Hg electrode

Adsorptive stripping analysis involves pre-concentration of the analyte, or a derivative of it, by adsorption onto the working electrode, followed by voltanmietric iiieasurement of the surface species. Many species with surface-active properties are measurable at Hg electrodes down to nanoniolar levels and below, with detection limits comparable to those for trace metal detemiination with ASV. 

When the potential of an electrode of the first kind responds to the potential of another ion that is in equilibrium with M"+, it is called an electrode of the second kind. Two common electrodes of the second kind are the calomel and silver/silver chloride reference electrodes. Electrodes of the second kind also can be based on complexation reactions. Eor example, an electrode for EDTA is constructed by coupling a Hg +/Hg electrode of the first kind to EDTA by taking advantage of its formation of a stable complex with Hg +. 

In hydrodynamic voltammetry current is measured as a function of the potential applied to a solid working electrode. The same potential profiles used for polarography, such as a linear scan or a differential pulse, are used in hydrodynamic voltammetry. The resulting voltammograms are identical to those for polarography, except for the lack of current oscillations resulting from the growth of the mercury drops. Because hydrodynamic voltammetry is not limited to Hg electrodes, it is useful for the analysis of analytes that are reduced or oxidized at more positive potentials. 

More recently, the curvature at air/solution interfaces has been accounted for by Nikitas and Pappa-Louisi98 in terms of a specific molecular model that predicts a linear dependence of (lM/ ) on (1/0). The same model also reproduces the behavior at metal/solution interfaces, specifically Hg electrodes, for which most of the experimental data exist. Nikitas treatment provides a method for an unambiguous extrapolation of the adsorption potential shift to 0= 1. However, the interpretation of the results is subject to the difficulties outlined above. Nikitas approach does provide... 

Theoretical C,E and C, a curves have been compared with experimental capacitance curves.359 In a wide range of potentials, the coincidence was good and it was concluded that the GCSG model is acceptable for Ga, In(Ga), Tl(Ga), and Hg electrodes in NMF + NaC104 solution. 

Metals can be inserted into metal-metal bonds during electrochemical oxidation reactions. The compounds Hg[M(CO),]2 (M = Mn or Re), Hg[Mo(CO)3Cp-T7 ]2 and Hg(Fe(CO)2Cp-i7 ]2 are formed during electrolysis of the carbonyl dimers in dimethoxyethanc with Hg electrodes . The electrolytic syntheses of Zn and Cd derivatives ... 

Electrochemical oxidation of the group-IIB metal can generate metal-metal bonds analogously to the manner described in 8.3.3.4. For example, cyclic voltammetry with Hg electrodes shows that M(CO)2dpam (M = Cr, Mo) reacts to form Hg—M bonds in two stages ... 

Similarly, electrochemical oxidation of RufCp-rj lj at Hg electrodes yields [(17 Cp)2RuHgRu(Cp-i7 )2] derivatives that can be isolated as salts . ... 

To overcome some of the problems associated with aqueous media, non-aqueous systems with cadmium salt and elemental sulfur dissolved in solvents such as DMSO, DMF, and ethylene glycol have been used, following the method of Baranski and Fawcett [48-50], The study of CdS electrodeposition on Hg and Pt electrodes in DMSO solutions using cyclic voltammetry (at stationary electrodes) and pulse polarography (at dropping Hg electrodes) provided evidence that during deposition sulfur is chemisorbed at these electrodes and that formation of at least a monolayer of metal sulfide is probable. Formation of the initial layer of CdS involved reaction of Cd(II) ions with the chemisorbed sulfur or with a pre-existing layer of metal sulfide. 

This equation applies to all reversible electrode reactions with soluble ox and red, so it includes cathodic chronopotentiometry of ions of amalgamating metals such as Cd2+, Cu2+, Pb2+ and Zn2+ at stationary Hg electrodes. For a redox couple such as Fe3+/Fe2+ the diffusion coefficients DTeA and Dox will not differ much, so that Etl4 is approximately equal to E° (770 V). 

A complicated situation arises when ox or red is insoluble, for instance in the reductive chronopotentiometry of an ion of a non-amalgamating metal then the metal precipitates on the stationary Hg electrode surface without further diffusion, so that... 

One can imagine that to go from the VDME to a streaming mercury electrode would be another obvious step many experiments in this connection have been carried out, e.g., the use of a jet-type Hg electrode as depicted in Fig. 3.70. However, the results obtained so far are not sufficiently reliable, so that the streaming Hg electrode has not found general acceptance. 

This technique is applied to mixtures of metal ions in an acidic solution for the purpose of electroseparation only the metal ions with a standard reduction potential above that of hydrogen are reduced to the free metal with deposition on the cathode, and the end of the reduction appears from the continued evolution of hydrogen as long as the solution remains acidic. Considering the choice of the cathode material and the nature of its surface, it must be realized that the method is disturbed if a hydrogen overpotential occurs in that event no hydrogen is evolved and as a consequence metal ions with a standard reduction potential below that of hydrogen will still be reduced a classic example is the electrodeposition of Zn at an Hg electrode in an acidic solution. 

The electrochemical investigation on an Hg electrode of the interaction between [Ir(Me2-phen)(cod)] and 02 gives evidence of formation of an unstable adduct.619... 

Instead of mercury, Ito et a/.18 examined systematically some sp metals, such as Zn Pb, Sn, In, and Cd (5N purity), as the cathode materials for C02 reduction (Fig. 1). Using an In electrode at 3.9 mA/cm2, the highest current efficiency (92%) for formic acid production was obtained in an aqueous Li2C03 solution the potentials at which C02 was reduced at an In electrode were ca. 400 mV less negative than those at an Hg electrode. 

Tetraazamacrocyclic complexes131 of cobalt and nickel were found110 to be effective in facilitating the reduction of C02 at -1.3 to -1.6 V versus SCE (Table 8). An acetonitrile-water mixture and water were used as solvents, while in dry dimethylsulfoxide no catalytic reduction of C02 took place. Using an Hg electrode, both CO and H2 were produced, where total current efficiencies were greater than 90%. The turnover numbers of the catalysts were 2-9 h 1. The catalytic activity lasted for more than 24 h and the turnover numbers of the catalysts exceeded 100. A protic source was required to produce both CO and H2, and the authors suggested that both products may arise from a common intermediate, which is most likely a metal hydride. The applied potential for C02 reduction was further reduced by using illuminated p- Si in the presence of the above catalysts.111... 

It is not generally acknowledged that, at about the same time, Kolthoff and Miller [25] also described reactions occurring at the dropping Hg electrode that exhibited current-potential curves in reaction mixtures that yielded mixed potentials, a term they also employed. 

By using Hg-electrode-based junctions, the electrical properties of the highly conjugated units of hexa-pen-hexabenzocoronene (HBC) have been characterized [171]. The HBC unit was anchored to the Au metal surface in an Hg-based junction by a decanonanethiol tether (C19), as depicted in Fig. 9. Comparison of the electrical behaviour of three different junctions, whose interfaces are schematized... 

Fig. 9 Scheme of different Hg-drop LAJs incorporating (a) SAMs of octadecanethiols (Cl8) on both electrodes (b) SAMs of C18 on the Hg electrode and of HBCS on an Au electrode (c) SAMs of HBCS on both electrodes (d) i—V curves measured for the three a, b, c junctions... 

Figure 5a schematizes LAJs based on two Hg electrodes. By bringing in contact the two Hg-drops inside an electrolyte solution and connecting them to a macroscopic reference electrode by a potentiostat (Figs. 15a and 16a), an electrochemical junction is created this junction allows for independent control of the potentials applied to the two Hg electrodes, so that the cathode can act as electron donor (source) and the anode as electron acceptor (drain) with respect to the redox centre. 

The reactions above provide the interpretation of very detailed polarographic measurements. The oxidation products of aryllead derivatives on Hg electrodes were identified in other studies on Ph4Pb, Ph3PbCl, Ph2PbCl2, Ph3PbOAc and Ph(OAc)3. These oxidations... 

A complicated reaction pattern is also observed with dichlorotetraphenylditin87. The electrochemistry of this compound compound on Hg electrodes involves formation of intermediate SnHg compounds by reduction (see also Reference88). The polarogram of Ph2ClSn—SnClPh2 (in methanol/LiCl, on Hg) shows an anodic peak and two cathodic waves at —0.4, —0.55 and —1.35 (vs SCE). The oxidation involves between one and two electrons as determined by coulometry, and the proposed reactions are ... 

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