Oxidant, pentavalent

In commercial units today, it is preferred to use the alkali metal metavanadates, particularly potassium metavanadate (KVO3). If de.sired, vanadium pentoxide (V2O5) can be added directly to the potassium carbonate solution, forming potassium metavanadate by reaction with potassium carbonate. For the corrosion inhibitor to be effective, the plant must be designed so that exposed carbon steel surfaces are wetted by the inhibited solution. In order to maintain the metavanadate in its fully oxidized (pentavalent) state, the licensors of the Benfield process recommend that potassium nitrite be injected into the solution on a carefully controlled basis (Sorell, 1990). 

Second, the oxidized pentavalent methylphosphonate P(V) linkage is susceptible to hydrolysis by iV-methylimidazole (NMI), which is the most commonly used capping reagent. This is solved by using dimethyl-aminopyridine (DMAP) (see Fig. 1), which does not readily cleave the backbone. 

Pentavalent phosphorus derivatives can be converted to phosphonyl halides or phosphine oxides by partial hydrolysis or by other oxygen donors. 

Rhenium Halides and Halide Complexes. Rhenium reacts with chlorine at ca 600°C to produce rheniumpentachloride [39368-69-9], Re2Cl2Q, a volatile species that is dimeric via bridging hahde groups. Rhenium reacts with elemental bromine in a similar fashion, but the metal is unreactive toward iodine. The compounds ReCl, ReBr [36753-03-4], and Rel [59301-47-2] can be prepared by careful evaporation of a solution of HReO and HX. Substantiation in a modem laboratory would be desirable. Lower oxidation state hahdes (Re X ) are also prepared from the pentavalent or tetravalent compounds by thermal decomposition or chemical reduction. 

Salts containing tetravalent tungsten have been prepared by various methods. The most important are the octacyanides, M(I)4(W(CN)g), which form yellow crystals and are very stable. They are isolated as salts or free acids and can be oxidized by KMnO in H2SO4 to compounds containing pentavalent tungsten, M(I)2(W(CN)g) (yellow). 

Vanadium Sulfates. Sulfate solutions derived from sulfuric acid leaching of vanadium ores are industrially important in the recovery of vanadium from its raw materials. Vanadium in quadrivalent form may be solvent-extracted from leach solutions as the oxycation complex (VO) ". Alternatively, the vanadium can be oxidized to the pentavalent form and solvent-extracted as an oxyanion, eg, (V O ) . Pentavalent vanadium does not form simple sulfate salts. 

For vanadium solvent extraction, Hon powder can be added to reduce pentavalent vanadium to quadrivalent and trivalent Hon to divalent at a redox potential of —150 mV. The pH is adjusted to 2 by addition of NH, and an oxyvanadium cation is extracted in four countercurrent stages of mixer—settlers by a diesel oil solution of EHPA. Vanadium is stripped from the organic solvent with a 15 wt % sulfuric acid solution in four countercurrent stages. Addition of NH, steam, and sodium chlorate to the strip Hquor results in the precipitation of vanadium oxides, which are filtered, dried, fused, and flaked (22). Vanadium can also be extracted from oxidized uranium raffinate by solvent extraction with a tertiary amine, and ammonium metavanadate is produced from the soda-ash strip Hquor. Fused and flaked pentoxide is made from the ammonium metavanadate (23). 

Perfluorinated organic bromides can be oxidatively fluonnated with elemental fluorine to derivatives containing tn- [124] and pentavalent [/25 126 127] bromine in yields up to 42% Perfluoroheptylbromine tetrafluoride has been used to fluonnate double bonds in halogenated alkenes [127]... 

Compounds of tnvalent phosphorus are generally oxidized to pentavalent phosphorus denvaPves bis(fluoroalkyl) and tns(fluoroalkyl) phosphites are oxi dized to the corresponding[106, lOI] (equation 92)... 

The introduction of divalent calcium and barium oxides into frits in preference to monovalent sodium and potassium generally increases water resistance. Furthermore, oxides of tetravalent and pentavalent metals have a favourable effect on the resistance of glasses and enamels to water. The influence of B2O3 and fluorine in the frit upon chemical resistance is variable and is dependent upon the content of them and the balance of the frit constituents, but they usually cause a diminution in resistance. In general, mill-added clay, silica and opaciher increase water resistance provided the firing or fusing of the enamel is at the optimum. 

The Table shows a great spread in Kd-values even at the same location. This is due to the fact that the environmental conditions influence the partition of plutonium species between different valency states and complexes. For the different actinides, it is found that the Kd-values under otherwise identical conditions (e.g. for the uptake of plutonium on geologic materials or in organisms) decrease in the order Pu>Am>U>Np (15). Because neptunium is usually pentavalent, uranium hexavalent and americium trivalent, while plutonium in natural systems is mainly tetravalent, it is clear from the actinide homologue properties that the oxidation state of plutonium will affect the observed Kd-value. The oxidation state of plutonium depends on the redox potential (Eh-value) of the ground water and its content of oxidants or reductants. It is also found that natural ligands like C032- and fulvic acids, which complex plutonium (see next section), also influence the Kd-value. 

Westheimer has also reviewed the induced oxidations by the Cr(VI)-As(III) couple of iodide, bromide and manganous ions vide supra). The induction factor of 0.5 for Mn(II) implies an intermediate tetravalent chromium species however, the factor of 2 for iodide points to a pentavalent chromium intermediate. Both... 

A variety of reagents can function as the electrophile E+ in the general mechanism. The most useful synthetic procedures for preparation of halides are based on the halogens, positive halogens sources, and diethyl azodicarboxylate. A 1 1 adduct formed from triphenylphosphine and bromine converts alcohols to bromides.15 The alcohol displaces bromide ion from the pentavalent adduct, giving an alkoxyphosphonium intermediate. The phosphonium ion intermediate then undergoes nucleophilic attack by bromide ion, forming triphenylphosphine oxide. 

Redox reactions may cause mobile toxic ions to become either immobile or less toxic. Hexavalent chromium is mobile and highly toxic. It can be reduced to be rendered less toxic in the form of trivalent chromium sulfide by the addition of ferrous sulfate. Similarly, pentavalent (V) or trivalent (III) arsenic, arsenate or arsenite are more toxic and soluble forms. Arsenite (III) can be oxidized to As(IV). Arsenate (V) can be transformed to highly insoluble FeAs04 by the addition of ferrous sulfate. 

Oxidation Trivalent arsenic citric acid or EDTA Potassium permanganate depending on chelating agent, metal chelate is either strongly sorbed to soil or is highly mobile and can be flumbed usinj water or dilute acid solutions. Oxidizes trivalent arsenic to pentavalent... 

Physical properties of binary or ternary Ru/Ir based mixed oxides with valve metal additions is still a field which deserves further research. The complexity of this matter has been demonstrated by Triggs [49] on (Ru,Ti)Ox who has shown, using XPS and other techniques (UPS, Mossbauer, Absorption, Conductivity), that Ru in TiOz (Ti rich phase) adopts different valence states depending on the environment. Possible donors or acceptors are compensated by Ru in the respective valence state. Trivalent donors are compensated by Ru5+, pentavalent acceptors will be compensated by Ru3+ or even Ru2+. In pure TiOz ruthenium adopts the tetravalent state. The surface composition of the titanium rich phase (2% Ru) was found to be identical to the nominal composition. 

In an analogous manner, 43 reacts smoothly with LiN(SiMe3)2, in THF at reduced temperature to afford the product of halide-atom metathesis 48, which upon oxidation with sulfur changed into the very stable pentavalent thionobisamide 49 with complete stereoselectivity (Scheme 17) [41—43]. 

Braman et al. [34] used sodium borohydride to reduce arsenic and antimony in their trivalent and pentavalent states to the corresponding hydrides. Total arsenic and antimony are then measured by their spectral emissions, respectively, at 228.8 nm and 242.5 nm. Limits of detection are 0.5 ng for antimony and 1 ng for arsenic, copper, and silver. Oxidants interfere in this procedure. 

If necessary a preconcentration was carried out on this solution to lower the detection limits of the method. Preconcentration was achieved by a method involving co-precipitation of the antimony with hydrous zirconium oxide in which the digest is stirred with 150mg zirconyl chloride and the pH adjusted to 5 with ammonia to coprecipitate antimony and hydrous zirconium oxide. The isolated precipitate is dissolved is 7M hydrochloric acid and 30% sulphuric acid. Antimony is then converted to the pentavalent state by successive treatment with titanium III chloride and sodium nitrite and excess nitrite destroyed by urea. 

In situ emission Mossbauer spectroscopy provides valuable information on the chemical structure of dilute metal ions at the metal oxide/aqueous solution interface The principles of the method are described with some experimental results on divalent Co-57 and pentavalent Sb-119 adsorbed on hematite. 

We now extend the work to in situ measurements on metal ions adsorbed at the metal oxide/aqueous solution interface. In this report, our previous results are combined with new measurements to yield specific information on the chemical structure of adsorbed species at the solid/aqueous solution interface. Here, we describe the principles of emission Mossbauer spectroscopy, experimental techniques, and some results on divalent Co-57 and pentavalent Sb-119 ions adsorbed at the interface between hematite (a-Fe203) and aqueous solutions. 

Adsorption of Pentavalent Sb Ions on Hematite. So far as we know, there are no experimental data on the adsorption equilibrium of dilute pentavalent Sb ions on metal oxides. Therefore, the pH dependence of the adsorption of pentavalent Sb ions on hematite was measured. Carrier-free pentavalent Sb-119 ions were adsorbed on 30 mg of hematite (prefired at 900°C for 2 hours) from 10 cm3 of 0.25 mol/dm3 LiCl solutions at 24 1°C. The amount of antimony employed in each run is estimated to be about 50 ng. The adsorption proceeds with a measurable rate and attains an apparent equilibrium after shaking for several hours. The reaction is second order with respect to the concentration of pentavalent Sb ions in the solution (13) The values given in Figure 4 are those obtained after 22 hours equilibration. As seen in Figure 4, strong adsorption of pentavalent Sb ions is observed below pH 7, while the percent adsorbed diminishes abruptly above that. Most of the Sb ions adsorbed on hematite from solutions of pH 2-5 are not desorbed by subsequent adjustment to alkaline conditions. Results on desorption of Sb ions pre-adsorbed at pH 4 are shown in Figure 4. 

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