Tungstate Anions Research Articles

The effect of the isopoly tungstate anion on the composition and conductivity of polypyrrole composite films

The polypyrrole composite films were prepared by the electropolymerization of pyrrole from the aqueous solution containing isopoly tungstate (poly-W) and the chloride anion (Cl −) at various ratios. The film formation was traced by electrochemical quartz crystal microbalance, and the tungsten concentrations in the films were measured by the inductively coupled plasma spectroscopy. The electric conductivities of the films were measured by the four-point probe method. In the ratio of poly-W concentration to Cl − concentration, ((poly-W)/Cl −)), higher than 1.26 × 10 −2 in the solution the concentration of tungsten in the films showed a constant value at 51 wt.% and the chloride anion was not included in the film. The conductivities of these films showed the constant value about 40 S cm −1. In the ratio lower than (poly-W)/(Cl −) = 1.26 × 10 −2, the conductivity decreased with the decrease of poly-W concentration and the composite films consisted of two layers; the inner layer initially formed contained larger amount of tungsten than that of the outer layer.

Formation of anodic alumina films in tungstate/ethylene glycol electrolyte

The growth of barrier-type anodic films at 50 A m−2 in 0.1 M tungstate/ethylene glycol electrolyte at 293 K has been studied using Rutherford backscattering spectroscopy and transmission electron microscopy. The resultant films are composed mainly of alumina and form at a Faradaic efficiency approaching 100%. The films are contaminated in discrete regions by incorporated species consisting of either directly-incorporated or derivatives of tungstate anions and solvent molecules. The tungsten species incorporated into the film are immobile during film growth, contrasting with the behaviour of tungsten species incorporated from aqueous electrolytes, which migrate outwards. The modified behaviour is attributed to incorporation of tungsten and solvent species into interstices in the alumina structure. Additionally, a mobile solvent species is present in the film which migrates inwards at about 60% of the migration rate of O2−/OH∼-ions.

Electrochemical quartz crystal microbalance study of polypyrrole and tungstate polyanion composite films

A composite film comprising polypyrrole (PPy) and isopoly tungstate (PW) anion was prepared by electropolymerization of pyrrole in an aqueous solution of polytungstic acid. The formation of the PPy-PW film was traced using an electrochemical quartz crystal microbalance (EQCM). The oxidation-reduction behavior of the film was also investigated by the EQCM and compared with that of PPy film prepared in acidic KCl solution. From the EQCM measurements, the growth rate of the PPy-PW film under constant current oxidation at 0.186 mA cm −2 was evaluated as 0.97 μg mC −1, which was 2.8 times as high as that of PPy-Cl film prepared in acidic KCl solution. From emission spectroscopic analysis, the composite PPy-PW film was found to contain 51 wt.% tungsten, or 67 wt.% meta-polytungstate anion, H 2W 12O 40 6− The PPy-PW film behaved like a redox polymer film in which the W(V)/W(VI) couple of the doped polytungstate anion functions as a reversible redox center. The doped PW changes the ionic permeating property of the polymer film; the PPy-PW film reveals protonic permeability, although the ordinary chloride ion doped PPy has anionic permeability.

Electrochemistry of the Transfer of Isopoly and Heteropoly Anions across the Water/Nitrobenzene Interface: Isopoly tungstate and Heteropolytungstate Anions

The transfer behavior of the heteropoly anion [H3PW11O39]4- and the isopoly anion [H2W12O39]4- across the water/nitrobenzene interface was investigated by cyclic voltammetry and chronopotentiometry with linear current scanning. The transfer processes were

Synthesis and x-ray structural characterization of oxo-persulphido dinuclear tungstate anions: {W 2O 2(μ 2-S)(S 2) 4} 2− and {W 2O 2(μ 2-S 2)(S 2) 4} 2−

The reaction of WOS 3 2− and elemental sulphur with the participation of NH 2NH 2·2HCl in DMF solvent at ambient temperature has led to the dinuclear complex anions [W 2O 2(μ 2-S)(S 2) 4] 2− and [W 2O 2(μ 2-S 2)(S 2) 4] 2− as a solid mixture in the structure of [Et 4N] 2{W 2O 2(μ 2-S)(S 2) 4} 0.77 {W 2O 2(μ 2-S 2)(S 2) 4} 0.23. Both anions are composed of two neutral [WO(S 2) 2] subunits bridged by either a sulphide (in W 2O 2S 9 2−) or an η 1,η 1-di-sulphide (in W 2O 2S 10 2−). Each anion contains tungsten(VI) ions with pentagonal bipyramidal environments, with the axial site occupied by a terminal oxo atom and the other one by the intramolecular approach of a sulphur from a disulphido group bonded to the other tungsten atom. The five equatorial atoms of each pentagonal bipyramid, which are defined by two η 2S 2 2− ligands and a coordinating sulphur atom, are almost planar. The possible pathway for the formation of the title complexes is depicted and the spectroscopic properties of the solid mixture are reported.

Surface functionalization with oxometallates entrapped in polymeric matrices: Part 3. Homogeneous vs. heterogeneous-phase formation and equilibria of isopolytungstates

The polymerization processes of tungstate anion in acidic aqueous solution are known to be very complex. The main species in these equilibria depend on several factors, including the ratio of protons to tungstate in the solution. Conditions have been selected in which decatungstate should be a major product. However, in homogeneous acidic media, pH 2, irrespective of the anion present (CF 3COO −, SO 2− 4 or C −), it was not possible to observe a clean voltammogram of decatungstate at room temperature. By contrast, it is shown that the influence of poly(4-vinylpiridinium) film (PVP) results in a remarkably stable decatungstate in the polymer matrix, in contact with an acidic aqueous electrolyte at room temperature. This observation attributable mainly to the polymer, and parallels the known stability of decatungstate in non-aqueous solutions. The influence of PVP on the electrochemical behaviour of decatungstate has been studied.

Surface modifications with heteropoly and isopoly oxometalates: Part III. electrode modification procedures. the necessity of proton interference during the electrodeposition of the h.e.r. catalysts

Heteropoly and isopoly oxometalates and tungstate anion can be used to modify persistently the surface of electrode materials. The test reaction chosen for demonstrating a satisfactory and useful derivatization is the hydrogen evolution reaction (h.e.r). From a practical point of view, the standard modification method in aqueous acid, which succeeds whatever the oxometalate, is obtained by poising the working electrode potential at -1.2 V vs. SCE. However, this method is too crude; to get a first insight into the various steps of such a modification, the gradual variation of the electrolysis potential has proved very helpful. The results show that no useful derivatization of the electrode surface for the h.e.r. is obtained, unless the potential of the working electrode is at least negative of a proton-dependent wave of the oxometalate. At more positive potentials, only adsorption products which are non-catalytic towards the h.e.r. are observed on the surface. Confirmation has been obtained by the use of a non-aqueous solvent such as dimethylformamide (DMF). These results, in water as well as in DMF, show that numerous nuances of the modification procedures can be devised, which all end up in a satisfactory derivatization, the choice depending on whether it is desired to optimize the duration or to try and understand some of the phenomena. In particular, direct adsorption of the oxometalate on the electrode surface prior to electrolysis in pure acidic solution has also proved successful. The possibility that the chemical integrity of the starting material be preserved during modification is discussed.

Epoxidation of olefins by hydrogen peroxide in the presence of supported tungstate anions: G. Gelbard, M. Quenard and V. Bonmarin, C.N.R.S., Vernaison, France

Epoxidation of olefins by hydrogen peroxide in the presence of supported tungstate anions: G. Gelbard, M. Quenard and V. Bonmarin, C.N.R.S., Vernaison, France

Surface modifications with heteropoly and isopoly oxometalates: Part II. Electrocatalytic behaviour of glassy carbon surfaces modified with 17-tungsto, 1-molybdo-diphosphate

Glassy carbon electrodes can be modified persistently with heteropoly and isopoly oxometalates and with tungstate anion. The present communication describes such a modification using 17-tungsto, 1-molybdo-diphosphate. In neat 0.5 M H 2SO 4, this modified material is characterized by a persistent surface redox couple. Activation of this modified glassy carbon surface with respect to the hydrogen evolution reaction, to the oxygen reduction process which is proton consuming or vis à vis the simple outer-sphere electrode reaction of hexammine ruthenium(III) cation, is demonstrated. For the hydrogen evolution reaction and oxygen reduction process, a platinized-platinum-like behaviour has been observed. For the ruthenium complex, a comparison has been made with freshly polished glassy carbon and with gold and it appears that on the modified surface, the apparent electron transfer rate constant is as high as on the gold surface. The activation is durable.

Chemical accessibility of sers active structures on anodized and electroplated copper surfaces

Paratungstate A adsorbed on an electrodeposited copper surface can be virtually completely decomposed to tungstate anion via contact with a basic aqueous solution. The decomposition can be monitored by observation of the SER signal in the WO stretching vibration region of the spectrum. Copper surfaces prepared by anodization in the presence of paratungstate A yield two populations of adsorbates: those than can be decomposed to tungstate anion and those that remain unchanged. It is proposed that some adsorbates are trapped under the surface, imbedded in the surface, or otherwise kept from contacting the bulk solution. The presence of such sequestered species produced by SER electrode preparation in the presence of the adsorbate must therefore be considered when interpreting the spectroscopic and electrochemical data obtained using such surface preparation procedures.

Electron spin resonance and optical resonance studies on copper(II) complexes with vanadate, molybdate and tungstate anions

A series of complexes of the form Cu(An) n L x · yH 2O, where An = vanadate, molybdate or tungstate anion, L = pyridine, α-, β- or γ-picoline, n = 1 or 2; x = 1−4 and y = 0−3, have been synthesized and characterized by electron spin resonance, photoacoustic and electronic spectral studies as well as infrared spectroscopy in the solid and solution state. The different coordination sites of the ligands and anions have been interpreted from infrared and electronic spectral data. Electron spin resonance spectral data show the square planar or distorted octahedral (in a few cases five-coordinate geometry) stereochemistry around copper(II) in these complexes. Parameters such as g ∥, g ⊥, A ∥, A ⊥, 〈 g〉, 〈 A〉, α 2 and β 2 1 calculated from electron spin resonance data indicate the presence of unpaired electrons in d x 2− y 2 or d z 2 orbitals. The results of electronic and photoacoustic spectral studies are in good accord with ESR data.

Raman-spectrophotometric determination of the tungstate anion and its isopolyanions in aqueous systems

The tungstate and some isopolyanions were determined by Raman spectrophotometry. Using the more intensive Raman active vibrational bands of these species and the bending vibration of water at 1,650 cm−1 as an internal standard the linear ranges for the measurement of concentrations were elucidated. The detection limits for tungstate, paratungstate-A, ψ-metatungstate and metatungstate are 4×10−3 M, 9×10−3 M, 10−2 M and 10−2 M, respectively. This technique was used to examine a mixture of tungstate and paratungstate-A.

Direct observation of the duplex nature of anodic barrier films on aluminum

Transmission electron microscopy of ultramicrotomed sections of anodic barrier-type films formed on aluminum in tungstate solution reveals directly the duplex nature of the anodic films. Immediately on exposure to the electron beam, a relatively dark region of film material is evident adjacent to the film-solution interface and separated from the metal substrate by a more extensive lighter region of film material. Subsequent investigation using a combination of scanning transmission electron microscopy (STEM) and energy-dispersive analysis of X-rays (EDAX) confirms the presence of tungsten in the outer darker region of film material, and induced crystallization shows that the inner region of relatively pure alumina crystallizes at a faster rate than the region in which tungstate anions are incorporated. Thus, depending on the anodizing conditions, three techniques are available for the determination of the extent of such anion incorporation readily in the electron microscope, which give further insight into the apparent transport numbers during film growth. For the situation in which anodizing is carried out in a solution containing an anion of a heavy metal with a relatively high electron-scattering cross section, and in which such species are incorporated within the film, apparent transport numbers can be determined directly without the need for induced crystallization and/or STEM-EDAX analysis.

The preparation and characterization of Ba 3Te 2O 9; a new oxide structure

The structure of Ba 3Te 2O 9 has been determined from x-ray powder diffraction data and by profile refinement of neutron diffraction data. We find tellurium octahedrally coordinated as expected and the same face-shared [B 2O 9] 6− unit as observed in Ba 3W 2O 9. The phase Ba 3Te 2O 9 has, however, a Cs 3Fe 2F 9 type structure ( P6 3 mmc , a = 5.8603(1) A ̊ , c =14.3037(6) A ̊ ) rather than the Cs 3Tl 2Cl 9 structure (R3c) found for the tungsten analogue. The two oxide structures have the same BaO 3 layer sequence but differ only in the spatial arrangement of the [B 2O 9] groups in the lattice. Infrared and Raman spectra confirm the different site symmetries associated with the different packing of the tungstate and tellurate anions in their respective structures.

Mixed valence and magnetically coupled vanadate domains in heteropoly tungstate anions

Mixed valence and magnetically coupled vanadate domains in heteropoly tungstate anions

Derivatives of the undecatungstogallate(III) anion. Cupro(II) and mangano(II) undecatungstogallate(III) preparation, properties, structure and salts

Methods have been developed for the preparation of both Na and NH 4Na salts of cupro(II)- and mangano(II)undecatungstogallate(III). X-ray structural studies are in agreement with a “Keggin-type” structure for both of these heteropoly tungstate anions. Visible spectral studies showed that the Cu(II) ions are octahedrally coordinated in its anion. Cryoscopic studies in saturated sodium sulfate decahydrate revealed that both heteropoly anions are quite stable (no detectable dissociation) in this solvent at 33°C. Potentiometric titrations showed that cupro(II)- and mangano (II)undecatungstogallate(III) anions react with 4.3 and 4.0 moles of protons per mole of complex, respectively.

Preparation and Characterisation of Some Pyridine Compounds of Cu(II) with Molybdate, Vanadate and Tungstate Anions

Preparation and Characterisation of Some Pyridine Compounds of Cu(II) with Molybdate, Vanadate and Tungstate Anions

The Anodic Oxidation of Tungsten in Aqueous Base

The kinetics of the anodic oxidation of tungsten metal in basic aqueous solution has been investigated using galvanostatic polarization. Anodic Tafel slopes were constant over all concentrations of hydroxide at 0.136 V/decade. Cathodic Tafel slopes ranged from 0.069 to 0.197 V/decade, increasing with hydroxide ion activity. Anodic exchange current density, obtained by extrapolation, averaged . Single ion activities, estimated by Harned's rule, in sodium hydroxide solutions buffered with sodium nitrate to constant ionic strength, yielded rate data indicating first‐order dependence on hydroxide ion activity over a concentration range of 0.02–6.00M OH−. A mechanism commensurate with the observed behavior is proposed involving oxidation to intermediate tungsten oxides, followed by nonelectrochemical dissolution to form the soluble tungstate anion. A brief discussion of the data for the hydrogen evolution reaction is also presented, in the light of recent studies on tungsten covered by oxide layers.

Luminescence of lead molybdate and lead tungstate. II. discussion

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Rapid acidification of solutions containing tungstate anions

Potentiometric titration of H+ against tungstate, WO42–, ions shows that the product formed in the first 0·01 s is not paratungstate A, HW6O215–, or any discrete ion intermediate between WO42– and HW6O215–, but is a mixture of a number of ill-defined products. These may be colloidal hydrated tungsten(VI) oxides formed very rapidly in local regions of high acidity before complete mixing can occur. These are then rapidly (ca. 1 s) converted into HW6O215–. The pH was measured by an indicator–spectrophotometric technique using a stopped-flow spectrophotometer.