Metals, accumulation potentials electrodes

Table II.7.3 Accumulation potentials and stripping peak potentials of some metals at graphite electrodes [2]...

The separation of the tested adsorptive species from electroactive non-adsorptive compounds in biological samples can be realized by medium exchange (e.g., by the flow injection method [171] or by electrode transfer [157, 158]. The scheme of this procedure is illustrated in Fig. 67. The extent of the interference depends on relative adsorbabil-ities of all species, on their bulk concentration, the choice of the accumulation potential and pH values of the solution. The electrocata-lytic activity of adsorbed substances is favorable for lowering the detection limit. For example, metal complexes of the Pt group extremely enhance the reduction of hydrogen ions in acidic and neutral aqueous media. Thus, the detection limit of these metals is decreased down to 10- " M [172, 173]. 

The function I is called the pseudopolarogram [25,26]. It is constructed by plotting the peak current in anodic stripping voltammetry as a function of the accumulation potential, because the peak current is linearly proportional to the concentration of metal atoms in the mercury electrode [27]. The half-wave potential of a pseudopolarogram depends on the mercury film thickness, the electrode rotation rate and the duration of accumulation. It can be used for the estimation of the optimal accumulation potential [28]. Besides, if metal ions form labile complexes with ligands in the solution, the half-wave potential of a pseudopolarogram depends on the ligand concentration, so that the stability constant of the complexes can be determined from this dependence [29,30]. If... 

In contrast to heavy metals mentioned before, there is apparently no practically applicable procedure that employed the accumulation of elemental Fe, Co, Ni, Mn, Cr, etc. at mercury, gold, platinum or (bare) carbon-based electrodes. The reason is poor or even impossible deposition of these metals at the electrode in ASV or PSA (see, e.g., the discussion in reference (111)). Thus, successful determination has to be accomplished via non-electrolytic accumulation with the exception of manganese which can be accumulated fairly well as Mn02 at positive potentials. 

Figure 18 shows the dependence of the activation barrier for film nucleation on the electrode potential. The activation barrier, which at the equilibrium film-formation potential E, depends only on the surface tension and electric field, is seen to decrease with increasing anodic potential, and an overpotential of a few tenths of a volt is required for the activation energy to decrease to the order of kBT. However, for some metals such as iron,30,31 in the passivation process metal dissolution takes place simultaneously with film formation, and kinetic factors such as the rate of metal dissolution and the accumulation of ions in the diffusion layer of the electrolyte on the metal surface have to be taken into account, requiring a more refined treatment. 

When the metal is in contact with an electrolyte solution not containing its ions, its equilibrium potential theoretically will be shifted strongly in the negative direction. However, before long a certain number of ions will accumulate close to the metal surface as a result of spontaneous dissolution of the metal. We may assume, provisionally, that the equilibrium potential of such an electrode corresponds to a concentration of ions of this metal of about 10 M. In the case of electrodes of the second kind, the solution is practically always saturated with metal ions, and their potential corresponds to the given anion concentration [an equation of the type (3.35)]. When required, a metal s equilibrium potential can be altered by addition of complexing agents to the solution (see Eq. (3.37)]. 

The metallic impurities present in an impure metal can be broadly divided into two groups those nobler (less electronegative) and those less noble or baser (more electronegative) as compared to the metal to be purified. Purification with respect to these two classes of impurities occurs due to the chemical and the electrochemical reactions that take place at the anode and at the cathode. At the anode, the impurities which are baser than the metal to be purified would go into solution by chemical displacement and by electrochemical reactions whereas the nobler impurities would remain behind as sludges. At the cathode, the baser impurities would not get electrolytically deposited because of the unfavorable electrode potential and the concentration of these impurities would build up in the electrolyte. If, however, the baser impurities enter the cell via the electrolyte or from the construction materials of the cell, there would be no accumulation or build up because these would readily co-deposit at the cathode and contaminate the metal. It is for this reason that it is extremely important to select the electrolyte and the construction materials of the cell carefully. In actual practice, some of the baser impurities do get transferred to the cathode due to chemical reactions. As an example, let the case of the electrorefining of vanadium in a molten electrolyte composed of sodium chloride-potassium chloride-vanadium dichloride be considered. Aluminum and iron are typically considered as baser and nobler impurities in the metal. When the impure metal is brought into contact with the molten electrolyte, the following reaction occurs... 

Tetra(o-aminophenyl)porphyrin, H-Co-Nl TPP, can for the purpose of electrochemical polymerization be simplistically viewed as four aniline molecules with a common porphyrin substituent, and one expects that their oxidation should form a "poly(aniline)" matrix with embedded porphyrin sites. The pattern of cyclic voltammetric oxidative ECP (1) of this functionalized metal complex is shown in Fig. 2A. The growing current-potential envelope represents accumulation of a polymer film that is electroactive and conducts electrons at the potentials needed to continuously oxidize fresh monomer that diffuses in from the bulk solution. If the film were not fully electroactive at this potential, since the film is a dense membrane barrier that prevents monomer from reaching the electrode, film growth would soon cease and the electrode would become passified. This was the case for the phenolically substituted porphyrin in Fig. 1. 

A method has been developed for differentiating hexavalent from trivalent chromium [33]. The metal is electrodeposited with mercury on pyrolytic graphite-coated tubular furnaces in the temperature range 1000-3000 °C, using a flow-through assembly. Both the hexa- and trivalent forms are deposited as the metal at pH 4.7 and a potential at -1.8 V against the standard calomel electrode, while at pH 4.7, but at -0.3 V, the hexavalent form is selectively reduced to the trivalent form and accumulated by adsorption. This method was applied to the analysis of chromium species in samples of different salinity, in conjunction with atomic absorption spectrophotometry. The limit of detection was 0.05 xg/l chromium and relative standard deviation from replicate measurements of 0.4 xg chromium (VI) was 13%. Matrix interference was largely overcome in this procedure. 

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