n-arylhydroxamic acids as reagents for vanadium(v) : Spectrophotometric determination of vanadium(v) with n-m-tolyl-p-methoxybenzohydroxamic acid

A study of the coloured complexes of 51 N-arylhydroxamic acids with vanadium(V) in hydrochloric acid media is described. The absorption spectra of the coloured chloroform extracts and the molar absorptivities are compared. The effects of different substituents attached to the carbon and nitrogen atom of the hydroxamic acid functional group are discussed. A rapid extraction-spectrophotometric method for the determination of vanadium(V) is described, employing the most promising of these reagents, N-m-tolyl-p-methoxybenzohydroxamic acid. The method is highly selective and tolerates large amounts of diverse ions usually associated with vanadium-bearing materials including iron(III), aluminium(III), chromium(III), nickel(II), cobalt(II), zinc(II) and manganese(II).     

Coulometric titration with quinquevalent uranium determination of vanadium(v)

Variadium(V) has been determined by reduction with generated uranium(V) with an accuracy of about ± 0.3%. Amounts as small as 15 μg were successfully titrated. The suggested ph range is 1–2.5. Iron(III) is reduced simultaneuosly with vanadium(V).     

The reaction of vanadium pentafluoride with phosphorus oxytrifluoride and with trifluoroacetic acid

Vanadium pentafluoride reacts via an oxygen exchange reaction with phosphorous oxytrifluoride to give VOF₃ and PF₅. The latter forms a weak 1:1 adduct. VF₅ reacts with trifluoroacetic acid to form trifluoroacetyl fluoride and VOF₃. Both reactions are indicative of the strength of the VO bond and its influence on the course of the reactions of VF₅ with oxygen containing materials.     

Reaction of vanadium(v) complexes with some adduct-forming substances in benzene

Vanadium(V) reacts with monobasic and bidentate extracting agents, i.e. 8-quinolinol, 2-methyl-8-quinolinol, salicylaldoxime, hinokitiol, benzoylphenylhydroxylamine, maltol, cupferron, dibenzoylmethane, salicylaldehyde, and phenylmethylbenzoylpyrazolone-5, to form 1:2 complexes containing a basic V=O group and an acidic V—OH group in the same molecule. The basic group reacts with acidic substances to give a hyper- and bathochromic effect; the acidic group reacts with basic substances to give a hyper- and hypsochromic effect. These reactions are observed in benzene solutions.     

Reactions of 3-functionalized chromones with triacetic acid lactone

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Spectrophotometric determination of vanadium(IV) with nitrilotriacetic acid

A new and convenient spectrophotometric method for the estimation of vanadium(IV) with NTA is described. The minimum ratio of metal ion to ligand, working pH, wavelength for maximum absorbance of the complex ion, and the effect of various cations and anions are described. The complex ion obeys Beer's law in the concentration range 1–32 mmol/liter of the vanadium(IV) ion. It is observed that iron(II), cobalt(II), nickel(II), copper(II), and oxidizing anions such as chromate and nitrite interfere in this determination, whereas managanese(II), chromium(III), iron(III), and anions like nitrate, chloride, bromide, iodide, thiocyanate, sulfate, and sulfite do not have any effect. Excessive amounts of acetate, phosphate, oxalate, tartrate and thiosulfate must also be avoided in this determination. Anions and cations which interfere in the determination of vanadium(IV) by NTA should not be present in the system.     

Spectrophotometric determination of vanadium (V) with 2-naphthohydroxamic acid

2-Naphthohydroxamic acid in methanol gives an intense and stable red-orange color with vanadium (V), sensitive to 0.009 μg V/cm², for log I₀/I = 0.001 abs. unit at wavelength 450 mμ. The value for σ is ±0.006 a.u., equivalent to ±0.08 p.p.m. V. The colored complex obeys Beer's law over the range 1–10 p.p.m. vanadium. The absorbance (in 1-cm cell) at 10 p.p.m. was so great that no data were obtained at higher concentrations. Under the conditions of the reaction, the combining ratio of vanadium and 2-naphthohydroxamic acid appears to be 1 to 2. Optimum conditions for the use of 2-naphthohydroxamic acid as a spectrophotometric reagent for vanadium-(V) were established; the procedure was applied to the determination of vanadium in steels and non-ferrous alloys with good precision and accuracy.     

Extract-photometric determination of vanadium(V) with 3-indole-acetohydroxamic acid

Summary A spectrophotometric study has been carried out of the violet complex 3-indole-acetohydroxamic acid-vanadium extracted into a solution of trioctylmethyl-ammonium chloride in toluene [_max 525 nm; =5381 l mol^–1 cm^–1; stoichiometry 1:3 (metal:reagent)]. A new method for the extract-spectrophotometric determination of V⁵⁺ in the range of 2–7 g/g is proposed and interferences by foreign ions were investigated. The method has been satisfactorily applied to the determination of vanadium(V) in fuel oil. The relative error is ±2.9%.     

The hexacyanomanganate(IV)-hydrogen peroxide reaction. Kinetic determination of vanadium

The reaction between hexacyanomanganate(IV) and hydrogen peroxide in acidic medium is strongly catalysed by vanadium. The stoichiometry has been proved to be Delta[Mn(IV)]/Delta[H(2)O(2)]=1. S-shaped absorbance-time curves are obtained for a wide range of conditions, which seems to be due to the transient formation of hydroperoxyl radicals, HO(2).; therefore, the reaction is autoinduced. The reaction kinetics is first order on the hexacyanomanganate(IV) concentration. Complex dependence of the initial rate on the H(2)O(2) as well as H(+) concentrations are observed both for the uncatalysed and for the catalysed reactions, but the rate laws are given and reaction mechanisms are proposed. The catalytic effect of vanadium has been used to determine traces of vanadium (the (IV) and (V) oxidation states are determined together). By applying the initial rate method a detection limit of 0.9 ng ml(-1) (18 nM), and a linear range up to 50 ng ml(-1) were found. The relative standard deviations for the 4 and 25.5 ng ml(-1) levels were 5.2 and 2.5% respectively. Positive kinetic interferences from Cr(VI), Hg(I) and Ag(I) have been observed; other interferences are less severe and Cr(IIl), Fe(II) and Fe(III), among many others, do not interfere. The Mn(IV) solution must be prepared daily and its conservation needs some care (0 degrees C) but the proposed method has been applied to the determination of vanadium in a phosphate rock (reference material, BCR No. 32) without previous separations. Recovery experiments have also been performed; excellent results were obtained in both cases.     

Anosovite-type v(3)o(5): a new binary oxide of vanadium

The reaction of either V(2)F(6)·4H(2)O or a mixture of 60 wt % VF(2)·4H(2)O and 40 wt % VF(3)·3H(2)O with a water-saturated gaseous mixture of 15-20 vol % hydrogen in argon leads to the formation of a new polymorph of V(3)O(5) crystallizing in the orthorhombic anosovite-type structure. Quantum-chemical calculations show that the anosovite-type structure is about 15 kJ/mol less stable than the corresponding monoclinic Magnéli phase. In addition, there are no imaginary modes in the phonon density of states, supporting the classification of the anosovite-type phase as a metastable V(3)O(5) polymorph. Susceptibility measurements down to 3 K reveal no hint for magnetic ordering.     

The solvent extraction of vanadium(IV) with HDEHP in benzene and kerosene The solvent-extraction of vanadium(IV) from sulphuric acid solutions with bis-(2-ethyl hexyl)-phosphoric acid in benzene and kerosene

The distribution of vanadium(IV) between sulphuric acid solutions and solutions of bis-(2-ethyl hexyl)-phosphoric acid (abbreviated as HDEHP or H₂A₂) in benzene and kerosene has been investigated as a function of the extractable metal ion, sulphate, acetate, nitrate and extractant concentrations, temperature and the nature of the diluents. The distribution coefficient has been found to increase with increasing concentration of the extractant, acetate, nitrate and temperature and with decreasing acidity and sulphate concentration. From the temperature dependence data, the apparent enthalphy change for the extraction reaction has been calculated to be ΔH = 4.74 kcal/mole. The molecular weight and the phosphorus-vanadium atom ratio of the solid complex have been found to be 2000 and 2:1 respectively. The mechanism of the extraction is discussed.     

Chiral dinuclear vanadium(v) catalysts for oxidative coupling of 2-naphthols

Preparation and structural analysis of chiral dinuclear vanadium(v) catalysts with high catalytic activity for the oxidative coupling of 2-naphthols are described.     

The OH(v = 9) + O3 reaction pathway

The oxygen-hydrogen system, including the reactive species H, O, H₂, O₂, O₃, OH, and HO₂, is very complex, and contains numerous reactions whose kinetics and branches have been insufficiently explored. In the present study we use computer modeling to simulate observations made in a 300-K ozone-hydrogen mixture, in which a critical H₂ pressure leads to rapid ozone decomposition, and generation of high concentrations of atomic oxygen. Initiation of the reaction chain involves heterogeneous O and/or H atom production, and the chain branching step is the reaction OH(v) + O₃ → OH + O + O₂, which is shown to be the predominant pathway for these reactants. The critical H₂ pressure (ca. 3 torr) sets important constraints upon the system kinetics.     

An analytical study of the vanadium(IV)-chromium(VI) reaction

Experimental conditions for reduction of chromium(VI) by vanadiuin(IV) are described. An excess of vanadium(IV) is necessary. The evidence indicates that the direct titration fails for kinetic rather than thermodynamic reasons, but that the back-titration procedure is made quantitative by the slow rate of the reverse reaction between vanadium(V) and chromium(III).     

The extraction of vanadium(IV) from hydrochloric acid solutions by di-(2-ethylhexyl)-phosphoric acid

The distribution of vanadium(IV) between hydrochloric acid solutions and solutions of di-(2-ethylhexyl)-phosphoric acid (DEHPA; HX) in organic solvent has been investigated under different conditions. Both the aqueous and organic phases have been examined spectrophotometrically. IR and electron spin resonance spectroscopies have been applied to the organic extracts. The mechanism of extraction is discussed on the basis of the results obtained.     

Extraction recovery of vanadium(IV) from acid sulfate solutions with di-(2-ethylhexyl)phosphoric acid

Extraction of vanadium(IV) with di-(2-ethylhexyl)phosphoric acid from acid sulfate solutions in the presence of sodium sulfate was studied. The composition of the complex being extracted was found, and the equation of the extraction reaction was determined. The equilibrium constant of the reaction by which vanadium(IV) is extracted with di-(2-ethylhexyl)phosphoric acid was found by taking into account the complexation of vanadium(IV) in acid sulfate solutions.