Tungstic acid, yellow

Yellow amorphous powder density 5.59 g/cm decomposes at 100°C insoluble in water and most acids soluble in hydrofluoric acid, caustic alkalies and ammonia solution. Freshly prepared tungstic acid containing a molecule of water of crystaUization is moderately soluble in water. 

Silver nitrate solution pale yellow precipitate of silver tungstate, soluble in ammonia solution, decomposed by nitric acid with the formation of white hydrated tungstic acid. 

Tungsten oxytetrachloride forms splendid transparent, ruby-red, needle-shaped crystals which melt at 210° C. and boil at 227-5° C., giving a red vapour of normal density. - By water it is decomposed, giving first the yellow oxychloride, and further, hydrochloric and tungstic acids. A certain amount of the yellow oxychloride is also formed from it by loss of chlorine on sublimation in air or oxygen. ... 

Tungstic acid, H2WO4, is formed as an amorphous yellow precipitate when an excess of hot hydrochloric acid is added to a solution of an alkali tungstate. If cold acid is used a white precipitate of the hydrate, H2WO4.H2O, results, from which the acid may be obtained either by boiling the mixture or by drying over sulphuric acid. It may also be prepared by the following methods ... 

A solution of the paratimgstate is boiled with excess of the yellow tungstic acid until the filtered solution no longer gives a precipitate with hydrochloric acid. It is then concentrated and allowed to crystallise slowly over sulphuric acid. 

Solutions of alkali tungstates are precijhtated h>y dilute mineral acids, yellow tungstic acid HgWOj or the white hydrate HgWO. HjO being formed (see p. 205). The precipitate readily dissolves in alkalies, and on reduction yields the blue oxide. 

To obtain WOg(cr) in a form soluble in the calorimetric solution, Spitsyn and Patsukova dehydrated H2W0 (cr) at the relatively low temperature of 250"C. The resulting WOg(cr) was a very fine lemon-yellow powder which may have excess surface free energy compared with the JANAF standard state. The latter is WO formed at high temperature in a calorimetric bomb. The actual enthalpy of formation of tungstic acid should be less negative if this effect is significant. 

The hexafluorides of molybdenum and tungsten arc volatile liquids (respective b.p. 35.0° and 19.6°), readily hydrolysed to molybdic and tungstic acids. WClg is a solid. The metals form oxide chlorides, MO.,Cl2. Chromyl chloride, familiar as a distinguishing test for a chloride, appears as a yellow distillate on heating a chloride with KgCrgO and concentrated H2SO4 ... 

Properties Yellow powder. D 5.5. Insoluble in water soluble in hydrogen fluoride and alkalies. A white form of tungstic acid exists, having the formula I I2W( )4 I I2(). This is formed by acidifying tungsten solutions in the cold. 

The most important compounds of Mo(VI) and W(VI) are the oxides and the molybdate and tungstate anions. White M0O3 (mp 1073 K) is usually made by reaction 22.34, and yellow WO3 (mp 1473 K) by dehydration of tungstic acid (see below). Both oxides are commercially available. 

II. A boiling solution of 200 g. of ammonium tungstate [composition 2 (NH4)30, 0.5 WO3, 3 H3O] in 4.48 liters of HsO is poured into 2 liters of boiling 35.4% HCl. The deep-yellow precipitate is filtered off and purified by a nine-day dialysis (until the wash water is free of Cl and the pH has reached a constant value of 4.4). Air drying of the residue gives a 55% yield of tungstic acid of composition WO3 1.18 H3O it still contains traces of NH3 and Cl. ... 

Long, lustrous, red needles, yellow in transmitted light. M.p. 209°C, b.p. 232°C. Decomposed at once by water, more slowly by atmospheric moisture, forming tungstic acid. 

A solution of 20 g. of NagW04 2 HgO in 200 ml. of HgO is prepared and an excess of yellow tungstic acid is added to it in portions. The suspension is boiled for about 1.5 hours, which produces white insoluble precipitates, settling out together with the excess tungstic acid. The pH of the solution after boiling and filtration is about 3. It is concentrated as described above. 

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