Cobalt, electrode potentials

It is interesting to note, as pointed out to me by Mr. J. L. Hoard, that these considerations lead to an explanation of the stability of trivalent cobalt in electron-pair bond complexes as compared to ionic compounds. The formation of complexes does not change the equilibrium between bivalent and trivalent iron very much, as is seen from the electrode potentials, while a great change is produced in the equilibrium between bivalent and trivalent cobalt. 

Vinyl substituted bipyridine complexes of ruthenium 9 and osmium 10 can be electropolymerized directly onto electrode surfaces The polymerization is initiated and controlled by stepping or cycling the electrode potential between positive and negative values and it is more successful when the number of vinyl groups in the complexes is increased, as in 77 A series of new vinyl substituted terpyridinyl ligands have recently been synthesized whose iron, cobalt and ruthenium complexes 72 are also susceptible to electropolymerization... 

SAQ 7.10 Consider the cobaltous ion cobalt redox couple. Write an expression for its electrode potential. 

Other commonly employed redox electrodes are metals such as copper, cobalt, silver, zinc, nickel, and other transition metals. Some p-block metals such as tin, lead and indium can also function as redox electrodes. However, s-block metals such as magnesium do not make good redox electrodes since the elemental metal is reactive and forms a layer of oxide coating, which leads to poor reproducibility, poor electronic conductivity and electrode potentials that are difficult to interpret, (see Section 3.3.1). 

Symbol Ni atomic number 28 atomic weight 58.693 a transition metal element in the first triad of Group VIll(Group 10) after iron and cobalt electron configuration [Ar]3d 4s2 valence states 0, -i-l, +2, and -f-3 most common oxidation state +2 the standard electrode potential, NF+ -1- 2e Ni -0.237 V atomic radius 1.24A ionic radius (NF+) 0.70A five natural isotopes Ni-58 (68.08%), Ni-60 (26.22%), Ni-61 (1.14%), Ni-62 (3.63%), Ni-64 (0.93%) nineteen radioactive isotopes are known in the mass range 51-57, 59, 63, 65-74 the longest-lived radioisotope Ni-59 has a half-life 7.6x10 years. 

Substrate Electrode electrode potential ( vs NHE) Nickel 0.6 Silver 0.85 Copper 0.7 Cobalt 0.6... 

Metal ion and halide impurities are an issue in ionic liquids with discrete anions. As we have demonstrated in Chapter 11.5 Li+ (and K+) are common cationic impurities, especially in the bis(trifluoromethylsulfonyl)amides which typically contain 100 ppm of these ions from the metathesis reaction. Although Li and K are only electrodeposited in the bulk phase at electrode potentials close to the decomposition potential of the pyrrolidinium ions, there is evidence for the underpotential deposition of Li and K on gold and on other rather noble metals. For a technical process to deposit nickel or cobalt from ionic liquids the codeposition of Li and/or K, even in the underpotential deposition regime, has to be expected. 

LAR/CER] Larson, J. W., Cerutti, P., Garber, H. K., Hepler, L. G., Electrode potentials and thermodynamic data for aqueous ions. Copper, zinc, cadmium, iron, cobalt, and nickel, J. Phys. Chem., 72, (1968), 2902-2907. Cited on pages 84, 332. 

Table 2 summarizes the electrode potentials of the electron transfers cited. At first glance it is not easy to discern an unequivocal trend in redox propensity after changing the metal atom, in that the substitution of one iron atom for one cobalt atom in [Fe3(CO)9(/<3-S)] results in the electron-transfer capacity changing from oxidation to reduction. Nevertheless, if, as indicated, one takes into consid-... 

TABLE 12.19 Electrode Potentials of Cobalt and Copper Species in Aqueous Solution... 

Table 5.2 Electrode potentials for some cobalt and iron complexes...

Figure 14.31 The potential-dependent SERS spectra of adsorbed pyridine from a cobalt electrode in 10 M pyridine/0.1 M NaC104. The electrode was roughened electrochemicaUy. The laser excitation line was 632.8 nm. Reproduced from reference (86) with permission from the Royal Society of Chemistry.

A voltaic cell is made from a Co (aq)/Co(s) halfcell and a Cu (aq)/Cu(s) half-cell. The cell potential was +0.62 V, with the copper half-cell positive. Calculate the standard electrode potential of the cobalt half-cell. 

Equation (7) has been studied in several articles and a summary is available in the literature [50,70], The mercury-cystine system is, however, more complex than what is implied in Eq. (7) and can, depending on the electrode potential, include adsorbed species such as (RS)2Hg and (RS)2Hg2 [70], The system has been studied and explored in detail [70], Cystine can also be reduced at carbon electrodes modified with conducting polymers containing fixed metal thiolate sites [50,71], different macrocyclic transition metal complexes (often containing cobalt and phtalocyanines or porphyrins) [50,55,57,72,73], or vitamin Bj2 [56], which lower the overpotential necessary for reduction. 

The stainless steels (types 303, 316, and 316LVM) as well as the cobalt-nickel-chromium-molybdenum alloy MP35N are protected from corrosion by a thin passivation layer that develops when exposed to atmospheric oxygen and which forms a barrier to further reaction. In the case of stainless steel, this layer consists of iron oxides, iron hydroxides, and chromium oxides. These metals inject charge by reversible oxidation and reduction of the passivation layers. A possible problem with these metals is that if the electrode potential becomes too positive... 

One of the difficulties encountered in studying anodic films is that in many cases the films are not completely removed by cathodic reduction. Once the iron, nickel, or cobalt electrode surface is subject to anodic potential to form passive layers, the ellipsometric readings on the surface usually do not return completely to the original values of the film-free surface on cathodic treatment, which indicates that a residual film remains. This makes it difficult to repeat the experiment on the same specimen of metal because there is no way of having a reference state assured. Szklarska-Smialowska et al reported that if EDTA is added to the electrolyte solution, complete removal of anodic film on iron is achieved on cathodic treatment. This was confirmed to work also... 

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