Vanadium ion

Nitroxyl radicals of diarylamines can also be obtained on oxidation with hydrogen peroxide in the presence of vanadium ions. Resonance helps stabili2e these radicals. Eor example, the nitroxide from 4,4 -dimethoxydiphenylainine [63619-50-1] is stable for years, whereas the radical from the unsubstituted diphenylamine caimot be isolated. Substitution in the ortho and para positions also increases the stabiUties of these nitroxides by inhibiting coupling reactions at these sites. However, they are not as stable as the stericaHy hindered tetramethylpiperidyl radical. 

It is evident from Fig. 22.2 that only in very dilute solutions are monomeric vanadium ions found and any increase in concentrations, particularly if the solution is acidic, leads to polymerization. nmr work indicates that, starting from the alkaline side, the various ionic species are all based on 4-coordinate vanadium(V) in the form of linked VO4 tetrahedra until the decavana-dates appear. These evidently involve a higher coordination number, but whether or not it is the same in solution as in the solids which can be separated is uncertain. However, it is interesting to note that similarities between the vanadate and chromate systems cease with the appearance of the decavanadates which have no counterpart in chromate chemistry. The smaller chromium(VI) is apparently limited to tetrahedral coordination with oxygen, whereas vanadium(V) is not. 

As in the case of ceric and vanadium ions, the reaction of organic compounds with Co(III) proceeds via formation of an intermediate complex. Such a complex decomposes and produces free radicals capable of initiating vinyl polymerization. However, only a few reports on Co(IIl) ion-initiated grafting onto cellulose fibers are available [38]. 

Consider some vanadium ions in aqueous solution. Pale violet solutions of vanadium(ii) salts contain the [V(H20)6] ion. The vanadium(ii) center is only weakly polarizing, and the hexaaqua ion is the dominant solution species. Aqueous vanadium(ii) solutions are observed to be unstable with respect to reduction of water by the metal center. In contrast, vanadium(ni) is more highly polarizing and an equilibrium between the hexaaqua and pentaaquahydroxy ion is set up. The of 2.9 means that the [V(OH2)6] ion (Eq. 9.17) only exists in strongly acidic solution or in stabilizing crystal lattices. 

Rhenium, molybdenum and vanadium ions yield orange, pink and yellow-colored chromatogram zones respectively on a colorless background. 

Reactions between vanadium ions A detailed study of the reaction... 

Upon reduction with methanol at 625 K, the vanadium ions in these samples exhibited an apparent, average degree of reduction of 0.5-0.8 electrons per atom (Table 3). This value... 

It was established in [96] that during light excitation of vanadium ions deposited on SiC>2 electrons and holes ( ) do not undergo a... 

The adsorption spectrum of aerosil containing admixture vanadium ions exhibits a maximum within the band 290 - 380 nm which was attributed by authors of [97] to the charge transfer transitions in oxygen-containing complexes of five valance vanadium = O -... 

In Rb2V308 the Rlr ions have coordination number 10 there are two kinds of vanadium ions, V4+ with c.n. 5 and V5+ with c.n. 4, and four kinds of O2- ions. The mutual coordination of these... 

Vanadium-catalyzed asymmetric epoxidation has recently been re-examined with a newly designed chiral hydroxamic acid (3).43-45 The hydroxamic acid (3) forms a 1 1 complex with vanadium ions and induces high enantioselectivity (Scheme 6). 

UPS studies of supported catalysts are rare. Griinert and coworkers [45] recently explored the feasibility of characterizing polycrystalline oxides by He-II UPS. A nice touch of their work is that they employed the difference in mean free path of photoelectrons in UPS, V 2p XPS and valence band XPS (below 1 nm, around 1.5 nm, and above 2 nm, respectively) to obtain depth profiles of the different states of vanadium ions in reduced V205 particles [45]. However, the vast majority of UPS studies concern single crystals, for probing the band structure and investigating the molecular orbitals of chemisorbed gases. We discuss examples of each of these applications. 

Another idea is to use a mediator such as molybdenum ions - or vanadium ions. Nakajima et clL. have proposed that a qrcling reaction involving lVfo(III)<—>Nfo(IV) mediates the methanol oxidation and have fovmd 100-200 mV gain to be achieved at lower cuirent densities. 

The lower oxidation states are stabilized by soft ligands e.g. CO (Prob. 3). The aquated vanadium ions represent an interesting series of oxidation states. They are all stable with respect to disproportionation and labile towards substitution. They undergo a number of redox reactions with one another, all of which have been studied kinetically. Many of the reactions are [H ]-dependent. There has been recent interest in the biological aspects of vanadium since the discovery that vanadate can mimic phosphate and act as a potent inhibitor (Prob. 4). 

Diffusion coefficients of vanadium ions in CMS (Neosepta made by Tokuyama Soda), CMV (Selemion made by Asahi Glass), and CMX (Neosepta made by Tokuyama Soda) cation exchange membranes were determined by measuring the ion-exchange fluxes of the vanadium ions with H3O+ ions using a dialysis cell. The lowest diffusion coefficients were observed in the CMS membrane for all vanadium ions. CMS membranes were found to be most suitable for V—RFB, as it was expected to prevent cross contamination of vanadium ions. ° ... 

Iodide ion-selective electrode The iodide electrode has broad application both in the direct determination of iodide ions present in various media as well as for the determination of iodide in various compounds. It is, for example, important in the determination of iodide in milk [44,64,218, 382, 442], This electrode responds to Hg ions [150, 306, 439] and can be used for the indirect determination of oxidizing agents that react with iodide, such as 10 [305], lOi [158], Pd(II) [117, 347,405] and for the determination of the overall oxidant content, for example in the atmosphere [393], It can also be used to monitor the iodide concentration formed during the reactions of iodide with hydrogen peroxide or perborate, catalyzed by molybdenum, tungsten or vanadium ions, permitting determination of traces of these metals [12,192,193, 194, 195]. The permeability of bilayer lipid membranes for iodide can be measured using an I"... 

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