Mercury determination with surface

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. 

For liquid electrodes thermodynamics offers a precise way to determine the surface charge and the surface excesses of a species. This is one of the reasons why much of the early work in electrochemistry was performed on liquid electrodes, particularly on mercury - another reason is that it is easier to generate clean liquid surfaces than clean solid surfaces. With some caveats and modifications, thermodynamic relations can also be applied to solid surfaces. We will first consider the interface between a liquid electrode and an electrolyte solution, and turn to solid electrodes later. 

A novel method for the determination of surface charge density at a HMDE coated with a self-assembled phospholipid mono-layer mimicking a biological membrane has been described by Becucci etal. [7] Charge density was calculated by integrating the capacitance current, which flows at the constant potential as a consequence of slight contraction of mercury drop. 

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. 

It remains to be determined to what extent the dye adsorption technique is applicable to other substrates. No evidence was obtained for Pseudocyanine adsorption to Mn02, Fe2Os or to pure silver surfaces, although this dye can be bound to mica, lead halides, and mercury salts with formation of a /-band (61). Not only cyanines but other dye classes can yield surface spectra which may be similarly analyzed. This is specifically the case with the phthalein and azine dyes which were recommended by Fajans and by Kolthoff as adsorption indicators in potentio-metric titrations (15, 30). The techniques described are also convenient for determining rates and heats of adsorption and surface concentrations of dyes they have already found application in studies of luminescence (18) and electrophoresis (68) of silver halides as a function of dye coverage. 

In the last two decades, mercury film electrodes (MFEs) have been used frequently in electroanalytical practice. Using such electrodes, metal ions present in the solution in trace amounts may be determined with satisfactory accuracy by their reduction on the surface of MFEs, formation of relatively concentrated amalgam, and anodic oxidation from MFEs of the preconcentrated metal in a final step (see Chap. 24). 

Porous materials are often analyzed with a mercury porosimeter. With a mercury porosi-meter we can measure the pore distribution of a solid. Thus, we can determine the specific surface area. Mercury is used because of its high surface tension (0.48 N/m) it does not wet... 

Adsorptive accumulation — Organic substances which exhibit -> surface activity and electroactivity can be electrochemically analyzed by adsorptive accumulation on the surface of a an electrode, e.g., mercury electrode, followed by the reduction, or oxidation of the adsorbate using -> voltammetry [i,ii]. Also, the adsorption of highly stable and inert -> complexes of metal ions with surface-active organic ligands is utilized for the determination of trace metals [iii]. In all these methods the maximum voltammetric response is linearly proportional to the surface concentration of the adsorbed analyte at the end of the accumulation period [iv]. In the majority of cases, the adsorption on mercury can be described by the -> Frumkin isotherm /icx=o = 0exp(ad)/(1- 9), where f is the adsorption constant, cx=o is the concentration of the dissolved compound at the electrode surface, 6 = T/rmax is the surface coverage, T is the surface concentration of the adsorbed compound, rmax is the maximum surface concentration and a is the Frumkin... 

From mercury penetration the surface area is determined knowing the surface tension and contact angle of mercury and the total volume of penetrated mercury. Measurements agree with the BET measurements below surface areas of 100 m /g. 

Open Ocean Mercury Determinations. In our initial studies concerned with the marine geochemistry of mercury, we obtained open ocean smrface samples by hand from a small work boat away from any adverse influence of the oceanographic research vessel. The concentrations of mercury in the open-ocean surface waters (western Sargasso Sea) were small (ca. 10 ng/1.) and rather imiformly distributed (26). However, to collect seawater to determine the concentrations of mercury at other depths, we needed an artifact-free sampling procedure. 

Sometimes mercury porosimetry and adsorption ofp-nitrophenol from an aqueous solution [245, 246] are also used to determine the surface area. The disadvantages of mercury intrusion technique will be discussed below. Regarding the method of adsorption firom solution, its drawback is associated, first of all, with the uncertainty in the determination of the cross-sectional area of the p-nitrophenol molecules, in which the benzene rings may adsorb either transversely or parallel to the surface. Also some sorbents swell in water, and this may lead to a misrepresentation of the surface area of the adsorbing resin in dry state. 

Adsorptive stripping voltammetry at mercury or solid electrodes provides very sensitive determinations with detection limits in the 10 to 10 M range for surface active compounds or their (metal)conjugates that cannot normally be accumulated electrochemically. ... 

CT = (7q - CV y where gq is the maximum value for the uncharged surface. Hence the c, V curve is a parabola. This was confirmed by Konig. Lippmann found that a current flows in the circuit if the size of the mercury surface in the capillary electrometer is altered by mechanical means this should cease when the mercury is uncharged and Pellat found that this happens when an electromotive force of 0 97 volt acts against the natural potential difference, agreeing with Lippmann s value of about i volt for the maximum of surface tension. Ostwald found that in different acids the surface tensions could differ by a ratio of more than i to 3. He emphasised that it is the charge on the mercury, and not (as Lippmann thought) the potential difference, which determines the surface tension. 

The adsorptiop of four apliphatic a-amino acids, DL-norleucine (NL), DL-norvaline (NV), DL-a-amino-n-butyric acid (ABA) and DL-a-alanine (AL), at the mercury-neutral aqueous solution interface has been studied by means of double-layer capacitance, potential of zero charge, and surface tension measurements [96]. The adsorption of NL, NV, and ABA obeys the Frumkin isotherm. The interaction parameter and the potential dependence of the adsorption energy were determined [96]. The shifts of p.z.c. with surface coverage are negative for AL and positive for ABA, NV, and NL, which can be explained by assuming the existence of a small dipole component perpendicular to the mercury surface. 

The porous structure of the sample was studied with mercury porosimetry (PoreSizer, Micromeritics) the specific surface area was determined with BET method using nitrogen. 

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