Vanadium complexes phosphates

Spectrophotometry also provides rapid, cost-effective measurement of total uranium concentrations, at levels down to tens of pgl . In this method, a chelating agent is added to the digested solution to form colored uranium complexes. The most common chelator is 2-(5-bromo-2-pyridylazo)-5-diethylamino-phenol (bromo-PADAP), which may require removal of chromium, vanadium, or phosphate interferences by solvent extraction, prior to analysis. 

Two years after implantation of femoral components made of Ti-6A1-4V, the titanium and aluminium concentrations measured in the synovial fluid were higher for cemented components than for the uncemented (200 p.m HA, or porous Ti coatings) components (Karrholm et ai, 1994). Table 9.10 gives the data for the synovial fluid and the aluminium concentrations in serum and urine. No significant concentrations of vanadium were found in any of the samples, which was also the case for titanium in serum and urine. Fast clearance of vanadium from the synovial fluid, due to high solubility of vanadium complexes, and formation of stable titanium compounds, e.g. titanium phosphates (Ribeiro et al, 1995), might be reasonable explanations for these findings. 

The first term is the law observed in the absence of catalyst. The effect is dramatic, however, with associated rate constants ko and ki being 50.3 1 mol s and 3.43 x 10 1 mol s respectively, and may be ascribed to the formation of a Mo -BrOa" complex with no evidence for any change in the oxidation state of the catalyst during the course of the reaction. The kinetics and mechanism of the bromate-ascorbic acid reaction have also been reported, and the effects of phosphate on the oxidation of iodide by chlorate in the presence of catalytic concentrations of vanadium(iv) have been described, the latter systems being considered to involve vanadium(iv)-phosphate complexes. 

Vanadium is found in about 65 different minerals among which are carnotite, roscoelite, vanadinite, and patronite, important sources of the metal. Vanadium is also found in phosphate rock and certain iron ores, and is present in some crude oils in the form of organic complexes. It is also found in small percentages in meteorites. 

Determination of uranium with cupferron Discussion. Cupferron does not react with uranium(VI), but uranium(IV) is quantitatively precipitated. These facts are utilised in the separation of iron, vanadium, titanium, and zirconium from uranium(VI). After precipitation of these elements in acid solution with cupferron, the uranium in the filtrate is reduced to uranium(IV) by means of a Jones reductor and then precipitated with cupferron (thus separating it from aluminium, chromium, manganese, zinc, and phosphate). Ignition of the uranium(IV) cupferron complex affords U308. 

Fig. 17. Structure of complex (VO)2P2C>7 in (010), viewed down the fr-axis. Vanadium octahedra and phosphate tetrahedra link together forming a three-dimensional network. Front (bold) and back (faint) layers are shown.

It is not clear whether V(V) or V(IV) (or both) is the active insulin-mimetic redox state of vanadium. In the body, endogenous reducing agents such as glutathione and ascorbic acid may inhibit the oxidation of V(IV). The mechanism of action of insulin mimetics is unclear. Insulin receptors are membrane-spanning tyrosine-specific protein kinases activated by insulin on the extracellular side to catalyze intracellular protein tyrosine phosphorylation. Vanadates can act as phosphate analogs, and there is evidence for potent inhibition of phosphotyrosine phosphatases (526). Peroxovanadate complexes, for example, can induce autophosphorylation at tyrosine residues and inhibit the insulin-receptor-associated phosphotyrosine phosphatase, and these in turn activate insulin-receptor kinase. 

The set of reactions defines the materials largely contained within this set of scientific studies. It deals mostly with vanadium oxides and vanadium phosphates followed by complex MMO phases and HPA. Figure 1.1 shows some relevant trends from the ISI database statistics. 

Leon-Lai, C.H., M.J. Gresser, and A.S. Tracey. 1996. Influence of vanadium(V) complexes on the catalytic activity of ribonuclease A. The role of vanadate complexes as transition state analogues to reactions at phosphate. Can. J. Chem. 74 38 -8. 

Her research interests originally focused on biological cell membranes, first working on phosphate transport in Escherichia coli and then the plasma membrane proton ATPase in Saccharomyces cerevisiae. While isolating vanadate-resistant mutants in yeast, she became fascinated with work showing that oral administration of vanadium salts alleviated symptoms of diabetes and switched her research focus to that area. She has pursued the insulin-enhancing mechanism of vanadium salts and complexes in cell culture, the STZ-induced diabetic rat, and human type 2 diabetic patients. The National Institutes of Health, the American Heart Association, and the American Diabetes Association have funded the work in her laboratory. Willsky has lectured all around the world and published both research articles and book chapters in this area. 

Vanadate(V) has been shown to block the activation of a progesterone receptor complex from avian oviduct104. Whether this effect involves phosphate metabolism is not known. A possible link between hormonal effects and the metabolism of vanadium in the reproductive physiology of rats has been investigated165"167. ... 

Vanadium phosphates have been established as selective hydrocarbon oxidation catalysts for more than 40 years. Their primary use commercially has been in the production of maleic anhydride (MA) from n-butane. During this period, improvements in the yield of MA have been sought. Strategies to achieve these improvements have included the addition of secondary metal ions to the catalyst, optimization of the catalyst precursor formation, and intensification of the selective oxidation process through improved reactor technology. The mechanism of the reaction continues to be an active subject of research, and the role of the bulk catalyst structure and an amorphous surface layer are considered here with respect to the various V-P-O phases present. The active site of the catalyst is considered to consist of V and V couples, and their respective incidence and roles are examined in detail here. The complex and extensive nature of the oxidation, which for butane oxidation to MA is a 14-electron transfer process, is of broad importance, particularly in view of the applications of vanadium phosphate catalysts to other processes. A perspective on the future use of vanadium phosphate catalysts is included in this review. 

Ballarini et al. (8) posed the question of whether vanadium phosphate catalysts for n-butane oxidation offer the scope for further improvements. They concluded that as a consequence of the complexity of the dynamic surface species present on the catalyst, optimization of such material will not be forthcoming without further fundamental investigations. Previous investigations have involved probing of a number of catalyst parameters, including the V P ratio, the content of metal ion dopants, and the method of preparation. These and related topics are evaluated in detail below. 

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