electroactive species, O, is generated by a reaction that precedes the electron transfer at the electrode. An example of the CE scheme is the reduction of formaldehyde at mercury in aqueous solutions. Formaldehyde exists as a nonreducible hydrated form, H2C(0H)2, in equilibrium with the reducible form, H2C==0 ... 

The case where the rate of adsorption at the electrode is governed by the adsorption process itself has also been treated, with the assumptions of a logarithmic Temkin isotherm and Temkin kinetics (4, 34, 50). The results of this approach have not been widely applied, although measurements of adsorption rates have been attempted (51). Delahay (34) concludes that the inherent rate of adsorption, at least on mercury from aqueous solution, usually is rapid, so that the overall rate is frequently governed by mass transfer. 

Mercury in aqueous solutions acts first as a mercury electrode, but, when cathodically polarized, all mercury ions in solution are deposited before H is discharged. Any conducting surface on which H ions are discharged acts as a polarized hydrogen electrode and can be so considered in evaluating a corrosion cell. 

Mercury layers plated onto the surface of analytical electrodes serve as Hquid metal coatings. These function as analytical sensors (qv) because sodium and other metals can be electroplated into the amalgam, then deplated and measured (see Electro analytical techniques). This is one of the few ways that sodium, potassium, calcium, and other active metals can be electroplated from aqueous solution. In one modification of this technique, a Hquid sample can be purified of trace metals by extended electrolysis in the presence of a mercury coating (35). 

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 polarographic determination of metal ions such as Al3 + which are readily hydrolysed can present problems in aqueous solution, but these can often be overcome by the use of non-aqueous solvents. Typical non-aqueous solvents, with appropriate supporting electrolytes shown in parentheses, include acetic acid (CH3C02Na), acetonitrile (LiC104), dimethylformamide (tetrabutyl-ammonium perchlorate), methanol (KCN or KOH), and pyridine (tetraethyl-ammonium perchlorate), In these media a platinum micro-electrode is employed in place of the dropping mercury electrode. 

Both lead ion and dichromate ion yield a diffusion current at an applied potential to a dropping mercury electrode of —1.0 volt against the saturated calomel electrode (S.C.E.). Amperometric titration gives a V-shaped curve [Fig. 16.14 (C)]. The exercise described refers to the determination of lead in lead nitrate the application to the determination of lead in dilute aqueous solutions (10-3 — 10-4lVf) is self-evident. 

Mercury cyanide, 5, 1062 Mercury electrodes potential range aqueous solution, 1, 480 Mercury fluoride, 5. 1059 Mercury fulminate, 2, 7, 12 5, 1063 Mercury halides, 5, 1049 Mercury iodate, 5,1068 Mercury iodide, 5. 1059 Mercury ions Hgf... 

The properties of anodic layers of HgS formed on mercury in sulfide solutions have been investigated in comparison with anodic sulfide layers of cadmium and bismuth. Also, the electrochemistry of mercury electrodes in aqueous selenite solutions has been studied (see Sect. 3.2.1). The problem with the presence of several cathodic stripping peaks for HgSe in acidic Se(IV) solutions has been addressed using various voltammetric techniques at a hanging-mercury-drop electrode [119]. 

The reduction of formaldehyde at a mercury electrode is an example of a system in which a chemical reaction precedes the electrode reaction. Formaldehyde is present in aqueous solution as the hydrated form (as dihydroxy methane), which cannot be reduced at a mercury electrode. This form is in equilibrium with the carbonyl form... 

An example of dimerization of the intermediates of an electrode reaction is provided by the reduction of acrylonitrile in a sufficiently concentrated aqueous solution of tetraethylammonium p-toluene sulphonate at a mercury or lead electrode. The intermediate in the reaction is probably the dianion... 

Udapa et al.16 showed that C02 was reduced to formic acid at a mercury electrode in a 0.05 M phosphate buffer (pH 6.8) solution. A current efficiency of 81.5% was obtained at a current density of 20 mA/cm2 and a cell voltage of 3.5 V. On the other hand, Bewick and Greener17 reported that malate and glycolate were produced at Hg and Pb electrodes, respectively, using aqueous quartenary... 

Figure 16.2 Interfacial tension 7 of a mercury electrode as a function of the electrode potential for 0.1 M aqueous solutions of several electrolytes at 18°C. The potential is given with respect to the pzc of a solution of KF. Data taken from Ref. 3.

After tq is passed, the second step starts by scanning the potential from Ed to a potential when all the deposited metals are re-oxidized (the reverse of reaction 25). The oxidation current recorded as a function of potential is the anodic stripping voltammogram (ASV). A typical ASY of three metals (Cd, Pb, and Cu) deposited on a mercury film electrode is shown in Fig. 18b.12b. The sensitivity of ASY can be improved by increasing the deposition time and by using the pulse technique to record the oxidation current. ASV in Fig. 18b. 12b was obtained by using the square wave voltammetry. In most cases a simple linear or step ramp is sufficient to measure sub-ppm level of metals in aqueous solution. The peak current of a linear scan ASV performed on a thin mercury film electrode is given by... 

A counter electrode of constant potential is obtained making use of a half-cell system in which the components are present in concentrations so high as to be appreciably unaffected by a flow of current through it. The saturated calomel electrode (SCE) is the most common example of such an electrode. As shown in Figure 7, it is comprised of a mercury pool in contact with solid mercury(I) chloride and potassium chloride that lie at the bottom of the KC1 saturated solution. The aqueous solution is thus saturated with Hgl+, K+ and Cl- ions, the concentrations of which are governed by the solubility of the respective salts. 

The description of the double layer reported in Figures 3 and 22 is only approximate the composition of the electrode/solution region is somewhat more complex. The double layer has been studied in most detail for a mercury electrode immersed in an aqueous solution. According to Gouy-Chapman-Stem there are several layers of solution in contact with the electrode, see Figure 25. 

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. ... 

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. 

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