Tungstate Complexes Research Articles

Comparative study of various polyols as complexing agents for the acidimetric titration of tungstate

Most polyols (L = alditol or carbohydrate) form dinuclear tungstate complexes according to the over-all equilibrium 2WO4(2-) + 2H+ + L equal equilibrium [W2O7L]2- + H2O. When the reaction is fast and complete, it allows the acidimetric titration of tungstate. The formation constants of the complexes of a series of polyols were determined by potentiometry. Their values were higher at low ionic strengths. Opposite structural factors govern the stabilities and the formation rates of complexes: alditols of threo configuration react with tungstate faster than those of erythro configuration, but the stability order is erythro greater than threo. Of the polyols investigated, only xylitol and D-glucitol (sorbitol) allowed a fast and accurate potentiometric titration. Using 0.02 M HCl as titrand, 0.04 mmol of tungstate was determined in a volume of 100 cm3. The interference of molybdate is discussed in detail.

Identification of tungstate complexes in lithium-tungstate-borate glasses by Raman spectroscopy

Changes in the Raman spectra of (Li 2B 4O 7) 0.5(WO 3) 0.5 glasses, when prepared under slightly different conditions, are interpreted as arising from tungstate complexes in the glass matrix; one of the observed extra vibrational modes can be correlated with a permanent blue color center which is reminiscent of the electrochemically induced tungsten bronze centers.

Stability constants and structures of homologous dinuclear molybdate and tungstate complexes of aldoses by potentiometry and 13C and 95Mo NMR

A potentiometric study showed that aldoses of the lyxo-series: d-lyxose, d-mannose and l-rhamnose, form dinuclear anionic complexes with tungstate or molybdate ions in acidic solutions. 95Mo NMR confirmed this result by revealing the presence of two non-equivalent metal atoms in the molybdate complexes. The corresponding formation constants were determined. The d-lyxose complexes had the higher stabilities, and the W VI complexes were more stable than their Mo VI homologues. The 13C NMR spectra were reassigned by means of a 2-D experiment and showed that all three carbohydrates were complexed in pyranose form. The chelating hydroxyl groups were strongly deshielded and displayed increased direct coupling constants 1 J CH. It can be concluded that the prevailing species in aqueous acidic solution cannot be furanose complexes similar to that characterized in solid state from molybdate— d-lyxose mixtures.

Chromatography of polyhydroxy compounds on cellulose impregnated with tungstate: determination of pseudo-stability constants of complexes

Impregnation of cellulose with tungstate affects the chromatographic migration rates of a number of polyhydroxy compounds, offering a means, on small and large scales, of resolution of mixtures not easily achieved by other methods. The effects have been correlated with the structure of the polyhydroxy compounds and their tungstate complexes. Pseudo-stability constants of the complexes have been determined.

Luminescent sites in CaWO4 and CaWO4 : Pb crystals

The transmission and emission spectra of CaWO4 and CaWO4/(1 × 10-4) Pb crystals are measured. In Pb-doped CaWO4 an additional relatively sharp absorption and broad emission band are observed. The absorption band is attributed to electronic transitions within the Pb2+-ion, the emission band to transitions in WO2-4-complexes stabilized by neighbouring Pb2+-ions. Quenching of CaWO4 crystals produces new emission bands which are ascribed to higher tungstate complexes. The optical transitions are discussed in a MO-diagram of tungstate complexes.

Electronic Structure of the Vanadate and Tungstate Complexes

The excitation and emission spectra of unactivated yttrium vanadate and calcium tungstate have been measured. Relative absorption spectra were derived from the excitation spectra. In order to aid in the interpretation of the absorption and emission, a series of semiempirical LCAO‐MO calculations were carried out on the vanadate and tungstate complexes. These calculations indicate that the transitions primarily responsible for absorption are the charge transfer transitions . The vanadate emission peak is accounted for by an increase of 0.15Aå in the vanadium‐oxygen distance for the excited state. Electrostatic splitting and the effects of lowering of symmetry from to are discussed.

Microdetermination of dl-alanine. II. Microdetermination of l-glutamic acid, dl-valine, and dl alanine in a mixture in one solution without separating

Abstract dl -Alanine was determined in micro amounts separately and in a mixture of dl -valine and l -glutamic acid in one solution directly. Sodium tungstate, gold chloride, and potassium tellurite form complexes with l -glutamic acid, dl -alanine, and dl -valine in the ratios of 1:2, 1:3, and 1:4, respectively. Potentiometric data and results in analysis support the complex ratios. Maximum errors in determinations of l -glutamic acid, dl -alanine, and dl -valine are 0.0, 1.4, and 0.16%, respectively. Ranges mg/liter) in which l -glutamic acid dl -alanine, and dl -valine were estimated in a mixture were 5.892 × 10 −4 to 14.730 × 10 −4 ; 4.6602 × 10 −4 to 18.6209 × 10 −4 ; and 11.6956 × 10 −4 to 29.2390 × 10 −4 respectively.

4. Complexes between polyhydroxy-compounds and inorganic oxy-acids. Part V. Tungstate complexes of acyclic polyhydroxy-compounds

4. Complexes between polyhydroxy-compounds and inorganic oxy-acids. Part V. Tungstate complexes of acyclic polyhydroxy-compounds

Complexes between polyhydroxy-compounds and inorganic oxy-acids. Tungstate complexes on sugars and other cyclic polyhydroxy-compounds

Complexes between polyhydroxy-compounds and inorganic oxy-acids. Tungstate complexes on sugars and other cyclic polyhydroxy-compounds

Tungstate complexes with catechol

Tungstate ion complexes with catechol to form anions containing tungstate and catechol in the ratios 1:1, 1:2 and 1:3; the compositions of the anions in solution have been obtained by Job's method and by conductometric and potentiometric titrations. On precipitation with heavy metal cations, the 1:1 complex is usually obtained, but the 1:2 complex is the most stable species in solution. The change from the 1:2 to the 1:1 complex can be followed by pH measurement. Possible structures of the complex anions are discussed.

934. Tungstate and molybdate complexes with glycollic and mandelic acid

934. Tungstate and molybdate complexes with glycollic and mandelic acid

717. Tungstate and molybdate complexes with tartaric, malic, and succinic acid

717. Tungstate and molybdate complexes with tartaric, malic, and succinic acid

The Mechanism of the Tungsten Alloy Plating Process

The mechanism of tungsten alloy plating was investigated. Absorption spectra indicate the possibility of cobalt tungstate complexes in the citrate cobalt‐tungsten plating bath. Polarographic evidence is given for the existence of complexes of tungstate and citrate. Calculations of energies of activation of iron‐tungsten alloy deposition show that the tungstate reaches the cathode by diffusion rather than by migration. The mechanism postulated for the plating of tungsten alloys includes (a) the deposition of a film of partly reduced tungstate on the cathode and (b) the catalytic reduction of this film by hydrogen in the presence of freshly deposited iron, cobalt, or nickel.