Vanadium oxidizing/reducing properties

The coordination chemistry of vanadium is strongly influenced by the oxidizing/reducing properties of the metallic centre, and the chemistry of vanadium ions in aqueous solution is limited to oxidation states +2, +3, +4 and +5, although V2+ can reduce water. Redox potentials are given in Table 1 and an E vs. pH diagram is shown in Figure 1. 

On metals that produce substantial yields of oxygenates, such as rhodium, promotion by reducible oxides (e.g., molybdenum or vanadium oxides) appears beneficial 76—78). The key property of the cation of the promoting oxide is a weak affinity for CO combined with a strong affinity for oxygen. An aftractive hypothesis to explain the role of the promoting cations is that the reducible cation provides a site for the O atom generated upon CO dissociation, whereas CO interacts only weakly with the promoting cations. 

It has recently been obtained industrially as the result of the oxidation of anthracene by air in the presence of vanadium oxide at about 450°. It is formed when calcium benzoate or calcium phthalate is distilled. Anthraquinone crystallizes from hot benzene in yellow prisms or needles, which melt at 27°, boil at 380°, and can be sublimed. It does not show the properties characteristic of quinones and is, therefore, not well named. It is not easily reduced, is not readily volatile, and does not possess a pungent odor. 

Similar to the PB-conducting polymer composites, composite films can be prepared with conducting polymers and transition metal oxides. Composite films have been prepared by the electrostatic layer-by-layer technique with polyaniline and vanadium oxide with the electrochemical properties dominated by the vanadium oxide layers and the optical properties dominated by the polyaniline layers [245,246]. Composites have also been made between the water-soluble poly(2-(3-thienyloxy)ethanesulfonic acid) and vanadium oxide by mixing the two materials at different mole ratios in aqueous solutions. These films switched between an orange color when fully reduced, to yellow-green at intermediate potentials, and to dark blue when fully oxidized [247]. 

Carbon content is usually about 0.15% but may be higher in bolting steels and hot-work die steels. Molybdenum content is usually between 0.5 and 1.5% it increases creep—mpture strength and prevents temper embrittlement at the higher chromium contents. In the modified steels, siUcon is added to improve oxidation resistance, titanium and vanadium to stabilize the carbides to higher temperatures, and nickel to reduce notch sensitivity. Most of the chromium—molybdenum steels are used in the aimealed or in the normalized and tempered condition some of the modified grades have better properties in the quench and tempered condition. 

The solids analysis described above can be taken to yet another level by correlating the color measurement to chemical properties. An excellent model system is vanadium pyrophosphate (VPO), which is a well-known catalyst for butane oxidation to maleic anhydride. During the synthesis of the catalyst precursor, solid V2O5 particles are dispersed in a mixture of benzyl alcohol and i-butanol. In this slurry phase, the vanadium is partly reduced. Addition of phosphoric acid leads to a further reduction and the formation of the VPO structure. With a diffuse reflectance (DR) UV-vis probe by Fiberguide Ind., the surface of the suspended solid particles could be monitored during this slurry reaction. Four points can be noted from Figure 4.4 ... 

The role of these interesting plasma membrane-dependent, vanadate-stimulated NAD(P)H oxidation reactions in cellular metabolism remains to be elucidated, although multiple interactions with cellular metabolism and components are possible including interactions with xanthine oxidase and lipid peroxidation [24], Decavanadate has been shown to enhance cytochrome c reduction [31], and cytochrome c release from mitochondria is associated with initiation of apoptosis. Perhaps the reduced cytochrome c is more readily released from the mitochondria. With increasing emphasis on the redox properties of vanadium being important in its pharmacological effects, it is quite possible that these reactions, either protein dependent or not, may play a role in therapeutic actions of vanadium. 

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