Potassium Tungstates

The anhydrous normal salt, K₂WO₄, is prepared by fusing together equivalent quantities of tungstic anhydride and potassium hydroxide or potassium carbonate, the resulting mass being taken up with water and allowed to crystallise; or by boiling a solution of potassium hydroxide or carbonate in presence of tungstic acid. Thin, needle-shaped, monoclinic prisms, with axial ratio,

a:b:c = 1.9702:1:1.2341; and β = 113° 15',

and isomorphous with potassium manganate and potassium molybdate,1 are obtained, which are readily soluble in water, decrepitate when heated, and melt at about 921° to 926° C. The salt, however, appears to be dimorphous, laminated crystals often accompanying the prisms.

The dihydrate, K₂WO₄.2H₂O, crystallises when the solution is concentrated over sulphuric acid at temperatures below 10° C. in lustrous monoclinic prisms and tables, with axial ratio

a:b:c = 0.9998:1:0.7880; and β = 110° 57'.

The crystals effloresce in dry air, but are deliquescent in moist air. Other hydrates have been described, for example, K₂WO₄.H₂O, slender needles, which was probably partly dehydrated dihydrate; and K₂WO₄.5H₂O, hexagonal prisms, which according to Ullik was a sodium-potassium tungstate.

The solution of normal potassium tungstate is strongly alkaline in reaction; its density at various concentrations at 15° C. has been determined as follows:

Concentration (per cent.)	2.42	6.57	8.72	16.19
Density	1.0202	1.0575	1.0781	1.1548

Various acid tungstates have been described. The compound formulated as the ditungstate, K₂O.2WO₃.3H₂O, in earlier literature has been shown to be the paratungstate.

Potassium paratungstate, 5K₂O.12WO₃.11H₂O or 3K₂O.7WO₃.6H₂O, is prepared by the action of acid on the normal tungstate; for example, (i) by saturating a solution with carbon dioxide, (ii) by adding acetic acid to a cold concentrated solution and boiling the resulting white precipitate with water, or (iii) by boiling the solution with a solution of tungstic acid. It may also be obtained by fusing together tungstic anhydride with potassium carbonate or potassium tungstate, and lixiviating the fused mass. The salt is deposited from solution as a crystalline powder consisting of iridescent scales. Electrolysis of a solution of the normal tungstate in a cell in which the electrodes are separated by a diaphragm also yields the paratungstate.

The crystals are triclinic pinacoids,

a:b:c = 0.9137:1:1.1362; α = 65° 36', β = 117° 22', γ = 115° 39',

and are isomorphous with ammonium paratungstate. The salt is somewhat soluble in cold water and dissolves readily in hot. Solubility data by Anthon, Riche, and Marignac are not in agreement. According to the last named, 1 part of the salt dissolves in 71 parts of water at 20° C., but when a saturated solution is prepared by boiling the salt with water for several days the cooled solution at 18° C. contains 1 part of the salt

after 1 day dissolved in 5.62 parts of water
after 26 day dissolved in 11.9 parts of water
after 153 day dissolved in 15.6 parts of water
after 334 day dissolved in 15.6 parts of water

It is insoluble in alcohol. When heated it loses water, and at a red heat fuses, undergoing decomposition,2 and on cooling sets to a yellow crystalline mass.

Two other hydrates of potassium paratungstate, 5K₂O.12WO₃.10H₂O and 5K₂O.12WO₃.14H₂O, or 3K₂O.7WO₃.8H₂O, have been described.

The salt, 2K₂O.5WO₃.4H₂O, is obtained as an amorphous precipitate by adding a cold concentrated solution of the normal salt to excess of glacial acetic acid; it is soluble in 20 parts of water, and the solution on evaporation at ordinary temperatures deposits prismatic tables. The solution decomposes on boiling. This salt may also be produced by the addition of potassium chloride to a hot solution of sodium paratungstate, or by crystallisation from a hot solution of potassium paratungstate.

Potassium tritungstate, K₂O.3WO₃.2H₂O, is obtained, by pouring a concentrated solution of the normal salt into a boiling solution of acetic acid, as a white precipitate, which must be filtered immediately and washed with alcohol in order to prevent the formation of metatungstate. It dissolves in 5 to 6 parts of water, and the solution on concentration yields fine needle-shaped crystals or prisms; but on boiling or prolonged heating decomposition occurs.

The acid salts, 5K₂O.14WO₃ and K₂O.8WO₃, have also been described.

The double tungstate, K₂Na₄(WO₄)₃.14H₂O, has been obtained by Ullik, whilst Hallopeau describes a tungstate of potassium and tungsten, of composition K₂O.WO₂.4WO₃, prepared by heating potassium paratungstate with tin for one hour at a temperature sufficiently high to melt the tungstate. On treating the residue successively with boiling water, concentrated hydrochloric acid, boiling potassium carbonate (50 per cent, solution), and finally hot water, the compound is obtained as dark blue prismatic crystals.