Tungsten blue reduction

Compounds intermediate between the di- and tri-oxides, and obtained from the latter by reduction, are known, and probably contain both tetra- and hexa-valent tungsten. These derivatives are generally blue in colour, and the existence of a definite blue oxide, of composition WjOj, appears to be established (see p. 199). Many of the blue reduction products appear to be mixtures of compoimds in various stages of oxidation. 

The effect of the addition of 0.1 mol of alkali metal to tungsten blue oxide is shown in Table 3.1. It can be seen that the influence in grain growth is considerable. Moreover, there are differences between the different metals which are mainly based on the properties of the metal. At the end of the reduction process the respective alkali tungstate is reduced and at 1000 °C the metal is evaporated. The vapor piessiue of Li is the lowest of the three metals and therefore the amount remaining is the highest. 

Folin-Denis method Reduction of complex polymeric ions formed from phosphomolybdic and phospholungslic heteropoly acids to complex molybdenum-tungsten blue. detection wavelength 725 - 770 nm recommended for uniformity 765 nm complexes and reagent are unstable in alkaline solution, formation of precipitates, controlled sequence and timing of the addition of reagents (reproducibility ), deviation from Beer-Lambert law (high phenol contents), reaction is stoichiometrically predictable 105,106,110... 

The blue oxides. These are also called molybdenum blue and tungsten blue and are obtained by mild reduction, for example, by Sn11, S02, N2H4, H2S, etc., of acidified solutions of molybdates and tungstates or of suspensions of Mo03 and W03 in water. Moist tungsten(vi) oxide will acquire a blue tint merely on exposure to ultraviolet light. 

The dark-blue, colloidal (W 020H)XW 03)i-4, Tungsten Blue , arising in an HCl mixture of with a little H28 or 8O2 provides an excellent test for tungsten or reductants ... 

Reduction to Metal Powder. The metal powder is obtained from APT by stepwise reduction with carbon or hydrogen. The intermediate products are the yeUow oxide, WO blue oxide, (see Tungsten compounds) and brown oxide, WO2. Because carbon introduces impurities,... 

For the production of lamp-filament wire, aluminum, potassium, and siHcon dopants are added to the blue oxide. Some dopants are trapped in the tungsten particles upon reduction. Excess dopants are then removed by washing the powder in hydroflouric acid. Eor welding electrodes and some other appHcations, thorium nitrate is added to the blue oxide. After reduction, the thorium is present as a finely dispersed thorium oxide. 

Tungsten pentachlofide [13470-13-8], WCl, mp 243°C, bp 275.6°C, is a black, crystalline, deHquescent soHd. It is only slightly soluble in carbon disulfide and decomposes in water to the blue oxide, 200 2. Magnetic properties suggest that tungsten pentachlofide may contain trinuclear clusters in the soHd state, but this stmcture has not been defined. Tungsten pentachlofide may be prepared by the reduction of the hexachloride with red phosphoms (9). 

Tungsten dioxide [12036-22-5] WO2, is a brown powder formed by the reduction of WO3 with hydrogen at 575—600°C. Generally, this oxide is obtained as an intermediate in the hydrogen reduction of the trioxide to the metal. On reduction, first a blue oxide, then a brown oxide (WO2), is formed. The composition of the blue oxide was in doubt for a long time. However, it has since been resolved that W2Q03g and W are formed as intermediates, which may also be prepared by the reaction of tungsten with WO3. 

T. WHken, C. Wert,J. Woodhouse, and W. Morcom, "Reduction of Blue Tungsten Oxide," in H. H. Hausner and P. V. Taubenblat, eds.. Modem Developments in Powder Metallurgy, Vol. 9, Plenum Press, New York, 1977, pp. 161—169. 

High-temperature reduction of Na2Ti03 with hydrogen produces nonstoichiometric materials, Na jTi02 (jr = 0.20-0.25), called titanium bronzes by analogy with the better-known tungsten bronzes (p. 1016). They have a blue-black, metallic appearance with high electrical conductivity and are chemically inert (even hydrofluoric acid does not attack them). 

Mechanistically the colour is formed by an optical charge-transfer between metal centres in the solid-state lattice, e.g. in tungsten trioxide this involves partial reduction of the pale yellow W" to the blue state. This reduction requires partial insertion of a balancing cation, as shown schematically in Figure 1.32, where M is usually lithium or hydrogen. 

Salts containing pentavalent tungsten may be obtained by the reduction of alkali tungstate in concentrated hydrochloric acid. Salts of types M(I)2(WOC1s) (green), M(I)(WOQ4) (brown-yellow), andM(I)(WOCl4 H20) (blue) have been isolated. Thiocyanato andbromo salts are also known. 

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